Quote Originally Posted by elmo_2006


I was recently in a forum and a member there mentioned that an aperature of a given lens behaves differently on different sensor sizes.


His example was based on the 85L f/1.4. The moment you putthis lenson an APS-C body the lens behaves like a 136mm f/2 lens. Is this correct (I understand the difference between FF and APS-C, it's the f/xx value that's got me wondering between the two)?


So confused!





It seems every time crop factor comes it gets more confusing by the time you get to the end of the thread than at the start.


But.....Check this out for yourself.


Go to the DOF calculator. Enter 100mm at F1.0 for both camera at 10 feet.


Note the DOF of both lenses. Then if change the F of the 7D to F1.6. At that point the DOF of both lenses at the same distance will be identical.


I think this was what that individual was referring to.


In that respect they will carry the same identical 1.6 to 1 ratio.


If you try and prove that ratio in other ways, like comparing a 160mm on a full frame to a 100mm on the crop, the DOF ratio is not there. Nor if you try 100' compared to 160', the results will not hold the ratio.


So the crop ratio is only true to the F stop and the field of view. Other factors will come in to play in the other situations and I would think those would be the dynamics of how the lens passes the light and the width of viewable light on the sensor.


Now if you put a 100mm lens that is a F1.0 lens (if there were such an animal) on your crop camera, it does not change the fact that it is a F1.0 lens.