Image stabilization can be done by the lens or by the camera sensor (or both, if the manufacturer has designed systems which interact rather than fight each other). The industry acronym for the latter is IBIS (In-Body Image Stabilization).
Here's a quick list of some other fundamental concepts:
f = Focal length (from backplane to focal plane, if focused at infinity).
d = Distance between the principal planes.
A = Aperture (diameter of transparent disk on backplane).
Almost all lenses used in modern photography have an adjustable aperture,so the "aperture" listed among the characteristics of a lens is really the maximal possible one (iris fully opened and lens focused at infinity). In addition to the above the following parameters are measured when a lens is focusedon an object at a finite distance:
D = Focusing distance (from object to focal plane).
r = Reproduction ratio (size of image divided by size of object).
Nikonand other manufacturers mayindicate the position of the focal plane by a grove on the bodies of their cameras. Without accessories (extension rings or bellows) D has a minimum value D0 corresponding to the normal use on the intended camera mount. The maximum value of r is a function of that.
Opticians often use following variables which are functions of the above.
p = Distance from the object to the frontplane (outer principal plane).
p' = Distance from the image to the backplane (inner principal plane).
The above definitions imply that p = D-d-p' The imaging equations for convex lenses are:
1 / f = 1 / p + 1 / p' and r = p'/p
Eliminating p' , we obtain the relation between D and r :
p = f ( 1 + 1/r ) = (D-d)) / (1+r) or f (1+r)2 / r = D-d
With extension rings (and/or bellows) of total length X, the maximumvalue of the reproduction ratio is thus the solution in r of the aboveequation with D = X+D0 which may be rewritten:
r2 + 2 r [ 1 - (X+D0-d) / 2f ] + 1 = 0
This equation has a (real) solution only when X+D0-d ≥ 4f I recommend expressing the (positive) solution with the followingnumerically robust form which is much moreconvenient, on modern scientific calculators, than the equivalenttraditional quadratic formula involving square roots:
r0 = exp ( sinh-1 [ 1 - (X+D0-d) / 2f ] )
For example, published specifications for the AF-SDX Micro NIKKOR 40mm f/2.8G give f = 40 mm, D0 = 163 mm and r0 = 1.0. The value of d is given by the equation:
d = D - f (1+r)2 / r = 163 - 40 (1+1.0)2 / 1.0 = 3 mm (The intended/correct value is 0 mm for a perfectly color-corrected lens.)
TheNikon F-mount features a distanceof 46.5 mm from focal plane to flange. That should be added to the published nominallength of this lens (64.5 mm when focused at infinity) to obtain the distance (111 mm) from the image of infinity tothe front of the lens. Subtract this from the aforementioned 163 mm and youobtain the largest extension size (52 mm) usable with this lens (corresponding to the dubious case of photographing a backlit objectnearly touching the front of a lens focused at infinity).
To copy old-school 35 mm film slides on a DX sensor,a reproduction ratio of about 1.5 is needed, which would be achieved using an extension ring of 6.7, mm with the lens on its fullest macro setting. Using the thinnest commercially available extension ring (12 mm) a reproduction ratio of 1.5 is obtained in the middle of the lens focusing range (it's 1.7 at full macro).
(2014-11-28) Nearest and farthest distances in focus at an acceptable sharpness.
When an object point on the optical axis is in sharp focus,the rays emanating from it converge to a singlepoint on the focal plane. If it's slightly out-of-focus, then they forma cone whose apex is not on the focal plane. That cone intersect the focal plane in a circle called the circle of confusion. When the diameter of that circle is small enough (typically defined as lessthan 0.030 mm in 35 mm photography) the object is in acceptable focus.
When a print of prescribed sharpness is desired using differentformats of negatives, we are imposed a constant ratio betweenthe focal length and the diameter of the circle of confusion. As a result, the hyperfocal distance is directly proportionalto the focal length or, equivalently, to the size of the negative.
Therefore, the larger the format, the tighter the depth of field.
(2014-11-29) The aspect of out-of-focus regions.
(2015-04-26) Controlling the spherical aberration of specialized lenses.
(2017-03-24) Smooth transition focus (STF) for perfect bokeh.
Markus Keinath (article quoted in footnote) has observed that STF could be achievedeasily by firmware control of the iris of any lens, by opening (or closing) the iris progressively during exposure (during a period of time when the shutter is fully open). This has never been done before, at this writing, and it would be a revolution for bokeh addicts.
An apodization filter may inhibit phase-detection autofocusing (it doesn't interfere with contrast-detection autofocus).
(2015-05-03) 2 colors (achromat) or 3 (APO, apochromat) or 4 (superachromat)...
The refracting index of glass (or any other medium) is subject to dispersion, which is to say thatit varies from one wavelength of light to the next. The different properties of an optical system at different wavelengths are collectivelyknown as color aberration (they translate into color fringes observed on sharply contrasted parts on an optical image). Mirrors are immune to it, lenses aren't.
Isaac Newton, who inventedthe reflecting telescope, once stated that it wasn't possible tobuild a refracting optical system free of color aberration.
It took thirty years to prove him wrong. Kinda. As early as 1729 (or 1733, according to some accounts) the amateur optician Chester Moore Hall figured out that different kinds of glass could be used to design an opticalsystem which forms identical images for red light and blue light (because the index of refraction increases with wavelength in some glasses and decreasesin others.
Apochromatic Lenses and Beyond :
Solving what happens at both extremities (red and blue) of the visiblespectrum may diminish the problem in the middle as well (green) butit doesn't quite solve it. It would take more than 30 years before someone would design a lenswith the same characteristics at three colors instead of just two (such a lens is now call apochromatic).
(2015-02-01) variable focal length and stable focusing distance.
A true zoom lens ought to be parfocal (i.e, its focusing distance remains stable when the focal length changes). The older term varifocal is thegeneral term still used for systems with variable focal length which need not meet this requirement.
The general theory of parfocal zoom lenses was worked out in 1958 (usingChebyshev polynomials) by Leonard Bergstein (1928-2008) who happens to be my own "scientific grandfather" (as the second doctoral advisor of Judea Pearl at the Brooklyn Polytechnic Institute, in1965).
(2014-11-27) Reacting to distance to mechanically adjust the focus of a lens.
Nowadays, all autofocus cameras use passive focus detection which works byanalyzing the light received from the scene (as opposed to the active sonar, most notably used with the SX-70 Polaroid camera,which computes the distance by sending an ultrasonic signal and measuring thetime it takes to bounce back from the subject). In low-light conditions, cameras may need to shine light from an auxiliary LEDfor the autofocus to work properly.
(2015-06-12) Just a minor issue in still photography. Critical in cinematography.
(2015-06-12) Extreme in macro-photography (with extension tubes and regular lenses).
(2014-11-29) The crop factor is 43.2666 mm divided by the diagonal of the image.
The image sensor used in manyDX Nikon cameras (D3300, D3400, D5300, D5500, D5600, D7100, D7200) is an effective array of 6000 by 4000 pixels (24.2 Mp)made by either Sony or Toshiba. It measures23.46 mm by 15.64 mm (the pixel pitch is thus 3910 nm).
This has the same aspect ratio (3/2) as a 36 mm by 24 mm full-frame. The crop factor is simply the scale between the two, namely 1.5345.
For dissimilar formats (different aspect ratios) the crop factor is defined as the ratio of the respective diagonals, since the angular coverage of a lens of given focal length always pertains tothe diagonal of the image.
For example, the Panasonic Lumix DMC-ZS25 (labeled Lumix DMC-TZ35 in Europe) has a sensor with a 4/3 aspect ratio (6.08 mm by 4.56 mm). The diagonal of the image is exactly 7.6 mm, which translates into a crop factor of 5.693. The native resolution is 4896 by 3672 (1242 nm pixel).
The big brother of the ZS25 is the Lumix DMC-ZS30 (a.k.a. DMC-TZ40) which has Wi-Fi, built-in GPS, a finer monitor and aslightlylarger sensor. They both feature the same Leica superzoom:
LEICA DC VARIO-ELMAR 1:3.3-6.4 / 4.3-86 ASPH
The full-frame equivalent of this lens is advertised as 24-480 mm forboth cameras. For the ZS25, it would be more accurate to say 24.5-490 mm.
The APS acronym in two of the above formats standsfor AdvancedPhoto System, the pompous name given to a large (technically misguided) effort for mass-marketing a small format of film photography (using economical 24 mm film) starting in 1996, just before the dawn of digital photography. The production of new APS cameras ceased in 2004 and the manufactureof APS film cartridges stopped completely in 2011.
Nevertheless, the format was a reference for a while. Just enough time for the next generation of smaller digital sensorsto be marketed as "APS-C" format, which now stands (although APS itself is all but forgotten).
APS allowed the film to record additional information besides the imageitself. Some of that could be printed on the back of the photosand there were also standardized instructions to the photofinisher to cropthe image in one of the three following ways (that could be overridden by special order, since the whole image was on film).
APS-H : The whole image ("High Definition") 30.2 × 16.7 mm.
APS-C : Cropped central part ("Classic") 25.1 × 16.7 mm
APC-P : Horizontal view ("Panoramic") 30.2 × 9.5 mm
The machines of photofinishers used paper rolls with a uniform width of 4'' to produce 4x7'', 4x6'' and 4x11'' prints, respectively. Throw-away cameras offered only a choice between "H" and "P", as "C" seemed less desirable. (Ironically, that's the only extantreference to "APS" now, although Canon's flagship DSLR once had an "APS-H" sensor, back in 2001.)
(2015-06-11) Make the shutter speed greater than the focal length in mm.
For example, with a handheld 300 mm telephoto lens, your shutter speedshould be 1/320 s of faster, or else you need atripod.
This traditional rule of thumb is only a starting point: You may use a slower shutter speed if you have a very steady hand. Usea faster one if you have less tolerance for blur and/or expect to produce larger prints.
This is all based on the acceptable blur induced by camera-shakefor a typical size of a finished printed image. With a smaller sensor,the same print size requires an additional enlargement bya factor equal to the crop factor. All told, your shutter must be faster in the same proportion.
Another way to state the same thing is to say that the above rule-of-thumbapplies to the full-frame equivalent of the focal length (which I like to call "reach" for short). A 300 mm lens with a Nikon DX camera (1.5345 crop factor) has a a 460 mm reach andmust, therefore, be shot at 1/500 s or faster. The aforementioned 1/320 s is just a little bit too slow (the correction would be far more relevant with larger crop factors).
(2015-05-09) The modern scale is the direct descendant of the ASA and DIN ratings.
In practice, the sensitivity scale we now use obeys the "Sunny 16 Rule", which states that a film will be correctly exposed on a sunny day if theaperture of the lens is f/16 and the shutter speed is the reciprocalof the sensitivity (e.g., 1/100 s for an ISO 100 sensitivity).
One degree of light sensitivity corresponds to 1/3 of an f-stop
ISO (ASA)
25
50
64
80
100
125
160
200
400
800
1600
3200
°ISO (DIN)
15°
18°
19°
20°
21°
22°
23°
24°
27°
30°
33°
36°
The DIN arithmetic progression is a logarithm of the ASA geometricprogression which doubles every third degree (it's approximately multiplied by 10 every 10-th degree). Strictly speaking, the above ISO numbers are just names for the terms of a geometricprogressionwhose common ratio is theDelian constant, which we may give with ludicrous precision:
21/3 = 1.259921049894873164767210607278...
If we assume that the round ISO values (100, 200, ...) are exact,the traditional ASA numbers 64 and 125 are not correctly rounded from the true values (62.996... and 125.992...) which beg to be rounded to 63 and 126 respectively. However, the traditional designations relate better to ASA sensitivitiesof 16 and 32 on one end and 250, 500, 1000... on the other. The choice of 160 to represent 23° merely makesthe aforementioned rule of thumb easy to apply (adding 10°gives an ISO number 10 times as large, namely 1600). This latter rule breaks down for the denominations of very high sensitivity. Thus, 45° is quoted as ISO 25600, by doubling 8 times from 31° (ISO 1000) rather than multiplying by 100from 25° (ISO 250).
Such minute details are needed only for programmers of photography-relatedsoftware, who must properly display traditional indications while workinginternally in exact logarithmic units of 1/3 of an f-stop (or binary submultiple thereof) for all three exposure parameters (ISO, aperture and shutter speed).
The unit used in Nikon firmware is 1/12 of an f-stop. The smallest units used by people are 3 or 4 times as large (i.e., 1/4 or 1/3 of an f-stop).
(2015-05-15) Chemistry of light-sensitive films and plates.
(2015-05-09) For a given electronic technology, sensitivity is proportional to pixel area.
(2015-06-13) In low-light, the number of photons received by each pixel fluctuates randomly.
(2015-05-23) How color-vision is given to an array of photodiodes.
Each photodiode is essentially a monochrome device. In scientific applications (aboard the Hubble Space Telescope,for example) arrays of identical photodiodes are only used to capturemonochrome images unrelated to human color vision. Uniform filters can be placed in front of the entire sensor to let itcapture the image for a specific part of the optical spectrum (call it a color ifyou must, but this can also be a slice of nearinfra-red (IR)or ultra-violet (UV). If needed, three exposures with differentfilters can be rendered in "false colors" by assigning arbitrarily a specificvisible color to each shot. True colors are just a special caseof this, engineered to reproduce the photopic (bright-light) color-response of the human eye.
In ordinary color photography, we can't proceed this way. For one thing, we'd rarely have the luxury of taking three differentshots of exactly the same object. We must use a single brief exposure to gather as much informationas possible about both the intensity and the color of the light received byevery pixel of the array.
For this, a special mosaic of small filters is used to makeneighboring cells react differently to light of different colors (just like the human retina has four kinds of light receptors withdifferent sensitivities and spectral responses).
Solid-state digital color cameras use almost exclusively the Bayer filter consisting of a regular pattern where each square offour adjacent pixels include one red, one blue and two greens. This mimic roughly the human eye, which is more sensitive to the middle ofthe visible spectrum (green) than to either extremity (red and blue). The was originally designed, in 1974, by Dr.Bryce E. Bayer (1929-2012) who spent his entire career (1951-1986) at Eastman Kodak.
The basic resolution of a sensor is the size of its elementary pixel (although the exact brightness and colorassigned to that pixel depend on what the photodiodes corresponding to neighboring pixels detect).
(2015-06-13) Information is also collected just outside the nominal active sensor area.
(2015-05-09)
(2015-05-02) Metering light. Reciprocity corrections for long exposures.
Before the digital era, a camera was normally loaded with film of a given ISO sensitivitywell before decisions were made concerning other means of controlling the exposure. For a given film, the proper exposure was thus measured asan exposure index (EI) defined as the product ofthe shutter speed into the square of the f-stop number.
The amount of light received by a unit area of the sensoris just proportional to the product of the exposure time into the square of therelative aperture (assuming a circular iris) divided by theoptical density of the system:
t A2 / d
A factor of 2 in exposure is traditionally called one f-stop. The term comes from the old-school construction of aperture rings with clickat regular intervals corresponding to a factor ofroot 2:
From one such stop to the next, the illumination doubles. Expensive lenses with apertures faster than f/1.4 have been produced,but they are rather rare.
Because it was natural to set an aperture ring "a little bit"above or below a full stop, the practice arose to divide f-stops into thirds as tabulated below.
Normalized aperture denominations (rounded values of 2n/6, for n = 0 to 41)
1.0
1.4
2
2.8
4
5.6
8
11
16
22
32
44
64
90
1.1
1.6
2.2
3.2
4.5
6.3
9
12
18
25
36
51
72
102
1.3
1.8
2.5
3.5
5
7.1
10
14
20
28
40
57
80
114
Manufacturers usually align the ratings of theirslenses on the highlighted entries of the above table. However, a few lenses have been made with apertures correspondingto half-stops (e.g., 1:1.2 or 1:1.7). and modern digital cameras can accommodate photographers who prefer half-stops:
Half-stop aperture denominations (rounded values of 2n/4, for n = 0 to 27)
1.0
1.4
2
2.8
4
5.6
8
11
16
22
32
44
64
90
1.2
1.7
2.4
3.3
4.8
6.7
9.5
13
19
27
38
54
76
108
210/3 = 10.0793683991589853181376848582...
The multiples of 1/6 of an f-stop would include all of the above. The internal operations of modern digital cameras byNikon (and, presumably, other manufacturers) rely on a unit exactly twice as fine (1/12 of an f-stop) which correspondsto an increase of 2.93% in the diameter of the lens iris:
2 1/24 = 1.0293022366434920287823718...
In theory, that unit could accommodate a user interface in terms of quarters of astop as well. However, I have never seen such a thing in actual use or evenheard of it, except on the Wikipediapage on that topic (I consider the relevant sectionutterly misguided). If it was used at all, a quarter-stop aperture scale couldn't possibly use 2-digit abbreviations without conflicting withthe above well-established ones.
In photography, narrow apertures (beyond 1/32 or so) are rarely used, if ever, because diffraction would then ruin the opticalquality of a lens. For all practical purposes, the above tables already represent an overkill.
Aperture Scales on the Rings of Old and New lenses :
The aperture ring of a modern lens bears the following numbers:
The maximal aperture, at one end of the scale.
Part of the above full-stop sequence: 1.0, 1.4, 2, 2.8, 4, 5.6, 8...
Before WWII, anold Germanaperture scale could be used instead. It was defined backward from a tiny aperture exactly equal to f/100.
The First Type of German aperture scale disappeared after WWII
1.1
1.6
2.2
3.2
4.5
6.3
9
12.5
18
25
36
50
71
100
The first of those abbreviations can be found in the second line of ourfirst table, which means that they represent apertureslocated very nearly 1/3 of a step above a modern full stop. In practice,that's good enough to use such lenses successfully with modern external light-meters.
To compute the precise difference between the two scales, let's divide by 100 the exactvalue of the counterpart of f/100 in our modern scale:
2 20/3/ 100 = 1.0159366732596476638410916...
Thus, apertures in the old German scale are about 1.6% larger than their matching moderncounterparts. (they let in about 3.2% more light). The difference is utterly negligible. It corresponds to 1/22 of an f-stop,which is about half of the smallest aperture unit (1/12 of an f-stop) used by digital cameras for internal computations.
As opposed to the current full-stop aperture scale, which was called international, the obsolete one was variously called European, German or continental.
(2016-12-20) The standard ways a lens can be designed to match a camera body.
The flange distance (FFD) of a camera mount is the distance from the focal plane to the outermost flat flange around the camera'sthroat (which mates with the rear flange near the back of all compatible interchangeable lenses).
Thread Mounts :
The FFD of T-mounts is 55 mm, which is greater than the FFDof the proprietary mounts of all major manufacturers of SLR 35 mm cameras (see below). Thus, manual T-mount lenses can be adapted to all proprietary mounts. The mini T-mount (M37, 0.75 mm thread) was released by Tamron (Taisei) in 1957. It's now abandoned in favor of the standard T-mount (M42, 0.75 mm thread) introduced in 1962 and still popularfor third-party optics, as adapters are cheap (the screw-in design of T-mounts only allowsmanual lenses, as no electrical or mechanical connection is possible between the lens and the body).
The M42 introduced by the East-German branch of Zeiss in 1949 (Contax, Pentacon) is incompatible with the T-mount because it has a different thread (1 mm). It became known as the Praktica Thread Mount or the Pentax Thread Mount. It has a fairly short flange distance of 45.5 mm.
C-mount was the de-facto standard for 16 mm movie cameras. It features a flange distance of 0.69'' (17.526 mm) and 1'' mouth (25.4 mm) with 32 threads per inch (i.e., 0.79375 mm pitch). The C-mount originated around 1929 as an evolution of the A-mount and B-mount previouslyused byBell & Howell (the C-mount was first found on their Filmo 70 cameras with serial numbers 54090 and above).
Bayonet Mounts :
Nikon's famous F-mount (three-lug bayonet) was introduced in 1959. It has a flange distance of 46.5 mm and a throatwith a diameter of 44 mm.
The flange distance of Canon mounts is shorter than that. For the old Canon FD-mount (1971-1992) it was only 42 mm. The current Canon EF-mount (EOS, introduced in 1987) has a flange distance of 44 mm.
It's thus possible to make mechanical adapters to fit Nikon lenses on Canon bodies, but not the other way around.
The Micro Four-Thirds system (MFT or M4/3) was introduced by Olympus and Panasonic in August 2008, for mirrorless cameras with interchangeable lenses. Its bayonet mount has a throat of 38 mm and a flange distance of 19.25 mm (that allows recessed adapters forC-mount lenses, although vignetting will occur). The image area of an M4/3 sensor has a nominal diagonal of 21.6 mm (so, its crop factor is very close to 2).
The Sony E-Mount has a flange distance of 18 mm and a 46.1 mm throat.
(2016-12-20) Mechanical specifications for mounting accessories on the front of a lens.
Screw-on photographic filters are rounds optical elements without any curvature (they consist ideally of a flat plate of uniform thickness).
They are normally used either for their spectral response (colored filter,cut filters) or for their ability to block one particular polarization of light (polarizing filter). They may also reduce the incoming light when a longer exposureis desired (neutral density filters).
Large-format photographers employ expensive filters to correct the vignetting oftheir wide-angle lenses (center filters,dark near the center and clear near the rim). Some other types of graduated filters can either produce artistic effects or prevent the overexposure of skies.
The (female) filter thread at the front of a lens can also be used to attach variousaccessories, includinghoods, close-up lenses, inverted lenses, carved grids (to produce diffraction stars),irregular surfaces (for soft-focus), molded prisms (for multiple images),split or drilled-out lenses, etc.
In addition to a male thread, most filters have a female thread on the opposite side, so several filters can be stacked (this is a good way to store them, but using several filters at once isn't recommended).
The pitch of a screw-on filter depends mostly on its diameter, according to the followingtable. However, it seems that the normal pitch of 0.75 mm is also used for smaller diameters and larger diameters. In the later case, the suffix c (coarse) can beused to specify 1 mm pitch unambiguously.
Three pitches are commercially available for screw-on photographic filters
Thus, a transmittance of 25% ( ¼ ) corresponds to the following density:
log ( 1 / 0.25 ) = log (4) = 0.60206
Using the usual approximation of 0.3 forthecommon logarithm of 2, this is always quoted asa density of 0.6. Several identifications are used for such a filter by different manufacturers, namely:
"ND 0.6" because the optical density is 0.6 (Kodak, Tiffen,Lee).
"102" or "2 BL" (B+W) since light isblocked by 2 f-stops.
"ND4", "NDx4" (Hoya)or 4x (Leica). Factor of 4 in shutter speed.
B+W (owned bySchneider-Kreuznach since 1985) now goes to the trouble of printing up to 4 markings (of all 3 above types). For example, the ring of their (52 mm millimeter diameter) filter with 0.1% transmission reads:
B+W 52 110 ND 3.0 - 10 BL 1000x E
Most manufacturers aren't this redundant. Normally, the clear differences in the above formats are sufficient to avoid ambiguities. However, the very common NDx2 and NDx4 filters (one and two stops, respectively) are often advertised as ND2 and ND4, which hasconfusedsome mail-order buyers looking for rare ND 2.0 or ND 4.0 dark filters (NDx100 and NDx10000 respectively; 6.6 and 13.3 f-stops).
Typical markings on neutral-density filters for a given transmittance (%)
50%
25%
12.5%
6.25%
3.125%
1.56%
1%
0.78%
ND 0.3
ND 0.6
ND 0.9
ND 1.2
ND 1.5
ND 1.8
ND 2.0
ND 2.1
101
102
103
104
105
106
107
x2
x4
x8
x16
x32
x64
x100
x128
ND2
ND4
ND8
ND16
ND32
ND64
The list goes on with a few very opaque filters like ND 2.6 = x400 (0.25% transmittance = 8.6 stops). ND500 is also found. Such opacities are close to the practical upper limit of what can beobtained with optical faders (variable filters consistingof two stacked polarizers).
One stop beyond that is the popular Big Stopper (or Big Blocker) 10-stop filter (0.1% transmittance) which can be marked ND 3.0, 110, 10 BL or x1000 (instead of x1024). Such extremely opaque filters allow tripod shots at low shutter speeds in bright conditions, so that waterfalls or foliage are just a blur in broad daylight...
Even more opaque filters are sporadically available, mostly in the square 100mm by 100mm format (4" by 4") from Lee, Kodak (Wratten) and others, which is often referred to as "Cokin Z Pro". You may occasionally find the following denominations, almost extinct:
x4000 or ND3.6 (12 stops, rarely called "112").
x10000 or ND4.0 (13.3 stops, for which "113" is a bit abusive).
Eclipsefilters for photographing the Sun (20 stops; ND6.0). It's dangerous to use such filters for optical viewing of the Sun,because a dilated pupil may let in too much damaging UV light.
The market for those tends to be rather small and prices can be prohibitive. Square gels sometimes sell for $100 or more.
(2018-32-20) What does it take to properly photograph the solar disk?
Cautious planning first and foremost. If you want to replicate the featdescribed below, make sure you know what you're doing. Aiming carelessly at the Sun with or without optics may damage your vision. This is not a tutorial!
This is so much out of the realm of everyday experience that we needsome back-of-the-envelope calculations to get an idea of what the properexposure is:
The solar disk and the full moon have roughly the same apparent diameter (or else we wouldn't have total solar eclipses lasting for just a couple of minutes). That's about 0.52°. A better number is given by modernastronomical data, provided by NASA.
Now, consider a perfectly white object illuminated by direct sunlight. By definition, it gives back all the luminous energy it receives in all directionsproportionally to its apparent area in that direction For a tiny surface element, that apparent area is justthe surface area multiplied into the cosine of the angle of observation . The total energy radiated back into the entire hemisphere is thus given by thefollowing expression, where I0 is the intensity radiatedper unit of solid angle perpendicularly to the element's surface.
I0 cos d = I0 cos sin d
If the light from the solar disk was spread out over the solid angle of one hemisphere, it would be correctly exposed according to the sunny 16 rule. The solar disk subtends a solid angle about four hundred timessmaller, which would mean that there's a difference of about 18 stopsbetween a sunlit surface and the apparent surface of the sun. With a 10-stop ND3.0 filter, you still need to expose 8 stopsbelow what the Sunny-16 rule says. Using 100 ISO and 1/4000 splaces us 5.3 stops below the Sunny-16 rule. If we stop down tof/32, we're still overexposing, but not by much. That would bewithin a stop or so from the correct exposure at which the Sun is no longerjust a white disk!
In such a photograph, the color balance must be manually set to direct sunlight (5600 K). It clearly doesn't getmore direct than this! Actually, a photo of the solar disk is a good check of thecolor calibration of a camera.
The solar disk is the brightest object accessible to ordinary photography. Lightning bolts are brighter but have smaller width. Atmospheric nuclear explosions are brighter and wider.
(2015-05-30) Selecting only part of the IR, visible and UV spectra.
The best known and cheapest ones are the mass-produced "UV filters" (L37) which photographersoften purchase as sacrificial glass to provide mechanical protection forthe front elements of their expensive lenses.
Hoya optical glass is transparent until 2700 nm or so,at which point the transmittance falls sharply to reach 50% at 2750 nm. Then, there's a hills-and-valleys decrease until perfect opacityis reached around 4500 nm.
Newcomer Zomei of Hong-Kong (XuzhouBingo Network Technology Co., Ltd., mainland China) uses RoHS-compliant HD glass from Schott.
Equivalences Between Some Common Longpass Infrared Filters
Proper infra-red photography produces an actual image of what the unaidedhuman eye can't possibly see. That point is lost on those who use touch-up software to produce fake infrared look-alikesfrom very ordinary pictures.
A color sensor behind an infrared filter may behave in unpredictable ways bycapturing some residual color information. Some amateurs have managedto use that as the sole basis for beautiful false-color renditions...
That endeavor creates a dubious temptation to use sub-standard IR filters (665 nm or 590 nm) instead of a proper 720 nm cut-filter. As more visible light is allowed in, the hope is that more color informationwill remain which might be usable... That's a bad idea, because this practice is very likely to overwhelm the redchannel and silence the other two. The picture below was taken in overcast weather at noon (June 2015, Los Angeles) through a 720 nm filter (100% "de-fading" in post-processing).
If you want real false-color infrared images, bite the bulletand make three separate exposures of the same subject through at least three different proper infrared filters (720 nm or longer). With the monochrome pictures so obtained, you may separate the infrared spectruminto several channels by subtracting from every exposure the one taken with the nextfilter (for the last channel, corresponding to your longest wavelength,there's nothing to subtract). Assign to each channel a visible color of your choice before combiningeverything into a single picture.
(2015-05-21) Direct sunlight is 5200 K (not 5800 K). Shadows are 8000 K.
The average temperature at the surface of the Sun is 5778 K. In the main, our star radiates like a blackbody at that temperature, but there are thousands of dark Fraunhofer lines in thesolar spectrum. The most prominent of these werefirst observed byJoseph von Fraunhofer(1787-1826) in 1814. (That great discovery is utterly irrelevant to photography.)
The atmosphere brings another level of complexity to sunlight, because Rayleigh scattering is more pronounced for short-wavelength light. Blue light is thus removed from direct sun rays and becomes visible in other directions. Yes, that's what makes the sky blue and the Sun yellow (or even red at sunrise/sunset, whenthe rays have to travel through a greater distance through the atmospheric shell). This effectively lowers the color temperature of direct sunlightdown to about 5200 K for the better part of the day. Conversely, shady areas on cloudless days are predominantly lit by the blue sky,which corresponds to a much higher color temperature (8000 K).
White clouds are lit from a combination of direct sunlight and skylightwhich essentially yields back the same color temperature as sunlight outsidethe atmosphere (5800 K or so). When the Sun is behind clouds but some patches of blue sky are showing,the resulting daylight has a typical color temperature of 6000 K.
6503.6 K is the correlated color temperature (CCT) of the standard illuminantD65 defined by the CIE in 1967 and corresponding to average cloud conditionsover Northern Europe. It was originally just a crude estimate of 6500 K but a recalibration of the c2 radiationconstant in Planck's law (1968) introduced a correction factor of 1.4388 / 1.438 which now stands. The CCT of the D65 illuminant is often quoted as 6504 K.
Because the surface of the Moon better reflects red than blue, the color temperature of moonlight is about 4150 K. If the Moon is low on the the horizon, the color temperature of moonlightcan be much lower.
The color temperature of candlelight is around 1850 K.
Incandescent light (as invented by Edison) is produced by a solid filament of tungsten, which melts at 3422 C (3695 K; the highest melting point of any metal). Therefore, no unfiltered incandescent light can possibly have a color temperaturegreater than 3695 K. (That's actually the color temperature of the bright flash emitted bya dying incandescent bulb, since its imminent failure is due to the meltingof the filament.)
Ordinary incandescent bulbs are 2400 K or 2550 K ("soft white"). Studio photofloods are typically 3200 K (or up to 3400 for survolted ones). 3200 K is thus known toold-school photographers as tungsten light.
Fluorescent light is entirely different because it's not produced by radiation from a hot body. Traditional light tubes contain mercury vapor and the light they produce isfrom the dominant light frequencies in the emission spectrum of mercury. To atrichromatic sensor, like a normal human eye or a standard color camera,this translates into a green tint; a color correction (CC) of +7, as explained in the next section.
Tint, Color Correction (CC)
Color Rendering Index (CRI)
The highest possible CRI of 100 is that of a perfect blackbody, at any temperature.
(2018-02-21) One snapshot will tell how dark and how blue it really is in the shade...
To avoid light emitted into the shadows by anything other than the sky, it's important to experiment away from buildings, trees, cliffs or anyobject above ground level except yourself.
On a cloudless day, put a large grey card on the ground (a calibrated 18% grey card is best but any whitesheet of paper will do; two smaller cards are even better than a large one). Set the white balance of your camera manually to direct sunlight (5600 K or so). Cover part of the card with your own shadow and takea properly exposed picture (making sure that the unshaded part of the card isnot overexposed).
Analyze the picture numerically and draw the conclusions.
In the nineteenth century that impressionists like Auguste Renoir (1841-1919) and Claude Monet (1840-1926) started to paint the shaded part of sunlit scenes with bluish tints. Before that, artists were just rendering shadows darker than the sunlit parts. Breaking away from that tradition was quite a revolution in the art world. The effect is very real but it went largely unnoticed until that time only becausethe human eye compensates for it.
(2015-06-09) Converting one type of color balance to another.
This type of filters has been made all but obsolete by the "white balance" settingof modern digital cameras. They survive in the form of gels which you can put in front of additional light sources to balance them with existing light. On the other hand, if you're shooting color film, you may need filters in front of your camerato match the loaded film with a type of light source different from the one specified by the film manufacturer. That's especially so with color slides, which lack the flexibility of applying color correction at printing time.
The traditional Wratten numbers are just reference numbers which are notbased on any particular piece of information. (The system was conceived well before fluorescent lighting existed and Kodak/Tiffenextended it with two trademarked mnemonics later.)
By contrast, the Hoya numbers correspond to differences between the "milred" ratings ofthe color temperatures involved ("micro reciprocal degrees"). The sign of that difference is specified either by an "A" for amber or a "B" for blue. Thus, the numerical rating for their conversionbetween the two common types of tungsten light is:
The first five filters listed above were commonly carried by most serious photographersin the film era. They can be stacked. For example, an FL-B filter is equivalentto an FL-D stacked with an 85b (except that the latter combination is darker).
(2015-05-10) The guide number (GN) is defined in distance units, assuming ISO 100.
When a light source emits a pulse of luminous energy in the direction of a object atdistance d, each unit of surface of the object (measured perpendicularlyto the direction of a light ray) receives an amount of light (luminous energy) inversely proportional to the square of the distance d.
On the other hand, the sensor of a camera observing an object at distance d' receives from it an amount of light proportional to the square of its relative aperture.
If the light source is a flash unit mounted on the camera,the distances d and d' are approximately equal. As d varies, for the sensor to receive the same amount of light (inversely proportional to its sensitivity measured in ISO units) the product of the aperture into the distance must be a constant,called guide number.
Since the relative aperture is a dimensionless number, the guide number has the dimension of a distance and is expressed in the same units as d. The more powerful the flash, the greater the guide number.
The above relationships of proportionality can be expressed by the followingformula involving a universal constant S with the dimension of a surface area,and actually proportional to the luminous energy of a flash pulse.
S / (ISO) = (guide number)2 = (distance x aperture)
For example, Nikon'sSB-500has a GN (at 100 ISO) of 24 m (or 78.74 ft, rounded to 79 ft). In metric countries, the unit of distance is often omitted (as it's understoodthat photographers ought to measure distances in meters). Knowing that the guide number is proportional to the square root of the ISO, we may tabulate it for various sensitivities:
Guide numbers (GN) of Nikon's SB-500 Speedlight for different ISO sensitivities :
ISO
64
80
100
125
160
200
250
320
400
500
640
800
1000
1250
1600
GN
19
21
24
27
30
34
38
43
48
54
60
68
76
86
96
To double the GN for a given flash, we must multiply the ISO by 4. Nikon says that the built-in flashof the D5500 DSLR has a standardguide number (at ISO 100) of 12 m. So, the SB-500 is 4 times more powerful.
The beam width of a flash unit is often given in term of thefocal length f of the widest lens whose field of view it would cover for a full-frame sensor (24 mm by 36 mm). Using the theorem ofPythagoras, the diagonal of a full-frame is do = 43.2666153...mm (or nearly 649/15). Therefore, the angular diameter of the beam is given by:
½ do/ f = t = tan ( /2) or = 2 atan ( ½ do/ f )
Thus, in the case of the aforementioned Nikon SB-500 AF Speedlight, the manufacturer specification f = 24 mm, translates into:
= 2 atan ( 21.6333/ f ) = 1.467 = 84°
Focused Flash Beams :
Flash units with zoom heads have several settings which can be selected eithermanually or automatically to match the angular field-of-view ofthe lens used by the camera. The automatic selection, involving a motorizedoptical system, is very useful when the flash is mounted on a camera with a zoom lens.
For a given source, if we let the angle varythe luminous energy received by an object within the focused beamis inversely proportional to the above solid angle of the bean.
This can be used to derive the guide number at any zoom-head setting from the manufacturer's rating atthe narrowest one (inbold below).
Examples of Guide Numbers (GN) at ISO 100,in meters or feet.
Beamwidth Setting
f
24 mm
28 mm
35 mm
50 mm
70 mm
85 mm
105 mm
96°
75.4°
63.4°
46.8°
34.3°
28.6°
23.3°
$125
Yongnuo YN568EX
28 m 92 ft
30 m 98 ft
39 m 128 ft
42 m 138 ft
50 m 164 ft
53 m 174 ft
58 m 190 ft
$380
Canon 580EX II
28 m 92 ft
30 m 98 ft
n/a
42 m 138 ft
50 m 164 ft
53 m 174 ft
58 m 190 ft
$200
Nikon SB-500
24 m 79 ft
No zoom head.
Nikon D5500 built-in flash
12 m 39 ft
No zoom head.
Table based on manufacturer specifications. Canon's580EX IInot tested.
Diffusers :
There are two types of diffusers, which serve different purposes:
Transparent diffusers just increase the angle of the beam(for use with a wide-angle lens, if the flash unit is mounted on the camera).
Diffusion screens and light boxes will, in addition to the above, increase the size of the light source (which transparent diffusers don't change much) which will soften theshadows created by the flashlight.
For example, when the built-in transparent diffuser of the YN568EX is used,the zoom goes automatically to its widest (24 mm) positionand the unit's LCD displays a focal length of 14 mm, corresponding to a beam diameter of 114°,as estimated by the manufacturer. The GN is then about 15 m.
Honeycomb Grids :
This is roughly the opposite of a diffuser. A grid narrows the beam of light in a specific way;the finer (and thicker) the grid, the narrower the beam.
Thisworks mostly by eliminating slanted rays,which have to undergo many imperfect reflections to go through the grid.
Flash Synchronization :
The first camera with a built-in flash socket, activated by theshutter,was introduced by Exakta, in 1935.
One mainstay of flash photography are small coaxial cables with3.5 mm (1/8") male connectors at both ends, matching PC sockets. The abbreviation stands forProntor/Compur and is named after two brands of camera shutters, made by two distinctmanufacturers of which Zeiss was a major shareholder (Compur from 1951, and Prontor from 1953 forward). The dimensions were standardized, as ISO 519, in 1974 and 1992. Electrically, the connection is simply an open circuit when inactive and a short circuit when active.
Several synchronization signals were generated by mechanical cameras for differentflash technologies. All of them were implemented in the legendary Nikon F. Only "X" synchronization survives today, to drive electronic flash units. The first three modes listed below (now obsolete) were designed for magnesium-burning bulbs, which reached their peaks a few milliseconds after ignition.
M (Medium). Active 20 ms before the shutter is fully open.
F (Fast). Active 5 ms before the shutter is fully open.
FP (Flat Peak). Long-burning bulbs designed for focal-plane shutters.
X (Xenon). Active as soon as the shutter is fully open.
Actually, the pulses of light emitted by modern Xenon tube are so short that they canbe emitted at any time the shutter is open. Doing it just before the shuttercloses is known as rear-curtain synchronization. That approach allows the dim motion trail of a moving object to be capturedduring a long exposure, ending with a sharp flash-lit image frozen in time.
AFP: Stroboscopic flashing for fast focal-plane shutters.
FP synchronization is often transliterated as focal plane. The specialized FP bulbs provided a constant illuminationbetween the time when the front curtainstarted and time when the rear curtain arrived. This way, every part of the filmreceived the same amount of light, even at high shutter speeds (achieved by allowing only a small slit between the two moving curtains).
Nowadays, the equivalent of an FP bulb can achieved by strobingan electronic flash very rapidly throughout the time the curtains travel. This is called AFP (Auto FP) by Nikon and HSS (High-Speed Sync.) by Canon.
AFP (HSS) solves a problem with no other solutions: A poorly-lit fast-moving subject in front of a bright background. A fast shutter is needed to freeze the motion (say t = 1/4000 s) but a flash in the usual X-mode can't be used to light up the subject becauseit the camera would then require a relatively slow shutter speed (say T = 1/200 s) which would overexpose the background.
Another case is often quoted where a blurry background is desired (hence wide aperture and fast shutter) with a static subject. However, this can be captured without AFP (usingneutral density filters).
AFP (HSS) effectively uses only a fraction of the energy deliveredby the flash unit. That fraction is equal to the nominal exposure time (t) divided by the time (T+t) during which stroboscopicillumination must be maintained because part of the image sensor is exposed.
Each point of the sensor receives only a fraction t / (t+T) of the total stroboscopic light emitted:
1/ (T/t + 1) = 1/ (4000/20 + 1) = 1 / 21
As every photographer who ever used this technique knows, it's thus very wastefulin terms of luminous energy. In our numerical example,the drain on the unit's battery is at least 21 times what would havebeen required to properly expose the subject with a single strobe at low shutter speed.
If the frequency of the flash unit is an exact multiple of 1/t, thenthe illumination will be perfectly uniform (regardless of the shapeof each pulse, assuming they are all alike). Now, notice that all standard exposure times above 1/8000 s in steps of half an f-stop are exact multiples of 1/24000. So, if the unit delivers its pulses at a frequency of 24000 Hz, that condition is met for any camera that aligns its shutterspeeds precisely at half-stops, using the nominal values:
Whole stops and half-stops exposure times as exact multiples of 1 ms / 24
3
4
6
8
12
16
24
32
48
64
96
1/8000
1/6000
1/4000
1/3000
1/2000
1/1500
1/1000
1/750
1/500
1/375
1/250
There's no need to go beyond that table, as the technique is utterlyuseless when ordinary flash photography applies (usually, at 1/200 s or slower).
On the other hand, if the unit's stroboscopic frequency is unrelated to the exposure time,it must be large enough to ensure that every point is exposed toan average number of flashes sufficient to make the contribution ofone extra pulse fairly irrelevant, in relative terms...
For example, if flash pulses at some frequency around 100 kHz are used with a 1/8000 s exposure time, each pixel sees between 12 and 13 pulses. The maximum deviation from the geometric mean of 12.49 is 4%, which corresponds to the following peak-to-peak amplitude, measured in f-stops:
log 2( 13 / 12 ) = 0.115477... (about 1/9 of an f-stop)
The resulting light-and-dark bands are barely detectable. Still, for AFP/HSS photography, it's a good idea to use stroboscopic light at 24 kHz (or a multiple thereof) to take advantage of the above numerical remark... If 96 kHz is chosen, there's no banding in 75% of the choicesof standard shutter speeds (including 1/8000 s and 1/6400 s). The worst banding is for 1/5000 s, if you absolutely insist on that shutter speed:
log 2( 20 / 19 ) = 0.074 (about 7.4% of an f-stop)
If the clock of the camera and the clock of the flash unit are slightly off, low-contrast bands are indeed produced, but they are much too wide to be noticed (wider than the picture itself, for crystal-controlled clocks).
Focal-plan shutters. Rear-curtain sync. Auto focal-plane (AFP) = HSS sync.
