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Does my crop sensor camera actually turn my lenses into a longer focal length?

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.

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marked as duplicate by mattdm, MikeW, Stan Rogers, rfusca, Rowland Shaw Feb 24 '12 at 13:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
But note that the Sony DSLR lenses are sold with their real focal lengths, not with a conversion applied. –  mattdm Feb 20 '12 at 3:32
    
@mattdm: So the Tokina lenses are actually the same as what Sony would describe as 28-80mm & 100-300mm? –  dlanod Feb 20 '12 at 5:06
2  
@dlanod: yes. See my answer to the linked question. The focal length is a property of the lens, independent of sensor (or film) size. The "crop factor" idea is just a tool to help you think about the change in field of view. –  mattdm Feb 20 '12 at 6:11

2 Answers 2

up vote 7 down vote accepted

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.

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+1 for the technical accuracy! –  Unapiedra Feb 20 '12 at 8:29

No, your 100-300mm lens is not equivalent to a new Sony 150-450mm.

"effective focal length" is just a way to say "the focal length that would give me the same field of view on a 35mm film camera or full frame digital camera"

So, if your 100-300mm on your new camera you will give you the same field of view as a new Sony 100-300mm - but that field of view will not be the same you got from the same lens on your old camera, to get the same field of view on your old camera you need a 150-450mm (that is why they say a 100-300 is "effectively" a 150-450).

A 100mm lens is still a 100mm lens, an old 100mm lens from the film days has the same focal length as a new 100mm lens designed for digital camera (after all, a millimeter remains the exact same length no matter what you are measuring) - and both of them have the same field of view and the same effective focal length.

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-1 for a rather confusing answer. I know what you are saying but you write in such a way that even I had to read and re-read several times to try to get the jist of it. The paragraph starting "So, if you put your 100-300mm...." is really poorly described. –  Mike Feb 20 '12 at 9:54
    
@Mike - thanks for the feedback, I've edited the answer to try and make it less confusing –  Nir Feb 20 '12 at 11:14

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