ANDFlow wrote in post #18505606
So for macro photography the F/stop is not real?
The f-stop is a calculated number. The nominal aperture is a real concept, and the effective aperture is also a real concept.
At macro conditions, the nominal f-stop (as sometimes marked on the barrel of a lens) becomes significantly different from the effective aperture. It's because the aperture moves away from the sensor (in classical macro photography where you extend the lens). The further it moves away, the dimmer the light is on the sensor. The more distant aperture also affects depth of field and diffraction.
So the effective aperture is more significant than the nominal aperture, if you are interested in exposure, DOF, and diffraction.
Normally in macro photography, you want to keep the effective aperture number at around f/22 or less for crop cameras, or f/32 or less for full frame cameras. These are just rough guidelines, feel free to experiment. Keep in mind that it is the effective aperture that matters.
If you don't have a Nikon and need to calculate the effective aperture, just multiply the nominal aperture by m+1 (where m is the magnification). (This ignores the effect of pupillary magnification.) It can make a big difference. If you are shooting at a magnification of 5X, then you would need to set the nominal aperture to f/4 to have an effective aperture of f/24.