Originally Posted by
Pere Casals
Hello Bernice,
For DIY the criterion is empiric, it consists in measuring resolving power at center, corner and mid while you unscreew. It is possible that image on Ground Glass (granularity) does not allow a perfect adjustment (with a magnifier) because GG do not show all possible sharpness. Of course a manufacturer may know a theoric increment in cells best spacing depending on focus plane distance.
Manufacturers like Rodenstock (for some models) were including a shim or more (or none) for some lenses, I guess they used fancy optical equipment to find optimal cell to cell distance for each unit, perhaps they also searched for good combinations of rear and front cells with compensating errors.
In the DIY realm, let me suggest 3 ways:
1)
Attach an eyepiece from a telescope or microscope (I use a x20 nikon eye piece from nikon microscope) in a lensboard, place that lensboard in the camera rear.
Place a USAF 1951 target at the intended distance, 1:30 magnification is good enough for distant subjects, from 1:30 you may not notice a change.
With rise-shift place the eye piece in the center, corner or mid of the image circle. Adjust tripod ball head to see the resolution target.
Check resolving power (group/element) for each half tour increment, check focus (belows extension) each time, as you unscreew you should check focus again.
This test do not check field flatness, as you get optimal focus for each reading, but it overcomes any camera alingment missmatch.
Then, if you know the good number of tours you also can know the shim thickness, just multiply the thread pitch by the number of tours.
2)
Another choice is using a DSLR in the back of view camera. Let me explain how I did some experiments for that.
I attached an extension ring (those for macro) ito a lens board, in that ring I place a Nikon D3300, this is better than a D3200 because the 3300 do not have the optical lowpass filter. A cheap D3300 has a high pixel density around 250 pix/mm IIRC.
Then the routine consists in placing a USAF 1951 target at the intended distance, for infinite distance optimization working at 1:30 magnification is enough. Then you unscreew half tour each time, then you have to measure resolving power (USAF 1951 group/element ) for center, mid and corner, so you use the camera rise/shift to place the camera sensor in each point, you check focus always before taking the digital shot, so you avoid camera alignment issues, as a view camera is not lab equipment. In this way you are not able to measure field flatness... but still you can find the best shim. You have a collection of digital images to find the right cell to cell distance.
3)
Another way, straighter, it consists in just taking series of shots with a sharp film (TMX, CMS 20...) , each with the front lens unscreweed more or less, but here you also have the effect of camera alignment, film flatness and film plane to GG matching.
Regards