Homemade 4X5 LF won't focus

[bruce_settergren]

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.

 

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Bruce Settergren

[james___]

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.

[steve_williams3]

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.

 

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If that doesn't work and nothing appears sharp anywhere on the

groundglass, then you are probably like me and need new glasses *grin*

 

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Good luck.

[bob_salomon3]

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),

 

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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).

 

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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.

 

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A recessed lensboard may work or a w/a bellows may work or you may

need both.

 

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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.

 

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Every lens manufacturer publishes their flange focal lengths for all

of their lenses so you should have no problem finding these for modern

lenses.

 

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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.

 

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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".