I was using an 20D for more than 4 years. I owned the EF-S 18-55 (original - no IS), bought a EF 28-135 IS and later a EF-S 17-55 f/2.8 IS.


Yesterday my new 5D Mk II arrived (body only) and I plugged the 28-135 to its body. I took a picture of my living room at 28 mm, took another picture with my 20D and the 17-55 at 17mm and the 28mm one looked wider. That's weird I thought...


The difference was visible even from the viewfinder, but I said ok, the 20D viewfinder is old, less coverage etc. etc. but the pictures showed the same thing.


I compared the two sensors' size (36x24) to (22.5x15) and they are EXACTLY 1.6x. I thought maybe the factor is not exactly 1.6x and they round it but... it is exactly 1.6X


17mm * 1.6 = 27.2 mm


28mm * 1.0 = 28.0 mm


So the 17 is wider! But not in actual life, where my 28 is wider than the 17. Both pictures are taken from the same position, with the same f/3.5 and the same focus point...


Then I plugged the 18-55. Ok its distortion at 18 is terrible and it is less wide than the 17, which makes sense.


What happens here? Where is the trick? Why the cropped frame lens is not exactly 1.6x the normal lens?