Have just been considering the effect of sensor size and the depth of field achievable for the same focal length and the same aperture. I have just been using an on-line depth of field calculator (www.dofmaster.com), which interestingly stated that for the same distance to subject, using the same focal length and aperture, the depth of field will be shorter on the cropped sensor camera compared to the full frame. However, one has to remember that on the cropped sensor camera, the effective field of view will be greater (x1.6 crop for example) resulting in a shorter DoF since the magnification will be greater. If however, the distance to subject is increased for the cropped sensor to provide the same effective field of view for both cameras (and to compensate for the magnification due to the cropped sensor). then the DoF will be shorter for the full frame sensor.
Example: (using the DoF master software)
Full frame 5DMKII with 50mm f/2.8 lens with a distance to subject of 10 feet:
Depth of field: 2.06 feet
APS-C 7D with 50mm f/2.8 lens with a distance to subject of 10 feet:
Depth of field: 1.29 feet
However, one must remember that the 50mm lens on the APS-C camera will behave more like an 80mm lens (50mm x 1.6), meaning the field of view will be "closer" and the DoF will be shorter (as DoF is also a product of focal length too). Therefore, if we "move" the APS-C camera further away from the subject to say 16 feet, then the field of view for both cameras, with the 50mm lens should be the same and hence the DoF on the APS-C camera should be deeper than the for the full frame.
A test video needs to be created for this.... watch this space!
Also, to provide approximately the same DoF for both sensors, as well as as moving the cropped sensor further away from the subject to provide the same field of view, one can decrease the aperture of the lens on the full frame sensor too.
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