Well, one thing has to be the secondary mirror, mounted on the front
element, which blocks light. If the secondary mirror were half the diameter
of the front element, I would think that the transmission would be reduced
by (lessee, 1^2 - (.5)^2 = .75) 25%. So that the T-value would be (for an
f4.5 lens) would be about f6 - a little more than one stop? But that's a lot
different than 3 stops.
Are those the correct calculations?
> 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
Mike
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