Mark,
The 20/2 and 38/2.8 are used on a bellows or (better) the telescopic tube.
This way they are extended far from the film plane. Every lens that is
extended that far 'loses' light. The formula for this light loss is
something like: effective f-stop = marked f-stop x (magnification + 1). At
3x magnification for the 38/2.8 this means that the effective open f-stop is
11.2. The 20/2 at 7x magnification has an effective open f-stop of 16. This
is the same for any lens, the 21/2 on the same magnification (could be a
slightly different extension, internal focus and all) gives you about the
same f-stops as the 20/2 macro.
I am not sure about the image angle (degrees of FOV?). I think this is also
normal for a lens: extend the lens and it gets more tele-like. It could also
be that these lenses are really reversed retrofocus wide-angles (?).
Daan
Pictures of insects:
www.kalmeijer.net
> 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.
>
> Mark Lloyd
>
< 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 >
|