Effective Aperture
The working f-number of a lens once it is focused close, which is always darker than the marked aperture because the lens is extended away from the sensor.
Formula / rule
N_eff = N × (1 + m)
Measured in
f-stops (EV)
In depth
As a lens focuses nearer, the distance from the exit pupil to the sensor grows, spreading the same cone of light over a larger area. At 1:1 magnification the effective aperture is two full stops darker than the marking. Cameras that meter through the lens compensate automatically, but flash calculations and studio setups must account for it manually.
Examples in practice
A macro lens set to f/8 at 1:1 magnification behaves like f/16 for exposure purposes, costing two stops of light.