Effective aperture and diffraction?

[Jim Ford]

I know that extending a lens with tubes or bellows reduces the light on the sensor, hence the effective aperture under these conditions is smaller than that shown on the lens, but does it make a difference to diffraction?

If a lens performs well at (say) f5.6, but diffraction softening starts showing at (say) f11, does extending the lens such that the effective aperture is f11 push it into the significant diffraction zone?

Jim

[rjlittlefield]

Yes. Diffraction depends directly on the effective aperture.

--Rik

[Jim Ford]

Thanks, I suspected as much.

Jim

[johnsankey]

I give formulae for diffraction and DoF for macros at http://johnsankey.ca/tripod.html#E