"Randall Ainsworth" wrote in message
In article , Brion K. Lienhart
As others have already noted, Randall, you're just flat wrong about this.
And your earlier, more absolute statement,
"F/2.8 lets the same amount of light through regardless of the lens
design. F/2.8 is f/2.8"
is even more absolutely wrong.
An uncoated f/2.8 lens for example will not let through as much light as a
coated f/2.8, and a multicoated one will do even a bit better.
All "f/2.8" says is that the effective aperture is 1/2.8 the focal length.
It doesn't say anything about actual transmission. T-stops do that, but they
have rarely been used on still-camera lenses.
At one time I had Minolta 50mm lenses in f/1.2, f/1.4 and f/1.7. From the
f-numbers you would suppose the f/1.2 wide open was a full stop faster, and
the f/1.4 half a stop faster, than the f/1.7 lens. But that simply wasn't
so. For that matter, I have never seen an f/1.4 lens that was really a full
stop faster wide open than it was at f/2.
Someone else mentioned the disparity between mirror and refractor lenses,
too. I can attest to that! I once had a Minolta 500mm f/8 mirror lens that
seemed to lose about a full stop (maybe more) compared to what an
all-refracting lens would have done.