Dagor Focus Shift?

[Lightbender]

Hello, I have two 8" Goerz Dagors.. one is an old uncoated in brass barrel. The other is a later coated "Kenro" branded.

Ive been testing some of my lenses lately and one thing I noticed with these two Dagors, there is a focus shift going from f6.8 to f16.
Is this normal for these lenses or did I just have bad luck.

Interesting, I also have a early g-claron dagor type and it does not seem to have an issue when stopping down. -But then it also starts at f9, so its not exactly a fair comparison.

[Nodda Duma]

You get a focus shift because spherical aberration is dependent on f/#. So the apparent best focus will shift as spherical aberration is better corrected at smaller apertures. Dagors don't have a lot of spherical aberration, but they have enough for you to see the effect.

[Teodor Oprean]

Rudolph Kingslake writes about the Goerz Dagor in his book "A history of the photographic lens", Academic Press, 1989. On page 92: "... it did suffer from a serious amount of zonal spherical aberration, which arose at the strong dispersive interface, with the result that the position of sharpest image tended to move along the axis when the lens was stopped down, a phenomenon known as focus shift. Nevertheless, the lens has always been popular with photographers, both professional and amateur."

[Teodor Oprean]

In the same book, the author lists several variations of Dagor lens designs. The quote from page 92 applies to the original Goerz Dagor. Later re-designs of the Dagor might be corrected sufficiently to eliminate focus shift.

[Peter Yeti]

Yes, it is normal that Dagors show a slight focus shift. There is hardly a lens of that period that doesn't. The difference between the older Series III f/7.7 and the later f/6.8 is only that the shift goes into the opposite direction but the amount is fairly constant. It also doesn't vary noticeably with subject distance. By contrast, a Tessar of that period (1920s) shows basically no focus shift at infinity but a significant one at 4'.

The g-Claron is a much later design and, though the early ones are roughly of the Dagor type, it is a completely different animal.

Peter