There is a slight confusion caused by the different detector sizes of digital cameras and therefore we sometimes have problems with stating the lens’s field of view at a particular focal length. To simplify things, we present a chart with the focal lengths’ correlation to the field of view (as measured diagonally) and by identifying the detector that has been used. We assumed that the lenses’ construction is based on the most popular projection, leaving the straight lines in their unchanged form (theoretically free of defects or distortions). In this projection the Alpha angle between the incoming beam and the system optical axis relates to the focal length f and the distance at which the picture was rendered R in the following formula:
R = f * tan (Alpha)
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When we recreate the formula, we learn that the view field is
presented in the following formula:
View field in degrees = 114.6*arctan(21.622/(S*f))
Where S is the focal length multiplier, being 1.3 for the APS-H
detector, while in the DX detector (used by Nikon, Pentax, Samsung and
Minolta) it is 1.5, in APS-C Canon detectors it is 1.6 and in the case
of the Olympus 4:3 detector system it is as much as 2.
The Symbol FF
(full frame) is the small frame film detector, which was used, for
instance, in the Canon 1Ds Mark II and 5D.
Finally, it is worth adding that the fields, especially for the wide
angles, are somewhat general because in the case of such lenses the
deviation from the set projection are often very distinct and the optical
defects (like distortion) add to this overall deviation.
