Focussing a Dagor

I have just got a 130mm f6.8 Goerz Berlin dagor in Dial-set Compur that has
been CLA'd.
I have been making some test negatives and wished to find the proper
technique for focussing.
Those who have a Dagor:
Do you coarsely focus by eye at f6.8, in order to compose, then stop down
and get best sharpness with a loupe?
Alternatively use a loupe at f6.8, stop down and then re-check.
It is very hard to focus at f16 or 22 in dull winter light, when you are
concentrating on a tiny detail under the loupe and se-saw back and forth to
judge max sharpness.
I have had two negs of the same scene one with the centre sharper than the
edge, the other where the edge was sharper than the centre. Is it better to
focus on detail mid-way between the centre and edge of the frame?

Get it right and this old lens is capable of resolving the wires of a pylon
three quarters of a mile off. I am using a re-rated MACO Genius Print film
developed in R09 (Rodinal) 1:50 for 8 min giving a speed of THREE ASA for my
tests.

"Neil Purling" wrote

It is very hard to focus at f16 or 22 in dull winter light

An old trick: Make a pencil mark on the ground side of the
GG where you want to focus and put a drop of oil over the
pencil mark - touch the tip of dropper to the glass so there is
just enough to make the GG clear.

Use a loupe to focus on the pencil line and move your head
back and forth a little bit. When the image isn't moving WRT
the pencil line you are in focus.

I use glass cleaner and a vinyl eraser to clean the GG up
when I get home.

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.nolindan.com/da/index.htm
n o lindan at ix dot netcom dot com

I put a Satin Snow after market GG screen into my Crown Graphic,which is
finer than the OEM screen.
Anyone use any other technique without using oil?

Neil Purling spake thus:

I put a Satin Snow after market GG screen into my Crown Graphic,which is
finer than the OEM screen.
Anyone use any other technique without using oil?

No, but your original guess is correct: focus with the lens wide open,
then stop down. Two reasons:

1. The lens' depth of field is smallest wide open, making focusing easier.

2. The lens lets more light through wide open, making focusing easier.


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My understanding what that there was likelihood of focus shift as a Dagor is
stopped down. Does this apply to early Dagors, only the German ones and not
Goerz America.
Mine is definitely German Goerz.

Better have another play methinks. This time i'll use some ancient Ansco
that lurks at the bottom of my film draw instead of my decent MACO.

"Neil Purling" wrote in message
. uk...
I have just got a 130mm f6.8 Goerz Berlin dagor in Dial-set
Compur that has been CLA'd.
I have been making some test negatives and wished to find
the proper technique for focussing.
Those who have a Dagor:
Do you coarsely focus by eye at f6.8, in order to compose,
then stop down and get best sharpness with a loupe?
Alternatively use a loupe at f6.8, stop down and then
re-check.
It is very hard to focus at f16 or 22 in dull winter
light, when you are concentrating on a tiny detail under
the loupe and se-saw back and forth to judge max
sharpness.
I have had two negs of the same scene one with the centre
sharper than the edge, the other where the edge was
sharper than the centre. Is it better to focus on detail
mid-way between the centre and edge of the frame?

Get it right and this old lens is capable of resolving the
wires of a pylon three quarters of a mile off. I am using
a re-rated MACO Genius Print film developed in R09
(Rodinal) 1:50 for 8 min giving a speed of THREE ASA for
my tests.
Most of the focus shift in a Dagor is gone by about
f/11. The image is still bright enough there to focus pretty
easily. The focus shift is caused by something called zonal
spherical aberration. All lenses have this to some extent
but its comparitively large in lenses of the "meniscus" type
like the Dagor and its variations like the Colinear. The
effect is a slight softness when wide open and a little
focus shift as they are stopped down. The slight softness is
usefull for portrait work.
Test for the amount of focus shift by simply focusing
wide open then stopping down to around f/11 and seeing how
much change is needed to re-focus.
Spherical aberration is dependant on the stop and
disappears pretty fast as a lens is stopped down.
Note that Spherical does not cancel in symmetrical
lenses. Rather, it adds, so the individual cells of a Dagor
have no more spherical than the assembled lens. What the
Dagor cells do suffer from is Coma. Coma is an off axis
aberration that results in a sort of tear-drop shaped smeary
blur spot. It is cancelled by the symmetry of the assembled
lens but is very noticable when a single cell is used. It
also is dependant on the stop, but, because the Dagor cells
are not corrected for it, rather small stops must be used if
the image from a single cell is to be reasonably sharp. An
assembled Dagor has very little coma which is one reason it
can have such a wide angle of coverage.

It is possible the last series of Dagors, made by Kern
in Switzerland, may have less Spherical than the older ones
but the defect is inherent in the type and all Dagors have
it to some degree.


--
---
Richard Knoppow
Los Angeles, CA, USA




--
Posted via a free Usenet account from http://www.teranews.com

This is very interesting. I will have ago focusing at f6.8 and making a
correction at f11, then stopping further down.

"Neil Purling" wrote in message
...
This is very interesting. I will have ago focusing at f6.8
and making a correction at f11, then stopping further
down.
You probably will not find a significant difference
between f/11 and smaller stops. At smaller than f/11 the
depth of focus can make the focus point difficult to see.


--
---
Richard Knoppow
Los Angeles, CA, USA







--
Posted via a free Usenet account from http://www.teranews.com

The old Dagors are among my favorites too, Neil. I finally settled on the
following focusing method, after having made test exposures. The correction
includes a best compromise of curvature of field in addition to the focus
shift with aperture-- at least for these lenses and a 5x7 format. A little
thought should lead to a good approximation for other focal lengths and
formats.

Compose and focus in the center of the frame using f/6.8. Then move the
rear standard back 0.5mm for a 7" Dagor; back 0.8mm for a 10 3/4 " Dagor.
Then stop down to the smaller taking aperture (f/22 is a good one for these
lenses) and take the picture. All this is for the case of no movements.
A good correction for maximum movements on 5x7 using the 10 3/4" lens is
back 1.4mm, I believe, for reasons I didn't document.

Using my old Korona, the ground glass position relative to the film is also
off a little-- worth rolling in an additional correction to the above
numbers in my case.

- Larry Whatley