focus shiftgreenspun.com : LUSENET : Large format photography : One Thread
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 etc....any solutions? Does the focus shift move closer or further from the film plane when stopped down? Thanks, Emile.
-- Emile de Leon (firstname.lastname@example.org), October 08, 2001
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.
-- Thomas Vaehrmann (TVaehrmann@web.de), October 08, 2001.
I have a 50 years old Schneider Super Angulon 90mm and on thad one I have a focus shift closer to the camera, but only in the 2 meterers and closer area. I would do it like so: Take the lens sharp open who the sharpness should be, take the halfe way down to f 11 overcorrect it a bit and close the rest down further. Or buy a very cheap lens like the Scheider G-Glaron which is corrected for 1:1 and it solved your problem! Good light!
-- Armin Seeholzer (email@example.com), October 08, 2001.
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.
-- Michael Briggs (MichaelBriggs@earthlink.net), October 09, 2001.
Thanks much for the info....Good Light! Emile.
-- Emile de Leon (firstname.lastname@example.org), October 11, 2001.