Nikon AF-S Micro-NIKKOR 60mm f-2.8G ED - Manual (Page 20)

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8. Exposure factor
The f-number of a lens indicates brightness of the image formed
by the lens set at “infinity” shooting distance. The higher the
reproduction ratio is, the more the brightness of the image will
decrease. The actual brightness of the image is called the “effective
f-number,” and the exposure compensation value for the change
of the effective f-number is called the “exposure factor.”
9. Effective f-number variation (P. 146)
With this lens, the higher the reproduction ratio is, the more the
brightness of the image on the film (image sensing device) will
decrease and the effective f-number will increase (the amount of
light that comes through the lens´ aperture will decrease). This
variation is automatically compensated for by the camera´s
exposure meter, so the user can determine exposure or operate TTL
flash shooting without having to consider the exposure factor.
The f-number displayed in the camera´s LCD panel or viewfinder is
the value automatically compensated.
10. Setting the aperture
Set the aperture on the camera body. When exposure (or shooting)
mode is set at “A” (Aperture Priority auto) or “M” (manual), the
camera will control to maintain the predetermined f-number (up to
approx. 1.6 stops) as the effective f-number varies with shooting
distance.