[dltorre]

Hi,

When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?
Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?

Thanks for your help

[jatrax]

Focal length is always focal length. So if you have a 50mm lens it is always a 50mm lens. The change between APS-C and FF (or any other format) is one of perspective not physical characteristics. A 50mm 'looks' different on APS-C but it is physically the same lens.

[TER-OR]

50mm - the actual register distance.

It's 40mm. I'm not sure if the 40XS covers a FF sensor, if so the FOV would be larger.

Just use the lens and see if you like it.

[dltorre]

Thanks for the clarification

[Na Horuk]

When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?

You input the number on the lens. Or closest one.

Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?

It is 40mm on both, regardless of whether you use film, aps-c, or ff. But! Since it is made for aps-c, it might have poor edge performance on ff or film. (vignetting, not as sharp). But only the parts that aren't visible on aps-c. The lens is always the same, crop just means the actual sensor is smaller in size and therefore cuts off some edges. Its only a problem is you strive for "equivalence" across mediums, but that is kind of pointless to do. There is a thread on these forums where people tested out the edge performance of DA (crop) lenses and some have great edge performance even on film, while others don't bother. The 40mm XS is a good lens, its tiny, fast AF, very sharp, and I think it can even be used on FF without vignetting. Just make sure you don't lose the lens cap! Or you can buy some generic pinch caps online

[Lowell Goudge]

Hi,

When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?
Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?

Thanks for your help

Focal length is a fundamental lens characteristic that does not change with sensor size. You always enter focal length. It is based upon the equivalent magnification, to a simple (single element lens) which has a distance from the lens to the focal plane when focused on an infinitely distant object.

50mm - the actual register distance.

no 50 is not the regestry distance . The regestry distance is the distance from the lens mount to the focal plane, which is 45.46mm for a Pentax camera. Regestry distance is only relevant whe trying to adapt a lens from one maker to a different camera body because you will not get the lens' full range of focus if the regestry distance is not the same

It's 40mm. I'm not sure if the 40XS covers a FF sensor, if so the FOV would be larger.

don't mix FOV with focal length. Adding a comment about FOV here does not help, the OP is asking what to put into the focal length setting

Just use the lens and see if you like it.

Best advise on the page

[TER-OR]

I stand corrected.

FOV is relevant because that's what the "75mm equivalent" means - and why it's so confusing to people. It's a useful reminder that the crop sensors do just that - crop the image. It's also irrelevant because this cannot be changed, but it's useful to remember the same lens will have different FOV on FF vs. crop sensors and if you're very accustomed to older 35mm lenses, you'll have to shift your thinking a bit.