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Re: [OM] Tamron SP 350mm f5.6 mirror lens

Subject: Re: [OM] Tamron SP 350mm f5.6 mirror lens
From: rdk@xxxxxxxxx
Date: Thu, 15 Mar 2001 13:45:40 +0100


Just a quick first impression...
I bought the lens this morning (for the agreed $50), and it was in excellent
condition. I compared it with my Tamron 60-300 at 300mm (f5.4), and the meter
reading on my OM-2 seemed to be the same! I will check it out properly tomorrow
when I have more time.....
It certainly balances well for a 350mm....

mvh / regards
Roger Key




: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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