I just finished the final assembly of a homemade 4X5 monorail camera. My excitement was spoiled by a complete inability to focus an image on the ground glass. The lens in use is a 90mm f6.8 Schnieder Angulon. My ground glass is a Bosscreen. The bellows is homemade, looks funky but appears to have no light leaks. Any suggestions as to what might be causing the image which I see to be totally unfocussed. Any suggestions would be greatly appreciated.

Bruce Settergren

-- Bruce Settergren (setterb@home.com), September 05, 1999

Answers

The bellows won't allow you to compress enough to use a 90mm. Try a 150mm. The front and rear standard aren't parallel thereby not focussing all the images' plane of focus at the same plane on the GG.

-- james (james_mickelson@hotmail.com), September 05, 1999.

The homemade monorail cameras I have seen don't compress enough to allow you to focus at infinity or where the bellows is extremely compressed. Try focusing on something very close where the bellows would be more extended. If it works then you know the problem. You might be able to cure it with a recessed lens board for your 90mm or you can use a lens of longer focal length.

If that doesn't work and nothing appears sharp anywhere on the groundglass, then you are probably like me and need new glasses *grin*

Good luck.

-- Steve Williams (sfw3@psu.edu), September 06, 1999.

To see if your camera will work properly with a given lens at infinity you need to know the Flange Focal Length or Optical Register at infinity (same thing),

For instance a 90mm 4.5 Grandagon N a flange focal length of 98mm and the 6.8 versions is 94mm and an Apo Sironar Digital 90mm 5.6 is 93mm (all in Copal shutter).

That means for the lens to focus at infinity the distance from the rear plate of the shutter to the film plane must be the flange focal length. If it is longer than that on your camera it will not focus at infinity unless you do something.

A recessed lensboard may work or a w/a bellows may work or you may need both.

If your flange focal length is less than the lens's than you do not need either a recessed board or a special bellows unless your bellows is too inflexible compressed to allow movements.

Every lens manufacturer publishes their flange focal lengths for all of their lenses so you should have no problem finding these for modern lenses.

if you don't have a lens with a publihed spec you simply point the lens out a window at a distant object while projecting the image onto a piece of white paper held behind the lens. Move the lens forward and back until a sharp image is projected on the paper. That is the distance you need to focus your camera with that lens at infinity.

Do be aware that a 90mm Angulon does not critically cover a 45 negative and has no room for corrections as it's coverage is limited. It was a lens whose specs from Schneider listed it as covering 9x12cm which is smaller than 4x5".

-- Bob Salomon (bobsalomon@mindspring.com), September 06, 1999.