InGaussian optics, thecardinal points consist of three pairs ofpoints located on theoptical axis of arotationally symmetric, focal, optical system. These are thefocal points, theprincipal points, and thenodal points; there are two of each.^[1] Forideal systems, the basic imaging properties such as image size, location, and orientation are completely determined by the locations of the cardinal points; in fact, only four points are necessary: the two focal points and either the principal points or the nodal points. The only ideal system that has been achieved in practice is aplane mirror,^[2] however the cardinal points are widely used toapproximate the behavior of real optical systems. Cardinal points provide a way to analytically simplify an optical system with many components, allowing the imaging characteristics of the system to be approximately determined with simple calculations.

The cardinal points lie on theoptical axis of an optical system. Each point is defined by the effect the optical system has onrays that pass through that point, in theparaxial approximation. The paraxial approximation assumes that rays travel at shallow angles with respect to the optical axis, so that$\sin \theta \approx \theta$,$\tan \theta \approx \theta$, and$\cos \theta \approx 1$.^[3] Aperture effects are ignored: rays that do not pass through the aperture stop of the system are not considered in the discussion below.

The frontfocal point of an optical system, by definition, has the property that any ray that passes through it will emerge from the system parallel to theoptical axis. The rear (or back) focal point of the system has the reverse property: rays that enter the system parallel to the optical axis are focused such that they pass through the rear focal point.

The front and rear (or back)focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. An object infinitely far from the optical system forms animage at the rear focal plane. For an object at a finite distance, the image is formed at a different location, but rays that leave the object parallel to one another cross at the rear focal plane.

Adiaphragm or "stop" at the rear focal plane of a lens can be used to filter rays by angle, since an aperture centred on the optical axis there will only pass rays that were emitted from the object at a sufficiently small angle from the optical axis. Using a sufficiently small aperture in the rear focal plane will make the lensobject-space telecentric.

Similarly, the allowed range of angles on the output side of the lens can be filtered by putting an aperture at the front focal plane of the lens (or a lens group within the overall lens), and a sufficiently small aperture will make the lensimage-space telecentric. This is important forDSLR cameras havingCCD sensors. The pixels in these sensors are more sensitive to rays that hit them straight on than to those that strike at an angle. A lens that does not control the angle of incidence at the detector will producepixel vignetting in the images.

The two principal planes of a lens have the property that a ray emerging from the lensappears to have crossed the rear principal plane at the same distance from the optical axis that the rayappeared to have crossed the front principal plane, as viewed from the front of the lens. This means that the lens can be treated as if all of the refraction happened at the principal planes, and rays travel parallel to the optical axis between the planes. (Linear magnification between the principal planes is +1.) The principal planes are crucial in defining the properties of an optical system, since themagnification of the system is determined by the distance from an object to the front principal plane and the distance from the rear principal plane to the object's image. Theprincipal points are the points where the principal planes cross the optical axis.

If the medium surrounding an optical system has arefractive index of 1 (e.g.,air orvacuum), then the distance from each principal plane to the corresponding focal point is just thefocal length of the system. In the more general case, the distance to the foci is the focal length multiplied by the index of refraction of the medium.

For a single lens surrounded by a medium of refractive indexn = 1, the locations of the principal pointsH andH′ with respect to the respective lens vertices are given by the formulas wheref is the focal length of the lens,d is its thickness, andr₁ andr₂ are the radii of curvature of its surfaces. Positive signs indicate distances to the right of the corresponding vertex, and negative to the left.^[4]

For athin lens in air, the principal planes both lie at the location of the lens. The point where they cross the optical axis is sometimes misleadingly called theoptical centre of the lens. For a real lens the principal planes do not necessarily pass through the centre of the lens and can even be outside the lens.

The front and rear nodal points of a lens have the property that a ray aimed at one of them will be refracted by the lens such that it appears to have come from the other with the same angle to the optical axis. (Angular magnification between nodal points is +1.) The nodal points therefore do for angles what the principal planes do for transverse distance. If the medium on both sides of an optical system is the same (e.g., air or vacuum), then the front and rear nodal points coincide with the front and rear principal points, respectively.

Gauss's original 1841 paper only discussed the main rays through the focal points. A colleague,Johann Listing, was the first to describe the nodal points in 1845 to evaluate the human eye, where the image is in fluid.^[5] The cardinal points were all included in a single diagram as early as 1864 (Donders), with the object in air and the image in a different medium.

The nodal points characterize a ray that goes through the centre of a lens without any angular deviation. For a lens in air with the aperture stop at the principal planes, this would be achief ray since the nodal points and principal points coincide in this case. This is a valuable addition in its own right to what has come to be called "Gaussian optics", and if the image was in fluid instead, then that same ray would refract into the new medium, as it does in the diagram to the right. A ray through the nodal points has parallel input and output portions (blue). A simple method to find the rear nodal point for a lens with air on one side and fluid on the other is to take the rear focal lengthf′ and divide it by the image medium index, which gives theeffective focal length (EFL) of the lens. The EFL is the distance from the rear nodal point to the rear focal point.

Thepower of a lens is equal to1/EFL orn′/f′. For collimated light, a lens could be placed in air at the second nodal point of an optical system to give the same paraxial properties as an original lens system with an image in fluid.^[5][6] The power of the entire eye is about 60dioptres, for example. Similarly, a lens used totally in fluid, like anintraocular lens, has the same definition for power, with an average value of about 21 dioptres.

The eye itself has a second special use of the nodal point that tends to be obscured by paraxial discussions. The cornea and retina are highly curved, unlike most imaging systems, and the optical design of the eye has the property that a "direction line" that is parallel to the input rays can be used to find the magnification or to scale retinal locations. This line passes approximately through the 2nd nodal point, but rather than being an actual paraxial ray, it identifies the image formed by ray bundles that pass through the centre of the pupil. The terminology comes fromVolkmann in 1836,^[7] but most discussions incorrectly imply that paraxial properties of rays extend to very large angles, rather than recognizing this as a unique property of the eye's design. This scaling property is well-known, very useful, and very simple: angles drawn with a ruler centred on the posterior pole of the lens on a cross-section of the eye can approximately scale the retina over more than an entire hemisphere. It is only in the 2000s that the limitations of this approximation have become apparent, with an exploration into why some intraocular lens (IOL) patients see dark shadows in the far periphery (negative dysphotopsia, which is probably due to the IOL being much smaller than the natural lens.)^{[citation needed]}

The optical center of a spherical lens is a point such that if a ray passes through it, the ray's path after leaving the lens will be parallel to its path before it entered.

In the figure at right,^[8] the pointsA andB are where parallel lines of radii of curvatureR₁ andR₂ meet the lens surfaces. As a result, dashed lines tangent to the surfaces atA andB are also parallel. Because two trianglesOBC₂ andOAC₁ are similar (i.e., their angles are same),$\frac{\overline{R_2}}{\overline{O{C}_2}}=\frac{\overline{R_1}}{\overline{O{C}_1}}\to \frac{\overline{R_2}}{\overline{R_1}}=\frac{\overline{O{C}_2}}{\overline{O{C}_1}}$. In whatever choice ofA andB, the radii of curvatures$\overline{R_1}$ and${\displaystyle \overline{R_2}}$ are same and the curvature center locations$C_1$ and$C_2$ are also same. As a result, the optical center locationO, defined by the ratio$\frac{\overline{O{C}_2}}{\overline{O{C}_1}}$ on the optical axis, is fixed for a given lens.

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The nodal points are widely misunderstood inphotography, where it is commonly asserted that the light rays "intersect" at "the nodal point", that theiris diaphragm of the lens is located there, and that this is the correct pivot point forpanoramic photography, so as to avoidparallax error.^[9][10][11] These claims generally arise from confusion about the optics of camera lenses, as well as confusion between the nodal points and the other cardinal points of the system. A better choice of the point about which to pivot a camera for panoramic photography can be shown to be the centre of the system'sentrance pupil.^[9][10][11] On the other hand, swing-lens cameras with fixed film position rotate the lens about the rear nodal point to stabilize the image on the film.^[11][12]

In optics,surface vertices are the points where each optical surface crosses the optical axis. They are important primarily because they are physically measurable parameters for the optical element positions, and so the positions of the cardinal points of the optical system must be known with respect to the surface vertices to describe the system.

Inanatomy, the surface vertices of the eye'slens are called the anterior and posteriorpoles of the lens.^[13]

Ingeometrical optics, for eachobject ray entering an optical system, a single and unique image ray exits from the system. In mathematical terms, the optical system performs atransformation that maps every object ray to an image ray.^[1] The object ray and its associated image ray are said to beconjugate to each other. This term also applies to corresponding pairs of object and image points and planes. The object and image rays, points, and planes are considered to be in two distinctoptical spaces,object space andimage space; additional intermediate optical spaces may be used as well.

An optical system is rotationally symmetric if its imaging properties are unchanged byany rotation about some axis. This (unique) axis of rotational symmetry is theoptical axis of the system. Optical systems can be folded using plane mirrors; the system is still considered to be rotationally symmetric if it possesses rotational symmetry when unfolded. Any point on the optical axis (in any space) is anaxial point.

Rotational symmetry greatly simplifies the analysis of optical systems, which otherwise must be analyzed in three dimensions. Rotational symmetry allows the system to be analyzed by considering only rays confined to a single transverse plane containing the optical axis. Such a plane is called ameridional plane; it is across-section through the system.

Anideal, rotationally symmetric, optical imaging system must meet three criteria:

1. All rays "originating" fromeach object point converge to a single and unique image point; imaging isstigmatic.
2. Object planes perpendicular to the optical axis areconjugate to image planes perpendicular to the axis.
3. The image of an object confined to a plane normal to the axis is geometrically similar to the object.

In some optical systems imaging is stigmatic for one or perhaps a few object points, but to be an ideal system imaging must be stigmatic forevery object point. In an ideal system, every object point maps to a different image point.

Unlikerays in mathematics, optical rays extend to infinity in both directions. Rays arereal when they are in the part of the optical system to which they apply, and arevirtual elsewhere. For example, object rays are real on the object side of the optical system, while image rays are real on the image side of the system. In stigmatic imaging, an object ray intersecting any specific point in object space must be conjugate to an image ray intersecting the conjugate point in image space. A consequence is that every point on an object ray is conjugate to some point on the conjugate image ray.

Geometrical similarity implies the image is a scale model of the object. There is no restriction on the image's orientation; the image may be inverted or otherwise rotated with respect to the object.

Afocal systems have no focal points, principal points, or nodal points. In such systems an object ray parallel to the optical axis is conjugate to an image ray parallel to the optical axis. A system isfocal if an object ray parallel to the axis is conjugate to an image ray that intersects the optical axis. The intersection of the image ray with the optical axis is the focal point F′ in image space. Focal systems also have an axial object point F such that any ray through F is conjugate to an image ray parallel to the optical axis. F is the object space focal point of the system.

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The transformation between object space and image space is completely defined by the cardinal points of the system, and these points can be used to map any point on the object to its conjugate image point.

• Film plane
• Pinhole camera model
• Radius of curvature (optics)
• Vergence (optics)

• Geometrical optics
• Geometric centers
• Science of photography

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