Dagor Focus Shift?

[john_sarsgard1]

I have an old (very) Dagor 300mm marked "Berlin Dagor." It seems to

produce consistently slightly mushy images when stopped down (f32).

I seem to remember hearing that Dagors sometimes exhibit focus shift

when stopped down. True? What would be the optimum aperture for

sharpness? Does one need to refocus when stopped down, or is this a

result of optical properties like spherical aberration? I could

test, but if somebody knows, would allow me to take real pictures

instead of testing (hate it). Thanks.

[ellis_vener_photography]

When i owned a 1960s vintage gold dot dagor, I found I got better

resolution ("sharper images") by composing/focusing with the

lens wide open but then stopping down a stop or two and

rechecking focus.

[david_goldfarb]

Be sure your darkcloth is really blocking out all extraneous light, let your eyes get used to the dark, and make your final focus adjustment as close to the taking aperture as you can. Yes, they do have some focus shift.

[dhananjay_n]

Focus shift is due to uncorrected zonal spherical aberration. The Dagors are supposedly notorious for focus shift. Basically, spherical aberration means that the rays of light passing through the edge/periphery of the lens focus to a different point than those rays passing near the optical axis of the lens. This means that there is actually a sort of range in which focus will be good. As you stop down, light from the edges is eliminated and the rays from the center dominate the image points. And that is the issue in focus shift - it is partly an optical illusion - the point of best focus does not really shift, it just appears to do so because one's judgment of it varies due to the effects of spherical aberration. If you judge the point of best focus primarily by contrast, you are likely to experience focus shift as you stop down. The easiest way out is to focus stopped down to the working aperture - you could rough in focus wide open but always refocus when stopped down. If you dislike checking focus on a dim image, I've found that a couple of stops down pretty much reduces the spherical aberrations a good deal. But to be really sure, take a loupe all over the image at working aperture. Good luck, DJ

[steve_hamley]

Is this also true of G-Clarons?

 

Thanks!

 

Steve

[dhananjay_n]

All lenses suffer from zonal spherical to some extent but often it is small enough not to be noticeable. Plasmat type designs allow the extra degrees of freedom to the designer which allow much better correction of zonal spherical (and other) aberrations. Modern lenses of this type, which includes the Symmar, G-Claron and Sironar, are nearly free of zonal spherical and thus focus shift. Cheers, DJ