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If a lens is designated by manufacturer as "designed for crop sensor" (i.e. DC in Sigma, DX in Nikon, DT in Sony), how would that affect the calculation of practical focal length (equivalent focal length for a full frame sensor)?

For example, if I were to purchase a 50mm lens designed for a crop sensor camera, would that given/advertised focal length already be adjusted for the size of the crop sensor, or would I still multiply by 1.5 (so that the 50mm design for crop sensor lens is actually effectively 50*1.5=75mm) to get the effective focal length?

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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.

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It does not affect the focal length or angle of view or anything else.

That "designed for crop sensor" statement means that it will be smaller (lighter, cheaper) and produce a smaller image circle that won't fully cover a full-frame sensor.

So if you are able to mount it on a full-frame you will see severe vignetting.

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  • \$\begingroup\$ I've never heard of the mount being modified in order to stop crop sensor lenses being mounted on full-frame cameras. Which manufacturers do this? Of course I know these crop sensor lenses don't project an image circle large enough to cover full-frame, but don't full-frame bodies just auto-crop an APS-C-sized image from the sensor when a crop sensor lens is mounted? \$\endgroup\$
    – osullic
    Mar 22, 2015 at 10:45
  • \$\begingroup\$ Yes, I removed that sentence. I was thinking about something else. \$\endgroup\$
    – Henk Holterman
    Mar 22, 2015 at 10:53
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    \$\begingroup\$ @osullic Some Canon EF-S lenses (designed for APS-C) protrude into the body such that they would interfere with the mirror on full-frame cameras. We have a question/answer about this around here somewhere. \$\endgroup\$
    – mattdm
    Mar 22, 2015 at 14:10

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