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Old September 29th 05, 05:32 PM
Jeremy
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"Randall Ainsworth" wrote in message
...

An f/2.8 lens is going to allow a specific amount of light to go
through regardless of what kind of glass it's made of.


I believe that you are incorrect. the F-stops are geometric measurements of
the aperture opening. If a lens has a large number of elements, like a zoom
lens, less light may ultimately reach the film plane (or the chip) than
would be the case if a lens with a lesser number of elements were
substituted, even if both lenses were set to the same aperture opening.

My SMC Takumar 50mm f/1.4 is an interesting example. As is well known, the
rear element on those lenses had Thorium mixed into the optical glass
formula, and the decaying atomic particles have yellowed the lens over the
decades. That lens, set at f/1.4, probably transmits only an amount of
light equivalent to another (non-yellowed) lens at f/1.8.

I admit, however, that most lenses, at any given f-stop, probably transmit
about the same amount of light to the film or chip. But it is not
absolutely guaranteed. And these days, with TTL metering, the exposure
values can be adjusted to compensate.