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According to CIPA guidelines, [2] 35 mm equivalent focal length is to be calculated like this: "Converted focal length into 35 mm camera" = (Diagonal distance of image area in the 35 mm camera (43.27 mm) / Diagonal distance of image area on the image sensor of the DSC) × focal length of the lens of the DSC.
Camera lens focal lengths are usually specified in millimetres (mm), but some older lenses are marked in centimetres (cm) or inches. Focal length (f) and field of view (FOV) of a lens are inversely proportional. For a standard rectilinear lens, FOV = 2 arctan x / 2f, where x is the width of the film.
Numerical aperture of a thin lens. Numerical aperture is not typically used in photography. Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written f/N, where N is the f-number given by the ratio of the focal length f to the diameter of the entrance pupil D :
The image sensor format of a digital camera determines the angle of view of a particular lens when used with a particular sensor. Because the image sensors in many digital cameras are smaller than the 24 mm × 36 mm image area of full-frame 35 mm cameras, a lens of a given focal length gives a narrower field of view in such cameras.
Angle of view can also be determined using FOV tables or paper or software lens calculators.
The crop factor is sometimes referred to as "magnification factor", [5] "focal length factor" or "focal length multiplier". [6] This usage reflects the observation that lenses of a given focal length seem to produce greater magnification on crop-factor cameras than they do on full-frame cameras.
Hyperfocal distance. Minox LX camera with hyperfocal red dot. Nikon 28mm f/2.8 lens with markings for the depth of field. The lens is set at the hyperfocal distance for f/22. The orange mark corresponding to f/22 is at the infinity mark ( ∞ ). Focus is acceptable from under 0.7 m to infinity.
With being the distance from the lens to the image, the height of the image and the height of the object, the magnification can also be written as: M = − d i d o = h i h o {\displaystyle M=-{d_{\mathrm {i} } \over d_{\mathrm {o} }}={h_{\mathrm {i} } \over h_{\mathrm {o} }}}
For a normal lens focused at infinity, the diagonal (or horizontal or vertical) field of view can be calculated as: F O V = 2 × arctan ( sensor size 2 f ) {\displaystyle \mathrm {FOV} =2\times \arctan \left({\frac {\text{sensor size}}{2f}}\right)}
When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", and "speedlights") is a measure photographers can use to calculate either the required f‑stop for any given flash-to-subject distance, or the ...