Are film lenses effectively longer focal lengths on cropped-sensor DSLRs?

Question

My wife has a pair of Tokina lens (28-80mm and 100-300mm, I believe, without the lens here to check) from her old Minolta film SLR that we've been using with Sony DSLRs. I was told that these aren't actually those measurements for a DSLR, but to multiply by 1.5 because of the cropped DSLR sensors. That would mean these two lenses are effectively the same as a 42-120mm and a 150-450mm (!) lens if we bought equivalent Sony DSLR lenses. Is that correct?

It seems too good to be true in the larger lens' case, so I'm looking for the catch.

Asked by dlanod

Answer

That is partly true. You won't actually get a lens with a longer focal length, but you get a lens with the same field of view as a lens with a longer focal length. The actual focal length remains the same, but as you are using a smaller part of the image circle you get a larger magnification, similar to what a longer focal length would give.

This way of multiplying the focal length to get an equivalent focal length is only applied to DSLRs, although the difference in how the field of view corresponds to the focal length has always existed, for example between medium format cameras and small format SLRs. It's only when there was a lot of consumers moving from SLRs to DSLRs that there was a need for an overly simplified way to compare how lenses work on different formats.

Answered by Guffa

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