Does an EF-S lens change focal length on an APS-C camera, or only its 35mm equivalent field of view?
Asked 8/23/2011
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If I mount an EF-S 10mm lens on a Canon APS-C camera with a 1.6× crop factor, does the lens become 16mm, or is its focal length still 10mm? Since EF-S lenses are designed only for APS-C bodies, why do people talk about a 35mm-equivalent focal length at all?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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The focal length is always the same (so in your example, the 10mm lens always has a focal length of 10mm). But on an APS-C camera the sensor, whilst the same distance from the lens as in a full-frame camea, is 1.6 times smaller. Hence the resulting photo is effectively a crop of just the middle part of the picture.
The 35mm equivalent lengths are just given as a convenient reference point for people who are familiar with full-frame 35mm cameras (such as film SLRs). It helps someone moving from a full-frame SLR to a cropped-sensor DSLR to figure out which lenses will give the same effective focal length they're already used to.
Originally by user3644. Source · Licensed CC BY-SA 4.0
user3644
15y ago
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Generated from our catalog & community — verify before relying on it.
The lens is still 10mm. Focal length is a physical property of the lens and does not change when you mount it on a crop-sensor camera.
What changes is the field of view. An APS-C sensor is smaller than full frame, so it captures a smaller central portion of the image circle. That makes a 10mm lens on 1.6× APS-C give a field of view similar to a 16mm lens on full frame.
So:
- actual focal length: 10mm
- 35mm-equivalent field of view: about 16mm
This applies whether the lens is EF-S or EF. EF-S lenses are simply designed for the smaller APS-C image circle, while EF lenses cover a larger sensor area. On an APS-C body, both still keep their stated focal length.
The reason people mention 35mm-equivalent focal length is convenience: it gives a common reference point for comparing angle of view across different sensor sizes, especially for photographers familiar with 35mm film or full-frame cameras.
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