There are a couple of factors at play here. One as Mike has pointed out is the
secondary mirror, which takes light out because it physically blocks the light
path. Then there is "silvering" which is hopefully silvering rather than
aluminizing. There are transmission differences between the two types of
coating. Then there is aging, where the mirror coating is less efficient.
Often mirror lens are discussed in terms of "T" stop which is a better
indicator of light transmission rather than "F" stop.
PS. I still love mine. Sharp, compact, and goes in my fanny pack.
Warren
<<<Hi Mickey-actually me-John Raymond, who has stated that a mirror lens and
it's f stop does not equate to a "normal-non-mirror lens" f stop.
Maybe someone here on the list can explain it to us both. Bending light and
the loss of light caused by this bending would be part of a lens's f stop's
rating I would think.
I recently took my (non mirror)Tamron SP400mm f4 out and metered it next to
my Vivitar 450mm f4.5 Series 1 mirror lens.
The mirror lens was about 3 stops+ slower than the Tamron "regular" lens. I
have read that there was a discrepancy about mirrors before but was suprised
at the difference. And, accordingly, the viewfinder was much dimmer with the
f4.5 mirror than the super bright f4 on my Tamron 400/f4.
I would have to assume that any mirror f8 lens out there is more like f11.
Looking forward to hearing other's thoughts.
John Raymond
Olympia, WA>>>
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