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Effective focal length (EFL) The effective focal length is the inverse of the optical power of an optical system, and is the value used to calculate the magnification of the system. [1] The imaging properties of the optical system can be modeled by replacing the system with an ideal thin lens with the same EFL. [2]
The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
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 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: f = F ⋅ ( 1 + m ) {\displaystyle f=F\cdot (1+m)}
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 length f ′ and divide it by the image medium index, which gives the effective focal length (EFL) of the lens. The EFL is the distance from the rear nodal point to the rear focal point.
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 ...
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: =. This ratio is related to the image-space numerical aperture when the lens is focused at infinity. [3]
where f c is the corrected focal length for the new lens, f is the focal length of the original lens, and x is the distance that the lens was moved. The value for x can be positive or negative depending on the sign convention. Lens power in diopters is the mathematical inverse of focal length in meters.