I just learned something new today, and I would like to verify this information with someone who has a good understanding of DOF.
I always thought FF had a shallower DOF than 1.6 crop, and it certainly is shallower if you are trying to maintain a same subject size to a frame size ratio, but if you ignore that ratio and keep the distance to the subject the same, then the 1.6 crop is in fact has a shallower DOF than FF, due to the smaller circle of confusion value on 1.6 crop.
For an instance, let's say that I used 100mm f/2.0 lens on the 5D, and I kept the distance of 10m to my subject to frame my shot.
5D's total depth of field from near limit to far limit would be 1.19m.
If I am to use my 20D to frame the same shot using the same exact lens, I would have to increase the distance to the subject by 6m to make it total distance of 16m, then I would have same subject to frame ratio size as prior shot.
And 20D's total DOF would be 1.94m, which is deeper of course.
However.. if I would have ignored the subject to frame ratio size and kept the distant to the subject to be same 10m, then the 20D's total DOF would be 0.75m, which is shallower.
To make it similar DOF as 5D, I would only have to increase the distance to my subject by 2.5m to make it total of 12.5m, then it yields DOF of 1.18m.
And I find that fascinating.
To calculate this, I used online DOF calculator from here http://www.dofmaster.com/dofjs.html.
Any remark would be appreciated.
Specially on the CoC calcuation.