I've tested a number of my lenses and I found that some exhibit noticeable focus shift.
For those who may not know.. for some lenses, when focused wide-open and then stopped down, the plane of focus shifts forward or backward.
Most noticeable is my Dagor, which is known for this.
I'm wondering if maybe the aperture is not in the correct position for the lens. Is it possible to shim the lens element s so as to reduce the focus shift problems?
What is the vintage of your Dagor? They almost cured that toward the very end, but not completely. It's an inherent design issue. Doubt there is any fix. I learned
to simply stop down a bit for acute focusing.
Originally Posted by Lightbender
I've the same issue, I use (and love) old convertible symmars, in particular the 360 converted to 620 f/12, and when converted it has some focus shift, and it has to be stoped /22 in general !
You have a very good solution: just focus at the aperture you are to shot, and then use a proven light tight protection to see the ground glass without stray light from the back.
I also wear dark sun glasses until I go under cloth to allow retina acomodate better. At night I use bright points to check focus, you can place your smartphone where you want to place focus...
Another option is to use a fresnel over GG (exterior side is straighter, no correction needed) to see a brighter image.
If you adjust distance between front and rear cell with a different shim or stacking shims you wont much modify (I guess) the focus shift, with that operation you may modify the way optic corrections work for near or distant objects.
Until I know, by modifying the shim thickness you can make it work a little better for near or distant objects, and perhaps you may see that it works better in the center, in the corners or in the middle point.
This is something interesting to test with a resolution target.
You could also get a different lens. My Dagors don't have a focus shift.
I have 5 Dagors as well as a few of its predecessors (Goertz Double Anastigmats) and don't have the problem. I arrange the image on the ground glass and get into general focus. Then I stop down to f 22 and recheck the focus of important areas. I was taught this way by an adult fried in the 1930's when focus shift was more prevalent, and have stuck with it. Why f 22? Because I know that this aperture used at the reciprocal of my EI works unless there is a cloud overlay. Then I adjust the shutter speed. I once had the opportunity to set up one of Ansel's 8x10's for a museum and noticed that he had screws limiting his aperture to a range of 3 stops. I guess that is almost the way I work.
The only Dagor I kept is a Kern. By f/11 any focus shift is gone, so that's what I use for critical focus. I might check again at a much smaller stop, but that would
be relative to depth of field issues.
You made a mark on your focusing standard and checked the mark was the same or different when you got the best focus with your loupe on the GG for each aperture, yes? If you exposed film for the test, you tested your ground glass placement.
Yes... this is the right test, using scene very bright points (near and far) to see if focus shifts when stopping
If it were possible would the manufacturer of the lens put the shim there for all in the first place???
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