More questiona about the a77 and a99

Started Apr 29, 2013 | Discussions thread
MediaArchivist
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Re: When is f/2.8 not f/2.8?
In reply to JerryCurtis, Apr 29, 2013

f/2.8 is a mathematical relationship between the focal length of the lens and the diameter of the iris. It is not a measure of light transmission. The more elements in a lens, the less light gets through (in general). Many telephoto zooms operate at their long end by creating  larger image circle with the same amount of light-- thus the sensor itself is getting less.

This is better explained here: T-stop.

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