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

Subject: Re: [OM] Tamron SP 350mm f5.6 mirror lens
From: "Mickey Trageser" <mickeytr@xxxxxxxxxxxxxxx>
Date: Wed, 14 Mar 2001 22:18:13 -0500
It never fails, just when I'm getting comfortable with what I know about the
mechanics of photography, I learn that I shouldn't be. This seems to remind
me that the 'f-stop' or wide open aperture rating of the lens is just a
ratio of two measures, but I can't recall the specifics. focal
length/smallest passage width of light? Even if it only slightly resembles
the truth, it explains why the f8 mirror is darker than a glass lens stopped
to f8. Hmmm... and yet the metering still works ok.  How can that be? Now
I'm truly muddled.
-Mickey
----- Original Message -----
From: <WKato@xxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Wednesday, March 14, 2001 9:00 PM
Subject: Re: [OM] Tamron SP 350mm f5.6 mirror lens


> 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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