<<
No, but you're right up to the point where you say 25%. That's only about
0.3 stops. The original post mentions 3 stops,
...
Someone also mentioned silver versus aluminium coating on the mirrors, but
the reflectivity is the same for all practical purposes and on par with the
transmittance in a multiple element refractive lens.
>>
I'm late on this thread, because I was on vacation last week (about 650
messages to read after an half day!).
On a mirror lens, there is the effect of the secondary element that blocks
light, but is quite negligible.
The other effect that could appens is a degradation on the
silvered/alumined surfaces. Aluminiun and/or silver if not properly
protected could loose reflectivity in some years (silver is less durable).
This situation could explain a 3 stops difference.
Ralf Loi
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