Focus shift when stopping down Goerz lenses

[Ole Tjugen]

First of all, Dagors and Artars are very different lenses of very different construction. Experience with one tells you exactly nothing about the other.

Most symmetrical lenses exhibit minimal focus shift. When used converted, the spherical aberration will increase the "focus shift". But even the Symmar isn't too bad! In nearly every case the "focus shift" is completely covered by the increase in DoF when stopping down. Only when planning extreme enlargements and super-critical sharpness should it be necessary to even think of it - and in those cases, you shouldn't be using half a lens in any case...

[robert_4927]

John, I have a 16 1/2" Dagor that is 1903 or early 1904 vintage before the Dagor name was used on the lens. It has shown no shift what so ever. At least none that I can notice even when viewing the negative through an 8x lupe. It is a great lens and it lives on my 8x20. I also use a 30" red dot and it is so sharp it will cut you. Now I also use a Blue Dot Trigor which many claim is the same as a Dagor. Bit when I examine the negatives it is sharper out on the edges than the Dagor. Amazing little lens. Covers like a Dagor but almost as sharp as an Artar. I have noticed here lately the larger Dagors seem to be harder and harder to find.

[N Dhananjay]

Also keep in mind that focus shift is partly an optical illusion. It results from uncorrected zonal spherical aberration i.e., off-axis rays are brought to a focus at a slightly different point than rays closer to the optical axis. Note that the actual point of best focus does not really change - when the lens is wide open, the point of best focus is overlaid with other apparent points of best focus. So, in a sense, the apparent point of best focus lies over a range. As you stop, down, this range shrinks because you are eliminating more and more of the off-axis rays. So, the actual point of best focus does not change but the range of best focus will seem to shrink. The one other thing that changes is that the local contrast of the image of the ground glass changes as you stop down. I suspect that if you judge sharpness based on contrast, you are more likely to experience focus shift. For example, it may not be unusual that one is more sensitive to the shadow areas when the lens is wide open and one is able to see into the shadows. As you stop down, the shadows become one mass of low luminance on the ground glass and the highlights actually become easier to focus on. Note that I'm not saying anyone focuses on different things wide open versus stopped down - merely that there may be differing sensitivities to regions as you stop down.

For what it is worth, I seem to remember Rudolph Kingslake in one of his books (I think it was 'History of Photogrpahic Lenses' but am not sure) describes the design of a Dagor and shows that the lens has substantial amounts of uncorrected zonal spherical aberrations, although I do not now remember whether he was describing individual elements or the full, symmetrical lens. However, I have never encountered focus shift in the Dagor lenses I have used. When used converted, these lenses certainly seem to have more spherical aberration, which might make them more vulnerable to the phenomenon. I have used convertible Protars (which are quite similar in principle) and I do find focusing with single elements more difficult than with the full lenses.

Cheers, DJ

[Joseph O'Neil]

8 and 1/4" Red Dot Artar. Nada a bit of focus shift on my lens.

This is going to sound like a totally stupid comment, but the past couple months, I've been noticing some trouble focusing. Just got my eyeglasses upgraded to a new prescription. No more focusing problems.

Not saying this isn't or hasn't been focus shift problems with some lenses in the past, but I wonder, after my recent experience, if some of the stories of focus shift in the past might not have something in common with my recent experience?

joe