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

Hi All,

I am very new to DSLR photography and have just recently purchased the T4i with the 40mm 2.8 lens.  To upgrade the lens kit that came with my camera 18-55, I am contemplating purchasing the 17-55 2.8 based on all the reviews I have read here and other sites.

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??)

Please forgive this very amateur question but it confuses me to no end.

Thanks in advance for the responses.

Ryll

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.).

ryllz wrote:

Hi All,

I am very new to DSLR photography and have just recently purchased the T4i with the 40mm 2.8 lens.  To upgrade the lens kit that came with my camera 18-55, I am contemplating purchasing the 17-55 2.8 based on all the reviews I have read here and other sites.

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??)

Please forgive this very amateur question but it confuses me to no end.

Thanks in advance for the responses.

Ryll

I think that the confusion is in that the equivalence is, in part, for people whose reference is to film and full frame.  For some people moving from film to APS-C, the conversion helps them to understand the field of view.  17-55 on a crop camera (APS-C) provides the same field of view as would a 27-88 on a full frame or 35mm film camera.    If you took an image using 35mm or full frame camera and a 50mm lens, the same image (using the same lens) from an APS-C camera would look as if the full frame/film image had been cropped...equivalent to taking the same image on a full frame/35mm camera with an 88mm lens....distance form subject being constant.  Thus APS-C cameras are called crop cameras.  You have the right camera for the 17-55. It just helps some folks moving from 35mm to know that the "crop factor" is 1.6, and that a lens, for instance the 17-55, provides the equivalent that a 27-88 lens  would on a full frame/35mm camera (although no one makes that focal range, nearest is 24-70).

Enjoy the 17-55 - it is indeed an excellent lens.

Best,

the focal length is really 17-55mm, but the image circle is small not for FF

so if u compare the result to the FF body u must multiply by 1.6

P&S cameras are marketed with equivalent focal length of their zoom lens.  It is a convenient way to compare with other brands.  For example, the S100 is said to have a 24-120mm zoom.  If any one actually look at the lens, it says 5.2-26mm.  But nobody cares.

1.6 crop bodies are meant to use both EF-s and EF lens.  It is easier to say your lens is 17-55mm, not because Canon think their customers are smart to do the math conversion to 27-88, but because it is easier to compare with other full frame lens.  You can know that a 50mm lens is about the same as the 17-55 zoom all the way in and a 85mm lens is a bit longer than this zoom.  If the 17-55 is marketed as a 27-88, people may think the 85mm has less reach.

My 17-55 is very sharp at 2.8 and throughout the stops.  From what I've read here over the years this seems to be the norm.