At 18:28 14.03.01 -0700, Michael Stahulak wrote:
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?
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, and the only way I can see
this happening is when the T-adapter isn't able to tell the camera that the
lens is permanently stopped down (been there, lots of overexposed pictures
and a wasted night by the telescope). Check the pictures, if they're
overexposed you have been warned about that adapter.
Also stop the Tamron down to about f/4.8 and use the stop down lever, the
viewfinder should be about the same with the two lenses.
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.
Regards,
Thomas Bryhn
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