[pure-silver] Re: 60 mm WA Enlarging Lens on an Omega B-8
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 24 Oct 2012 19:05:13 -0700
----- Original Message -----
From: "K W Hart" <kwhart1@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, October 24, 2012 6:10 PM
Subject: [pure-silver] Re: 60 mm WA Enlarging Lens on an
Omega B-8
Good suggestion. A question: if you had an enlarger exposure
meter, so that you could meter the light at the center and
the corners, would that eliminate the need to make a print?
Ken Hart
kwhart1@xxxxxxxxxxxx
----- Original Message -----
From: Bob Younger
I think a print gives you a more exact idea of what is
going on. An enlarging exposure meter may be somewhat
sensitive to the direction of the light meaning that its
readings away from near the center of the field may not be
accurate. OTOH you may be able to see how uniform the field
is by following Ken's suggestion, that is set up as you
would to make a print. Then take the negative out and look
at the light on the easel. Begin with the lens wide open
and stop it down. Even a normal lens will show fall off
when wide open due to vignetting of the pupils so you will
see the relative brightness from center to edge change as
you stop down. You will get some idea of how far you need to
stop down to eliminate the vignetting. You can also look
back through the lens from the point where the easel is
(have to move the enlarger head) and see how far small the
iris must be for you to see it all from a corner.
All lenses that project a geometrically correct image
have an inherent fall off. In normal lenses its cos^4 theta,
where theta is the half-angle of the image (the distance
from the center to one side). A retrofocus lens of certain
designs can reduce this to cos^3 theta but will still fall
off. Its possible to make lenses with no fall off but other
image characteristics will be affected. For instance, a
fish-eye lens does not follow the cos^4 rule. A center
filter can be used but they are not available for many wide
angle enlarging lenses.
I use a 135mm lens for 4x5 and must burn in the corners
when making a print from a full negative. IMO, for good
uniformity of illumination you want a slightly long lens but
sometimes they are impractical. You may have to cut a round
dodger to use with this lens.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
=============================================================================================================
To unsubscribe from this list, go to www.freelists.org and logon to your
account (the same e-mail address and password you set-up when you subscribed,)
and unsubscribe from there.
Other related posts:
