Equivalent focal length (EGF) Is a conditional characteristic that provides information about the angle of view of the system "Camera with a cropped matrix + lens (including specially designed for a camera with a matrix cropped relative to the full 35mm frame) = image angle equivalent to a full 35mm frame"... When installing a full-frame lens on a crop, this information is calculated to understand what angle of view the system has, in terms of the widespread and familiar to everyone 24x36mm.

(from Latin: aequivalentis - here: equalizing value + focus - hearth, fire)
a term used to characterize a complex optical system consisting of several components, where a component refers to both a single lens and several glued lenses, or lenses, the surfaces of which are connected in pairs by optical contact.
~~ in digital photography: an invented virtual characteristic lens.



The history of the term originates from the depths of the XX century, when the widespread film of the 135th type, which had a frame size of 24x36mm, was in use. At the time, it was normal to judge the angle of view by focal length. For example, a lens with a 50mm focal length gave a 45 degree field of view. With the advent of the digital camera era, it has become more difficult to assess the viewing angles, since the matrices of photographic equipment have completely different sizes, and the focal length of the lens, printed on the body, cannot adequately report the viewing angle. For example, some compacts are marked 4,5-12mm, which will be 18-55 in full frame. And the person who shot on film at the sight of a 4,5mm focal length will not understand at all what angle of view the system gives. It is good, of course, if the size of the matrix is ​​proudly indicated. But that doesn't happen often.

Visual representation of differences in sensor sizes of digital cameras
Visual representation of differences in sensor sizes of digital cameras

For zoom lenses, the minimum and maximum values ​​are indicated (equivalent focal length range), for fixed lenses - the only one (the one for which it is designed). Confuse "Effective Focal Length" and "Equivalent focal length" not worth it, since these are completely different concepts. "Effective focal length" is a specific value and defines one of the most important characteristics of a lens system.

How to calculate EGF? Simple enough if you know the crop factor of the system on which the lens will be used. So, for example, "full-frame" digital and the vast majority of film cameras will have a crop factor equal to one (we are actually starting from it). Canon DSLRs with APS-C sensors have a crop factor of 1,6 (Canon 60D as an example). EGF for a lens with a focal length of 50mm at 1.6 crop will be calculated as the product of the lens focal length multiplied by the crop factor. At the same time, the output data should tell us about the viewing angle that the system with the lens installed in it will have from the calculation. Rough example:

efr chto eto takoe liveviewer.ru 2

The picture above - the angle of view is equal to the 50mm lens set to full frame (crop factor equal to 1). The frame in the center is the same lens, but already mounted on a camera with a smaller sensor (with a crop factor of 1,6).

What is this all for

The general message for the calculation EGF is to obtain information about how much the angle of view of a given lens will decrease on a matrix with a smaller area than the nominal value. The crop factor of the camera's matrix is ​​usually indicated in the technical specifications. In general, the information indicated on the lenses of digital cameras is compared with the focal length of full-frame matrices (24x36mm format films).

The main purpose for which information on the EGF is used is to represent the angle of view, which will give the installation of one or another lens under a full frame on a camera with a smaller sensor area. We will build on the data obtained further, in the process of working with this lens.

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