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

Subject: Re: [OM] Tamron SP 350mm f5.6 mirror lens
From: Pandionhalietius@xxxxxxx
Date: Wed, 14 Mar 2001 20:15:26 EST
In a message dated 3/14/01 4:26:37 PM Pacific Standard Time, 
mickeytr@xxxxxxxxxxxxxxx writes:

<< Roger- You have me very confused. How is a f5.6 anything slower than f8?
 What do you mean about the f4.5 mirror that is about a f7.5? Are you saying
 that mirror lenses have overly optimistic ratings? Or is there something
 else that I missed earlier in the thread?
 -Mickey
  >>
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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