[rollei_list] Re: OT: Feedback on early 240mm f6.8 Goerz Dagor
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 8 Feb 2005 08:37:47 -0800
----- Original Message -----
From: "Fox, Robert" <RFox@xxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, February 08, 2005 7:07 AM
Subject: [rollei_list] OT: Feedback on early 240mm f6.8
Goerz Dagor
> Lens Gurus,
>
> I'm a newbie to large format, but I'm having great time
> with the Speed
> Graphic. I have the opportunity to buy an early (circa
> 1902) Goerz
> Berlin Dagor 240mm/f6.8 in beautiful condition, already
> mounted on a
> custom made lenscone and Graphic lensboard. The glass is
> like new.
> Apparently this lens has really good coverage, enough for
> 8x10, and I'm
> planning to use this on a 4x5 Speed and want to be able to
> use all the
> movements.
>
> Does anyone have any feedback on how this lens might
> perform? I know
> it's a double anastigmat, but I don't have any experience
> with these
> lenses. I'm most interested in using it for portraiture
> (both "soft" and
> "sharp").
>
> TIA,
> R.J.
>
Sounds like this is an original made by Goerz in
Germany.
The Dagor is essentially a wide angle lens. When stopped
down to f/45 it will cover 87 degrees with decent quality at
the corners. Light fall off is the full cos^4 theta since it
does not have any compensation.
Dagors share with other double meniscus lenses a rather
large amount of residual zonal spherical aberration. This
results in some softness at larger stops and some focus
shift. Focus shift is the apparent change in the position of
sharpest focus with change of f/stop. When used at stops up
to about f/11 the lens should be critically focused at the
stop to be used. By about f/16 the spherical is gone and the
image is quite sharp. The softness at large stops is of a
sort that gives a pleasant diffusion to portraits. Mostly it
shows up as a softening of highlights.
The optimum stop depends on the angle. up to about 60
degrees it is f/22, at the widest coveage it is f/45.
The single cells can be used as long focus lenses. The
focal length of a single cell is about 1.8 times the
combined focal length. Speed depends on its location: when
used in back of the stop it is about f/13, when used in
front it is about f/12, the difference is due to the
magnification of the stop by the lens when its in front.
Correction is theoretically better when its used behind the
stop but in practice there is little or no difference.
Because the principle points of a meniscus lens are
displaced toward the convex side the bellows draw is
significantly shorter when teh lens is in front of the stop.
The individual cells are not corrected for coma. When
used in combination the symmetry eliminates the coma but its
plentiful with the individual cell so it must be stopped
down considerably (f/36 or more) for reasonable sharpness.
The complete lens is well corrected for color.
Because there are only four glass-air surfaces flare in
minimal even in uncoated lenses.
Dagors vary in quality like other lenses but Goerz, both
German and U.S. manufacture, had a very good reputation for
consistency of quality.
Dagors were the work horses of commercial photography for
several decades.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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