Back Focus Revised - New Info...

Started Dec 29, 2005 | Discussions thread
Rinus Borgsteede Veteran Member • Posts: 3,207
No idea about focus shift

There are two instances where focus shift seem to occur and neither is explainable by me. I have personally observed a major focus shift in one of my enlarger lenses. I believe it was the 50mm Rodenstock lens that had a 2.8 aperture. I was used to stopping the lens down during enlarging and started to see soft images even though stopping down should get me some depth of field that counters the focus shift.

I learned that I needed to focus this lens stopped down which without my rather expensive Omega focusser would have been impossible. It may have been the reson the original owner sold it in the first place. (Used lens) The grain focusser made for a world of difference.

The other was a zoom lens but I cannot remember which. The stop down also solved the problem but focussing was done with the lens wide open and a stop down was already fuzzy. 2 stops down you could observe back focussing when making large prints.

The 12-24 Sigma I have is probably the best lens to see if the focus is correct. Except for the wide angle problems photographing a test tarhget it is most critical as slight incorrect focussing the sharpness field is quickly completely out of focus.

I wonder if visible sharpness compares with IR sharpness as I heard someone mentioning.

I trust my eyes for the focus only some of the time but I disagree with the field lens theory of these view finders. It is ground glass so the error cannot be in the eyes if the diopter adjustment is set correctly. Well, I wished I could see a bigger image in dark areas so the autofocus is still my best solution. I see that all of you are doing a good job of taking the variables out.
Rinus of Calgary/Kelowna

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