Nikon AF-S DX Micro NIKKOR 85mm f/3.5G ED VR User Manual page 17

■ Exposure factor
The f-number of a lens indicates brightness of the image formed by the
lens set at "infinity" shooting distance. The greater the reproduction ratio,
the more image brightness decreases. 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."
■ Effective f-number variation (P. 148)
With this lens, the greater the reproduction ratio (the shorter the
shooting distance), the more the brightness of the image projected onto
the image sensor decreases and the more the effective f-number
increases (the amount of light passing through the lens' aperture
decreases). 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.
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