focus shift

[e]

Hi, Does any one know the details of the shifting of focus in macro/closeup as a lens is stopped down? I've heard that the xenars do this ... you have to refocus at stopped down aperture.This is a pain because the image is so dim at f22-32 e tc....any solutions? Does the focus shift move closer or further from the film p lane when stopped down? Thanks, Emile.

[Thomas Vaehrmann]

Hi, I've read what Ansel Adams wrote about that, he mentions the Dagor and similar lens designs. But never heard about that shifting as shooting with Tessar-types nor made any such experiences with that lenses.

[Michael S. Briggs]

As far as I know, the main lenses that suffer from focus shift are Dagors, dagor-related lenses and Imagons. These lenses all have significant spherical aberration, intentionally in the case of the Imagon. The Dagor, Angulon and Super Angulon are discussed in Rudolf Kingslake's book A History of the Photographic Lens. Kingslake's diagrams show the Angulon (patent 1930) to be what he calls a reversed Dagor. The Super Angulon (patent 1957) is an unrelated design.

My suggestion for testing whether your lens has focus shift in closeups: focus on the filament of a clear-glass lightbulb, perhaps using a dimmer. Then stop down, turn up the brightness, and recheck the focus. If it is a problem, in the studio you can try using extra illumination for focusing. In the field, sometimes placing a flashlight as a focus aid is possible.

[e]

Thanks much for the info....Good Light! Emile.