Does a full-frame lens gather more light than an APS-C lens on the same APS-C camera at the same f-stop?
Asked 11/3/2015
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If two lenses on the same APS-C camera give the same framing and are both set to the same f-stop, does a full-frame lens let the crop sensor use a faster shutter speed because its image circle and entrance pupil are larger? For example, compare a 36mm f/2.8 full-frame lens and a 24mm f/2.8 APS-C lens on the same APS-C body, both aimed at the same scene and framed similarly. Does the full-frame lens deliver more usable light to the APS-C sensor, or is any extra light outside the crop sensor simply unused?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Your last sentence gets it right. The extra light is wasted because it falls outside the sensor area. In the approximation that your lens is a single thin lens (it is not, but it is a useful way to think about it) the rays that pass through the center of the lens are not changed in direction. In full frame, you need rays that pass at a wider angle so you can cover the larger sensor.
Originally by user14486. Source · Licensed CC BY-SA 4.0
user14486
10y ago
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On the same APS-C camera, if two lenses give the same framing and are set to the same f-stop, the crop sensor does not get a shutter-speed advantage just because one lens is designed for full frame.
At a given f-number, image brightness at the sensor is essentially the same. A full-frame lens projects a larger image circle, but the APS-C sensor only records the central portion of that image. Light falling outside the sensor area is not captured, so it does not make the exposure brighter.
So in your example, a 36mm f/2.8 full-frame lens and a 24mm f/2.8 APS-C lens on the same APS-C body would give essentially the same exposure at the same ISO and shutter speed, assuming similar transmission.
Why full frame often performs better in low light is mainly due to the larger sensor collecting more total light overall, not because a full-frame lens somehow sends extra usable light to a smaller sensor.
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