Nikonian72 Loc: Chico CA

What is Effective Aperture for Macro?

Per http://www.cambridgeincolour.com/forums/thread1358.htm
A rule of thumb is that at 1:1 you lose about 2-stops of light beyond what you would otherwise get at a given aperture. The effective aperture (for gauging exposure) is therefore 2-stops smaller than that set on your lens; an aperture of f/2.8 therefore becomes more like f/5.6, and f/8 more like f/16, etc. For other magnifications, a very crude estimate is as follows:

Effective Aperture = Lens Aperture x (1 + Magnification)
Example:
if you are shooting at 50% magnification, then the effective aperture for a lens set at f/4 will be somewhere between f/5.6 and f/6.3;
if you are shooting at 1:1 magnification (100% = true macro), then effective aperture for a lens set at f/4 will be about f/8.

The above formula works best for prime lenses near 50-mm focal length. Using this formula for macro lenses with much longer focal lengths, such as 105-mm or 180-mm, will tend to slightly underestimate the difference between effective and actual lens aperture.

For those analytical types who are interested in getting super accurate results, you will need to know the pupil magnification of your lens. In that case a slightly more accurate formula would be:

Effective Aperture = Lens Aperture x (1 + Magnification / Pupil Magnification)

Canon's 180-mm macro lens has a value of 0.5 at 1:1 mag, for example, so at 1:1 the above formula would give a 50% larger f-number for the effective aperture than if you were to use the simpler formula. However, using the pupil magnification formula probably isn't practical in the field. More just an interesting fact. The biggest problem is that pupil magnification changes depending on focusing distance, which introduces yet another formula.

Any camera that has through the lens metering (TTL) will automatically adjust the exposure to compensate for this (using available light), and will do so much better than the above equation. This covers the vast majority of current cameras. Other cameras, such as those using a hand held light meter, will need to use the above formula to estimate the necessary shutter speed and/or ISO.

Also, if pupil magnification is less than unity (most macro lenses are), then you end up gaining a little depth of field. However, if you require a given shutter speed, you will end up losing depth of field when all things are considered. Overall, it is an advantage though, because you will get a little more depth of field before hitting the diffraction limit.