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EF-S 17-55mm f/2.8 IS USM - Focal Length Confusion

Started Oct 10, 2012 | Discussions thread
PC Wheeler
PC Wheeler Forum Pro • Posts: 17,902
Re: EF-S 17-55mm f/2.8 IS USM - Focal Length Confusion
1

ryllz wrote:

My question is this: If the EF-S 17-55 was specifically made for the APS-C DSLRs and is not compatible with the FF cameras then why would the equivalent focal length still be at 27-88mm on the APS-C DSLR??  This confuses me a whole lot and i've read quite a bit.  I still have not found the explanation.

Based on my question above then what type of camera should I buy to take advantage of the actual 17-55mm focal length that I am planning to buy (obviously not an APS-C and this does not work on a FF so what are my other choices??)

Welcome to the DPR forums!  There are two separable issues above, lens mount and crop factor.

  • The APS-C cameras (all but the 10D, I believe) will use both EF and EF-s lenses. FF DSLRs can use only EF lenses.
  • The eff. focal length of a lens depends on the sensor size. For Canon APS-C DSLRs the crop factor is 1.6.  1.6x (17-55) is approx 27-88 mm. That has ​nothing​ to do with the EF vs. EF-s mount.  So if you used a 17-40L EF lens on an APS-C body the focal length would become 27-64 mm.

So you can use that lens, the very fine 17-55 f/2.8, on any Canon APS-C body (60D, 7D or one of the Rebel or xxxD series) but not on a full-frame body (6D, 5D, etc.).


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Phil

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