Olympus-OM
[Top] [All Lists]

Re: [OM] Identifying a filter and how to use it

Subject: Re: [OM] Identifying a filter and how to use it
From: Lars Bergquist <timberwolf@xxxxxxxxxx>
Date: Sun, 19 Apr 1998 07:55:43 +0000
Wrote Ian:

>Thank you Lars, and the others who responded to my query.  My
>confusion arises from my earlier understanding that only circular
>polarisers can be used on those (non-Oly) autofocus cameras where the
>lens rotates as the autofocus operates.  (My son has one :-{  )
>
>I assumed that this was because the orientation of the filter became
>unimportant when it was a circular pol - obviously I'm mistaken.

Polarizers are of course a pain in the neck to use on lenses with front
cell focusing. You must always adjust the filter after focusing the lens,
not before. The introduction of polarizers in the 'thirties was in fact
one important reason why manufacturers changed to double helical
focusing, in order to avoid the turning motion.

I would also like to touch on the subject of vignetting. Pola filters because
of their double rings often have somewhat deeper mounts than normal.
Also, OM wide angles with 49 mm filter rings often have quite wide
front elements for their filter diameter. Hence the unusually thin OM
filters. -- Now, contrary to non-optical intuition, vignetting increases
with stopping down. (This is true both for shades and filter rings.) The
reason is that with increasing depth of field, more of the ring or shade
comes close to focus. At medium to large aperture, there may well be
no noticeable effect at all.

To find out if there is a risk of vignetting, mount the suspect device on
the lens and turn the lens out to maximum close focus. Open the
camera back and also open the shutter on B (a lockable wire release
is convenient). Look through an extreme corner of the film aperture
at such an angle that only a small part of the bright lens opening is
visible through it. Now stop down to minimum aperture with the pre-
view button. If the bright patch darkens, there is vignetting at that
aperture. You may then experiment with other apertures to find out
if there is a range of stops which still can be used. But of course the
safe way is to change the filter or shade -- you may in the heat of
battle forget what you are doing.


Vänliga hälsningar/Best regards
Lars Bergquist
Välkommen till/Welcome to ...
<http://www.bahnhof.se/~timberwolf/>



< 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