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35 mm equivalent focal lengths are calculated by multiplying the actual focal length of the lens by the crop factor of the sensor. Typical crop factors are 1.26× – 1.29× for Canon (1.35× for Sigma "H") APS-H format, 1.5× for Nikon APS-C ("DX") format (also used by Sony, Pentax, Fuji, Samsung and others), 1.6× for Canon APS-C format, 2× for Micro Four Thirds format, 2.7× for 1-inch ...
The rear focal length f ′ is the distance from the rear principal plane H ′ to the rear focal point F ′. Front focal distance (FFD) The front focal distance (FFD) (s F) is the distance from the front focal point of the system (F) to the vertex of the first optical surface (S 1). [1] [3] Some authors refer to this as "front focal length".
A 100 mm focal length f /4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f /2 lens has an entrance pupil diameter of 50 mm. Since the area is proportional to the square of the pupil diameter, [6] the amount of light admitted by the f /2 lens is four times that of the f /4 lens.
The effective focal length is nearly equal to the stated focal length of the lens (F), except in macro photography where the lens-to-object distance is comparable to the focal length. In this case, the magnification factor (m) must be taken into account:
where is the focal length, is the distance from the lens to the object, and = as the distance of the object with respect to the front focal point. A sign convention is used such that d 0 {\textstyle d_{0}} and d i {\displaystyle d_{i}} (the image distance from the lens) are positive for real object and image, respectively, and negative for ...
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.
[3] [5] The magnification here is typically negative, and the pupil magnification is most often assumed to be 1 — as Allen R. Greenleaf explains, "Illuminance varies inversely as the square of the distance between the exit pupil of the lens and the position of the plate or film. Because the position of the exit pupil usually is unknown to the ...
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. This is an advantage in, for example, bird ...