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From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.rec.photo
Subject: Re: Flashes and telephoto lenses
Message-ID: <2683@sdcrdcf.UUCP>
Date: Sat, 15-Mar-86 11:06:02 EST
Article-I.D.: sdcrdcf.2683
Posted: Sat Mar 15 11:06:02 1986
Date-Received: Mon, 17-Mar-86 04:04:51 EST
References: <1587@watmath.UUCP> <2579@genat.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Distribution: net
Organization: System Development Corporation R&D, Santa Monica
Lines: 32

>>I have also heard that f4.5 is f4.5 no matter which lens you use.

In an ideal world, this might be true, but in actual practice, there is
enormous variation. 15-20 years ago, Nikor lenses were very careful to have
the marked F-stops very close across the lens line because through the lens
metering was not yet common.  Needed if you wanted your handheld (or
Photomic-F) meter readings to come out consistent across a collection of
lenses.  Thus my ancient Nikor 500/5 mirror lens is 122 mm in diamater
(enough to call 4.2 in todays market!) to compensate for the large hole in
the middle of the lens.
The worst lens I have in this regard is an off-brand cheapo 85-210/3.8 zoom
which meters a full stop slower than my Nikor 200/4 (i.e. it's a full 1.3
stops slower than "marked").  A combination of lots of reflection loss from
the high number of elements, probably limited use of coating (claims
"Tri-Coat" but maybe thats in the whole lens?), and marketing bias in
roundoff (engineer says it's formulated at 3.85000, sales says round "down").
This is also noticable in many mirror lenses now (500/8s taking 72mm
filters, indeed!
[I originally thought the problem with the cheapo zoom was poor sharpness
because the viewfinder image looked so awful, till I noticed it was metering
a stop slower; the 200/4 Nikor looked the same stopped down to 5.6]


-- 
Darrel J. Van Buer, PhD
System Development Corp.
2525 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA