Olympus-OM
[Top] [All Lists]

[OM] Question on 20/2 and 38/2.8 macro lenses

Subject: [OM] Question on 20/2 and 38/2.8 macro lenses
From: Eric Pederson <epederso@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 29 Jul 2002 15:38:23 -0700 (PDT)
> From: "M. Lloyd" <royer007@xxxxxxxxx>
>
> When I read about these two lenses one thing I never
> really understood was how they and their slower manual
> counterparts could be considered 20 and 28 mm lenses
> or how they have the f2, 2,8, 3.5 rating when for the
> 20mm the 1-12 screen is the only screen that shows the
> image with any brightness. Based on the specs shown on
> e-sif their true specs are more like 400mm (based on
> degrees of FOV) f22 or something like that. Thnaks in
> advance for your enlightenment.

f-stop is determined by using the focal length when focused at infinity. You
should find that f2 is quite bright when close enough to the film to actually
be f2. Otherwise, the "bellows factor" of the extension reduces the effective
aperture with close focussing. Obviously this will be most acute with a
lens like the 20/2.
Since the 20/2 is used for greater than 1:1 photography, it has an effective
max f-stop of about 8 or smaller diameter depending on the actual
extension.
=====
Eric Pederson
epederso@xxxxxxxxxxxxxxxxxxxx (no "n" in "epederso")
Professional home page:
        http://logos.uoregon.edu/uoling/faculty/pederson/pederson.html
Personal home page:
        http://darkwing.uoregon.edu/~epederso/


< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz