Anthony,
Thanks, that's very enlightening. It did occur to me that the mirror or
screen were misaligned, so I did some test shots using a super-telephoto
lens, but everything seemed OK. Your statement that under certain
circumstances the eye focuses at the level which is comfortable is the most
convincing explanation I've heard, so I think I'll stick with it!
Richard.
--- original message ---
Don't confuse depth of field with depth of focus. Depth of focus is the
tolerance for focussing error on the film plane (rather than at the subject
plane which is where depth of field occurs). It is extremely small with
wide angle lenses.
The common mechanical cause for depth of focus errors is a misaligned reflex
mirror or focussing screen, but operator error is just as likely especially
with screens designed for autofocus. These screens are designed for maximum
brightness with slow zoom lenses and as such are not so strongly etched as
the screens for manual focus cameras, in many cases the centre spots are
almost clear, similar to an aerial image finder (like an enlarging
focus-magnifier). The trick with aerial images is that you need cross hairs
to train the eye onto the correct focal plane, otherwise your eye will find
focus at any level that is comfortable for it. Autofocus cameras in general
don't have cross hairs on the central spot so that is the probable cause of
focus inaccuracy.
It's best to use the ground glass outside of the centre spot for manual
focussing.
Regards,
Anthony Farr
----- Original Message -----
From: "Mishka" <[EMAIL PROTECTED]>
>I've heard a number of times about people having difficulty focusing
>wide lenses. Which I find kind of strange, since at 15mm, the DOF is
>pretty much from nothing to infinity. 30mm has quite a bit of DOF as
>well. I would imagine even a rough guesstimate of distance should be
>OK.
>
>Mishka
home page: www.richard-seaman.com
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.
-
This message is from the Pentax-Discuss Mail List. To unsubscribe,
go to http://www.pdml.net and follow the directions. Don't forget to
visit the Pentax Users' Gallery at http://pug.komkon.org .
