[font color=\'#000000\']Here is my attempt to answer Ray, who asked:
>If I use a standard 50mm lens, designed for full frame 35mm, with >the D60 at a specific aperture, say F8, then that would be equivalent >to using an 80mm lens with full frame 35mm at the exact same >distance from the subject. Understood. But what should the f stop >setting of the 80mm lens be to get the same DOF and the same sized >circles of confusion?
To get the same DOF, meaning the same COC size on prints of the same size, for different camera formats (image sizes) and lenses giving the same angular field of view (so focal length proportional to image size):
a) Use the same aperture DIAMETER (not aperture ratio)
or equivalently
Adjust the f-stop in proportion to the camera image size or "focal length factor".
So in your case
D60 format with f=50, aperture f/8 = 6.25mm
is equivalent to
35mm format, f= 80mm (=1.6x50mm) f-stop of f/13 since 1.6x8=13, which still gives aperture diameter of 80/13 mm = 6.25mm =50/8 mm (all calculations approximate!)
Why?
The underlying optical fact (simple enough, but not so often mentioned in camera literature) is that:
To get a COC whose diameter is the same fraction of the image (diagonal) length, and so enlarges to the same size COC on prints of the same (diagonal) size, when recording images of the same angular field of view with camera of various formats, an aperture opening of the same diameter should be used.
Since focal length then must be varied in proportion to the diagonal length of the image size of the camera, the f-stop (ratio of focal length to aperture diameter) must vary in proportion to focal length and so in proportion to image diagonal length. I could dig up the actual COC diameter formula if anyone desparately wants to see it.[/font]