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<blockquote data-quote="WayneF" data-source="post: 519337" data-attributes="member: 12496">Actually, no. If at the same ISO and shutter speed, then the same f/stop means the exposure is the same, regardless of all else.Definition of f/stop = focal length / aperture diameter.The purpose of f/stop is to keep the light the same.So comparing say a 100mm and a 400mm lens,The 400 magnifies size 4x, so the area seen is 1/16 the area&nbsp; (1/4 W x 1/4 H).&nbsp; You seem to be calling that diminished light, however the light per unit area stays the same.&nbsp; Because (to be the same f/stop number), the aperture diameter of the longer lens is 4x larger, allowing in 16x more light (at the same fstop number).&nbsp; The magnified subject area is 16x larger.&nbsp; The light per unit of area (the exposure) stays exactly the same... regardless of 100 vs 400 mm, regardless of FX vs DX, etc.The entire concept of f/stop is that the exposure stays the same on all lenses set to same f/stop number, regardless.&nbsp; This lets cameras and exposure meters be useful.Some troublemaker will pipe&nbsp; up and say that not all lenses exactly accomplish this (T-stops, etc).&nbsp; Modern coatings make that normally pretty minor though, and a whole different subject...&nbsp; The concept is that f/8 is f/f8, regardless.</blockquote>

[QUOTE="WayneF, post: 519337, member: 12496"] Actually, no. If at the same ISO and shutter speed, then the same f/stop means the exposure is the same, regardless of all else. Definition of f/stop = focal length / aperture diameter. The purpose of f/stop is to keep the light the same. So comparing say a 100mm and a 400mm lens, The 400 magnifies size 4x, so the area seen is 1/16 the area (1/4 W x 1/4 H). You seem to be calling that diminished light, however the light per unit area stays the same. Because (to be the same f/stop number), the aperture diameter of the longer lens is 4x larger, allowing in 16x more light (at the same fstop number). The magnified subject area is 16x larger. The light per unit of area (the exposure) stays exactly the same... regardless of 100 vs 400 mm, regardless of FX vs DX, etc. The entire concept of f/stop is that the exposure stays the same on all lenses set to same f/stop number, regardless. This lets cameras and exposure meters be useful. Some troublemaker will pipe up and say that not all lenses exactly accomplish this (T-stops, etc). Modern coatings make that normally pretty minor though, and a whole different subject... The concept is that f/8 is f/f8, regardless. [/QUOTE]

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