Close focus distance vs depth of field

[KeithB]

I am in the process of doing some test shots to show that small-sensor cameras have greater depth of field than larger sensor cameras.  (Thanks for the depth_of_field article, Bob, though you have too many depth of field calculators - I have trouble getting back to the one I need at the time!)

Anyway, if I focus at the closest focusing distance will the depth of field equations still hold, or will the depth of field fall off more sharply?

[Bob Atkins]

The depth of field equations hold at all focus distances. The complication you might run into is in defining distances when you are very close, since with complex lens designs you're really measuring to a point somewhere within the lens.

At high magnification (really close focus), it's better to use the equations based on magnification rather than distance.

[KeithB]

Thanks for the answer, though I think I figured it out for myself.  I was thinking that if you focus at the closest focusing distance, things would get blurry in the near field because you were closer than the closest focusing distance.

I realized that the closest focusing distance is really a mechanical thing.  The lens properties will stay the same and depth of field will not not follow the equation.

[nikonian]

You have to differentiate between close focus and macro. The formula for macro is different to close up. Your DOF for macro is down to mm.