I don't know of a DOF calculator that works very well for macro.Is there a good macro DOF table about? Preferably one which attempts to quantify the effects of diffraction?
Bob Atkins has a DOF calculator program that includes diffraction.
Norman Koren has a long web page dedicated to DOF and diffraction.
Ted Clarke has a Modern Microscopy article addressing the question of how to get maximum DOF.
All of these articles are likely to be confusing unless you have some basic intuition, which is really pretty simple:
- Geometric blur shrinks in direct proportion to the effective f-number.
- Diffraction blur grows in direct proportion to the effective f-number.
- Optimum DOF occurs when the geometric and diffraction blurs are equal, at which point the effective DOF is about 70% of what would be calculated from the geometric blur alone.
- If you close the aperture 1 f-stop below optimum, DOF drops to zero because diffraction blur exceeds the allowed circle of confusion even in the plane of best focus.
- Decide how sharp you need the image to be.
- Stop down until the entire image is too fuzzy from diffraction even in the plane of best focus.
- Open up 1 f-stop from there.
- In case of doubt, open up. You get more DOF at 1/2 stop wider than optimum than you do at 1/2 stop narrower than optimum.
At best, the calculations rely on questionable assumptions and on numbers that you're not likely to have at hand, such as pupillary magnification factor and exactly what the effective f-number is for a particular camera and lens setting.
The above procedure, in contrast, does not rely on any measurements except your judgment about what is sharp enough, and it does not rely on any assumptions except that diffraction blur is what's responsible for making your images fuzzy as you stop down too far.