View Full Version : Lens spacing issue/optics question

Kevin Crisp
7-Mar-2006, 07:58
If a classic lens like a Protar VIIa is remounted and the lens elements are moved somewhat futher apart at the end of that process (say, about 0.012") would that cause the lens to develop focus shift even when both lens elements are used together? If so, why would this happen? Thanks in advance.

Michael Gudzinowicz
7-Mar-2006, 19:13
The VIIa will focus shift without changing the design parameters. I'm not sure whether it will get worse with increased spacing. Usually spacing affects astigmatism with that type of lens. When using it, refocus after stopping down. At small apertures, you may find that focusing on an aerial image is easier (see the "ground glass Korona" thread).

Ernest Purdum
9-Mar-2006, 15:04
Rudolf Kingslake has this to say on the subject. "If an occasion arises to remount the two halves of a photographic objective in a new barrel or shutter, it is essential to ensure that the optical axes of the two halves are coincident and that there is the correct separation between the two components. Some lenses are highly sensitive to the space between the halves, and an error of as little as 0.5% of the focal length can often be quite serious. For instance in a lens of the Dagor type, a small error in the separation by as little as 0.4% has a considerable effect on the field curvature, and also a lesser effect on the coma, an increase in the airspace leading to an inward-curving field. On the other hand, in a lens of the Rapid Rectilinear type, an error as large as 1.0% has only a negligible effect on the field curvature and almost no effect on the coma, an increase in the airspace leading to a backward-curving field. In general it is usually found that the better the lens the greater is the effect of an error in the central airspace."

Note that he makes no mention of focus shift. To consider the seiousness of a 0.012" error. you would have to put it into the term of a percentage of the focal length.

Kevin Crisp
9-Mar-2006, 17:12
Thanks, that was interesting. I suppose I could just unscrew the elements some to increase the distance and see what happens.