shimming a linhof TK ground glass - front-focus correction

**tadler** · 8-Jun-2012, 03:53

i made again more accurate focus tests with my TK 45s (and another 4x5 camera for reference), making a few exposures on both close and distant objects, wide angle lens, aperture fully open, moving the focus 0.5mm in both directions to checking my front-focus problem i have. so yes, this GG definitely has a front-focus problem. however the 0.5mm correction was way too large. i calculated i need to shim the glass only around 0.125 mm or so. now the question is (Bob maybe?), with those little four screws that shim the metal pieces behind the GG, how do i know what amount of turn pushes the glass by 0.125mm? i suspect a whole turn might be too much, but have no clue.
now, i don't feel like spending the whole month turning 1/8 turn, making exposures, sending to lab and then repeating the procedure, just to find out. will be happy to know if there is already the data regarding those little screws.
does anyone know?
thanks,
tal

**Bob Salomon** · 8-Jun-2012, 05:00

Is your ground glass in properly with the grain side facing the lens and any fresnel placed on top of it?

Who played with the shims otherwise?

If you need help because the shims have been played with contact Linhof in Munich. They are probably closed today but will be open on Monday.

**Tim k** · 8-Jun-2012, 16:38

Just measure it. The frosty side of the glass should be the same distance back as the film in your holder.

**IanB** · 9-Jun-2012, 04:25

Adjustment on the TK ground glass is carried out by shimming alone - the screws are not adjusters. In principle there is a spring which pushes the GG forward, and it's position is fixed by the holding plates - these are shimmed to the correct position, and held firmly by the screws. You either add or remove shims to get the correct position overall. Shims can be of any material which is convenient and reasonably un-compressible.

Hope that helps!