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From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.rec.photo
Subject: Re: Wide angle lens question
Message-ID: <2552@sdcrdcf.UUCP>
Date: Thu, 2-Jan-86 12:47:38 EST
Article-I.D.: sdcrdcf.2552
Posted: Thu Jan  2 12:47:38 1986
Date-Received: Fri, 3-Jan-86 08:25:09 EST
References: <174@decwrl.DEC.COM>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation R&D, Santa Monica
Lines: 45
Summary: viewfinders distort

In article <174@decwrl.DEC.COM> bennison@clt.DEC (Victor Bennison - DTN 381-2156) writes:
>---
>...  I'm curious about some things.  All my
>Nikon lenses, the above mentioned and my 50mm f1.8 series E, exhibit
>noticable pincushion (is that the right term?). ...line
>exactly along the bottom of the viewfinder, it arches slightly into
>viewfinder in the middle.  Is this normal?  I note that my Kiron
>28-85 f2.8 when set at the 28mm setting exhibits NO noticable
>pincushion.  Should I be really impressed with my Kiron?  I thought
>Nikkor lenses were supposed to be great lenses.  Am I confused or
>what?
This prompted a quick experiment with a Nikon FE2, Nikkormat FTN and ground
glass in the film plane.  Results: a Nikkor 28/2.8 showed noticable
pincushion in the FE2 finder, slight pincushion in the Nikkormat finder and
almost none at the film plane (even a 135/2.8 Nikkor shows pincushion in the
FE2 finder!! and tele's seldom have such problems).  I suspect a combination
of framing mask which is not in the same plane as the focusing screen and
the strong lenses used to bring most of the light to your eye.
>I do notice that the Nikkor 28mm is brighter then the Kiron at the
>28mm setting.  Am I correct in assuming that f2.8 refers to the SIZE of the
>opening and that the brightness is a function of how much glass is between
>the light meter and the subject?
Yes, the f stop is the ratio of the effective light gathering aperature to
focal length.  In extreme lens designs, there may be no real aperature of
that size, only the projection of a virtual aperature that size.  Lenses T
stop (transmission based speed) varies from the real F stop  for a number of
reasons: large number of elements (with attendant reflection loss), extent
of lens coating, manufacturers' rounding biases (e.g. suppose it's really
2.93 --would you call that 2.8 or 3.2 standard marking).  Even age of lens
design may: before wide use of TTL meters, there was a tendency to round
smaller to compensate for other losses.
>
>                               Vick Bennison
>                               ...decvax!decwrl!rhea!tools!bennison
>                               (603) 881-2156

--
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