Inphotography,stopping down refers to increasing the numericalf-stop number (for example, going fromf/2 tof/4), which decreases the size (diameter) of theaperture of a lens, resulting in reducing the amount of light entering theiris of alens.[1][2]: 112
Reducing the aperture size increases thedepth of field of the image. In the case offilm cameras, this allows less light to reach thefilm plane – to achieve the sameexposure after stopping down, it is necessary to compensate for the reduced light by either increasing the exposure time, or using aphotographic film with a higherfilm speed (ISO). In the case ofdigital cameras, stopping down the aperture allows less light to reach theimage sensor – to achieve the same exposure, it is necessary to compensate for the reduced light by either increasing the exposure time, or increasing thesignal gain of the sensor by increasing the camera'sISO setting. Alternatively, more light can be added to the scene by increasing the amount of light illuminating the scene, such as by using or increasing the strength ofelectronic flash or other light sources.[3]
As a lens is stopped down from its maximum (widest) aperture, most lens aberrations (spherical aberration,coma andastigmatism) are decreased, but lensdiffraction increases. The effect is that for most lenses, the balance between the decreasing aberrations and the increasing diffraction effects of stopping down the lens means that lenses have an optimum aperture for best results, often about three stops closed down from maximum aperture, so for a lens with a maximum aperture of ƒ/2.8, ƒ/8 would be the optimum aperture.[2]: 180
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