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View Full Version : Is my ground glass mis-seated?



Christopher Condit
23-Feb-2004, 23:11
Recently I upgraded from 4x5 to 5x7, and I am unconvinced that my first dozen or so shots are accurately focussed. In particular, 11x14 enlargements from the 5x7 are not as sharp as those from my 4x5. I do not believe that weather, technique, lens, or film differences are at fault.

Amongst the various other variables at play, the one that intrigues me today is, how do I know that the film in the filmholder, when inserted, is precisely where the ground glass was when I focussed on it? Were it not, how can I determine which is at fault, the camera back or the filmholder? (Hint: the camera is brand new and the filmholders are quite used ;-)). If it is the camera back, what can I do about it? And finally, how likely is this to be be the cause, or am I following a red herring here?

Not being a craftsman, and not owning a micrometer, I think my first step will be to buy a pair of brand new filmholders, maybe that will solve the problem...

Has anyone else encountered similar focus problems and successfully resolved them? How?

TIA,

Michael S. Briggs
24-Feb-2004, 00:19
The archives contain a surprising number of reports of people with used cameras with incorrect focusing. Apparently previous owners have changed the ground glass in an incorrect manner. But your camera is new and so the probability that the ground glass is correctly located is high. 5x7 filmholders have been standardized for many years, so unless yours are in poor condition, or are really old, probably they are OK.

Since you don't have experience with micrometers etc., probably the best test is a photographic one. The simplest version is to photograph a fence at an angle, marking (e.g., with some tape) the position on the fence that you focused on. Examine the negative for the part of the fence in best focus -- is it the part that you focused on? If not, either your cameras or filmholders are off in some way, or you need to improve your technique with this camera. More elaborate versions of this test can be made in which one carefully postions objects at several distances from the camera, with the separations between the objects being ranging below and above the expected depth of field.

If you find a discrepancy between your back and holders, probably the simplest resolution would be to buy some new holders and test again. If there is still a discrepancy, the back would probably be the culprit. Another approach would be to use a depth micrometer and compare the measured distance to the film or ground glass to the correct value. If your back were to be off (unlikely), the ground glass can be moved further back with shims. If the ground glass needs to go forward, the repair would be somewhat more difficult.

Andrew O'Neill
24-Feb-2004, 00:42
I use the ruler and toothpick approach. If the back of your camera can be removed you should be able to do this. Get a ruler and clip a toothpick at a right angle in the middle of it with pointy tip down. Lay the camera back down on a table with the inside facing up. Lay the edge of the ruler across the camera back making sure that it is sitting unobstructed. Push the toothpick down until it just touches the inside of the ground glass. Lay the ruler on the table making sure not to disturb the toothpick. Next, place a sheet of film you don't need in a holder and slip the holder into the camera back. Don't bother putting the slide in. Take your ruler again that you preset with the toothpick, lay it along the back and see if the toothpick just touches the film. You can check the sides and corners too. Primitive but it works and I'll bet you have a straight edge and toothpicks within arms reach!

Christopher Condit
24-Feb-2004, 09:31
Two good ideas that sound easily within my scope. Thanks, guys.

Unfortunately the back of my Toho does not in fact come off, but just for fun I will try the toothpick technique from the front (entire back+bellows+front comes off standards). The angled picket-fence shot I will probably also try, I'd heard of it but forgotten.

Bill_1856
24-Feb-2004, 10:31
An easier way: Just photograph a ruler close-up at an angle (lens carefully focused at 6" and shot wide open). If your camera is set up properly, the 6" mark will be sharpest. If some other mark is sharper, you've got problems. If you've got Polaroid, it won't take 5 minutes to have your answer.

George Pappas
24-Feb-2004, 13:30
Chris,

It sounds like you are on the right track to diagnosing your problem. If these tests show that you film back position and groundglass are at the same plane, there may be another explanation.

If you are using a longer lens that what you used for your 4x5 work, you may be having a depth-of-field problem. Just like when you step up to 8x10, you have to pay a little more attention to depth-of-field with a longer lens. This will be easier to see with certain subjects than others and, of course, does depend on how you use movements.

I found that when I moved up to 5x7, I used a 305mm lens as standard instead of the 210 I used on my 4x5. As a result, I had to pay a little more attention to depth-of-field issues....

Best Regards,

George

Michael Kadillak
26-Feb-2004, 20:40
I would enjoy hearing what you concluded from your post after further review. Were the older holders at fault or was it the ground glass on the camera? Cheers!