Does an APS-C-only lens change its stated focal length on a crop-sensor camera?

Asked 3/21/2015

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If a lens is labeled as made for APS-C/crop sensors (such as Sigma DC, Nikon DX, or Sony DT), is the printed focal length already adjusted for the smaller sensor, or is it still the lens’s true focal length? For example, would a 50mm APS-C lens behave like a 50mm lens, or like a 75mm full-frame equivalent on a 1.5x crop camera?

lenses full-frame crop-factor aps-c focal-length

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

11y ago

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The advertised focal length is the actual focal length of the lens and not the Full frame equivalent focal length. The focal lengths are reported like this as the focal length of a lens is a physical property of the lens that is not changed by the size of the surface onto which it projects an image.

Originally by user10405. Source · Licensed CC BY-SA 4.0

user10405

11y ago

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The stated focal length is the lens’s actual focal length, not a full-frame equivalent. So a 50mm APS-C lens is still a 50mm lens.

To compare its field of view to full frame, you still apply the camera’s crop factor. On a 1.5x APS-C body, a 50mm lens gives a field of view similar to a 75mm lens on full frame.

What “designed for APS-C” changes is mainly the image circle: the lens is built to cover the smaller sensor area, which can make it smaller, lighter, and cheaper. It does not change the focal length itself.

If you mount an APS-C-only lens on a full-frame camera (when physically possible), it usually won’t cover the whole sensor, so you’ll typically get strong dark corners/vignetting.

Does an APS-C-only lens change its stated focal length on a crop-sensor camera?

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