[newarts]

Here's what theory has to say about optimizing macros - it is simple but took me a long time to realize as the detailed theory is seldom tied together.

When a lens is used for macro photography, DOF depends only on magnification, m, and effective f-number Neff.

DoF=2CNeff/m^2

For a particular Neff both exposure and diffraction disk size (resolution) are the same.

If you use two different perfect lenses at the same macro magnification and adjust f-number for the same exposure time (constant lighting & iso), DoF & resolution will be the same.

In practice it implies for each lens one should find the highest f-number setting you can use for your magnification before diffraction becomes a problem & use that f-number for all your macros - this will give you the best combination of resolution and DoF.

========== math details follow===============

This is true because Neff takes the actual optical distance from lens to image plane into account. For a lens with pupil magnification ratio, p, (exit pupil)/(entrance pupil),

Neff = N(1+m/p)

Diffraction spot diameter = 2.44 x Neff x wavelength

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Last edited by newarts; 10-14-2011 at 11:46 PM.

[SpecialK]

What is C in that equation?

[newarts]

What is C in that equation?

C is called "Circle of Confusion" it is the smallest spot that can be seen by the observer (like a single pixel on a computer display) scaled down to the Pentax sensor's size; for a 1000 pixel wide picture on a computer display, about 0.02 mm.

[Chex]

There are a handful of app's you can get for smartphones with all sorts of these DoF, exposure, min. shutterspeed calculators

[newarts]

There are a handful of app's you can get for smartphones with all sorts of these DoF, exposure, min. shutterspeed calculators

Have you seen any using pupil magnification for macros?

I was mostly trying trying to point out that for macros, dof (and resolution) is essentially the same for all good lenses.

[Chex]

No, can't say I have off hand.. I'll keep an eye out though.