Quote Originally Posted by btaylor
So the effective focal length becomes 27.2 - 64mm and aperture remains f/4.0.

I agree with you about the focal length: the full frame equivalent is indeed 27.2-64mm. However, I do not agree with you about f-number. If you are going to apply the crop factor to focal length, then you must also apply it to f-number.


No matter what body you put a lens on, the focal length always stays the same. 17mm is always 17mm, no matter if you use it on a 1.0X, 1.3X, 1.6X, or 7.0X body (such as the Canon XL-H1, which has a 1/3" sensor and EF lens mount). That is why I think the term "effective focal length" is very misleading -- it is much more clear to say "equivalent angle of view". For example, these are correct:
  • 17mm on 1.6X has the same focal length as 17mm on full frame.
  • 17mm on 1.6X has the same angle of view as 27.2mm on full frame



Same thing applies to f-number. No matter what body you put the lens on, the f-number is always the same. However, just as "angle of view" changes between format sizes, many factors affected by f-number change as well: total light gathered, noise, dynamic range, diffraction, depth of field, and more. Of course, some of these are affected by the sensor as well. But in the case where the sensors in both cameras have the same performance per area, such as the case of the 5D2 and 7D, then things become very simple:
  • f/4 on 1.6X has the same f-number as f/4 on full frame.
  • f/4 on 1.6X has the same exposure as f/4 on full frame.
  • f/4 on 1.6X has the same depth of field as f/6.4 on full frame.
  • f/4 on 1.6X has the same diffraction as f/6.4 on full frame.
  • f/4 on 1.6X has the same total amount of light as f/6.4 on full frame.
  • f/4 on 1.6X has the same noise level as f/6.4 on full frame.



Most people just stop at "exposure". They only consider what happens when you use the same ISO and same f-number on both cameras, in order to get the same exposure. But it's important to also consider that the larger sensor allows you to use a different exposure (i.e. higher ISO and less exposure) and still get the same (or better) results. After all, it's the results that matter.


I've done some demonstrations of this equivalence here:


http://thebrownings.name/images/2009-10-5d2-equivalence/


Here is a 1.6X image taken at 70mm f/4 ISO 640.





Compare that with the following image which is 111mm f/6.3 ISO 1600:





They have the same noise, diffraction, and depth of field, despite being at two completely different f-numbers and ISO settings. That shows that f-number scales with format size for all the important factors in photography.