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

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.

Nobody really answered your question. Basically it comes down to why lenses are named what they are. The focal length of the lens is ​defined ​​as what distance behind the lens the rays converge to a focus when the lens is focused at infinity. This measurement is independent of sensor size. Focal length is focal length, ​always​.

However, for a very long time, the dominant photographic format was 35mm film (equivalent to DSLR "full frame"), so a lot of people are used to thinking about focal lengths in terms of the field of view a given focal length provides ​on a 35mm frame​. When you have a sensor smaller (or larger!) than a 35mm frame, people talk about "effective focal length". The smaller sensor sees a smaller part of the image circle created by the lens, and so acts like a crop or zoom in on the central part of the field of view. So your 50mm lens on a full frame has the same ​field of view​ (NOT focal length!) as an 80mm lens on a 35mm frame if that same 50mm lens is used on a Canon 1.6x crop factor camera (i.e. all of their DSLRs except their full-frame models).

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??)