Is a lens’s stated focal length a physical property, or a 35mm-equivalent value?
Asked 5/3/2022
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I’m confused about focal length versus 35mm equivalence. A given lens gives a different field of view on different sensor sizes, and some lens designs don’t place the optical center exactly one focal-length distance from the sensor. Does the marked focal length describe a 35mm-equivalent value, or is it an actual property of the lens? If you had a lens of unknown focal length, what determines or defines that focal length?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
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The focal length is a physical attribute of the lens, regardless of sensor size.
The focal length coupled with the sensor dimensions gives you the angle of view, also called field of view, or FOV.
Angle of View = 2 * tan-1(d/(2 * f))
where d is the one of the dimensions of the sensor (X,Y, or diagonal) and f is the focal length of the lens (units must be the same, i.e. millimeters).
You will see equivalent focal length used on crop sensor cameras as referenced to a full frame sensor. This equivalency gives a sense of reference for those who are used to shooting with full-frame cameras.
Originally by user98537. Source · Licensed CC BY-SA 4.0
user98537
4y ago
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Generated from our catalog & community — verify before relying on it.
A lens’s stated focal length is a physical property of the lens itself, not a 35mm-equivalent number.
What changes with sensor size is the angle of view: the same focal length projects the same image, but a smaller sensor crops more of it, so it looks narrower. “35mm equivalent” is just a comparison tool to help photographers relate one format’s field of view to full-frame/35mm.
So, for example, a 50mm lens is still a 50mm lens on APS-C or medium format; only the field of view changes.
Focal length is defined by the lens optics. For a distant subject (effectively at infinity), it is the distance from the lens’s optical center/principal plane to the focused image plane. In simple terms, if you focus a very distant light source onto a surface, the image distance at focus corresponds to the focal length.
Lens designs such as retrofocus or telephoto can make the physical length of the lens differ from the focal length, but that does not change the lens’s actual focal length marking.
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