Ground glass film alignment?greenspun.com : LUSENET : Large format photography : One Thread
After having some eradict focussing issues, I decided to investigate the issue of ground glass / film alignment. I read the last thread, however it did not address some of the issues I thought of. For consistent critical focus I realize the film must sit in the exact same plane as the ground glass, i.e. so the film sees exactly what I see in the ground glass. However, when studying how to test this, I came up with a good system, but many questions. Using a depth micrometer with a large straight milled metal bar, I can measure from the rear standard plane down to the glass. Comparing that distance to the film holder in place with film would make sense. However this raised several questions which I could not find answers to.
1) Since the ground glass and film are different thicknesses, where do you measure the micrometer to for proper comparison? How about fresnel lenses, how do they affect the measurement?
2) Is the actual focus point on the ground glass on the back where your eye is, or in the front towards the lens?
3) Once it is determined what you should be measuring to, then what would be a reasonable tolerance between film holders, quick loads, polaroids, roll film backs, etc? The micrometer for $30 reads to .001 inch.
This seems like such a critical issue, I find it hard to beleive this is not tested more often and a micrometer kit is not sold for 4x5 and 8x10's with instructions. Anybody have experience at this? It will save me a lot of headaches, thanks in advance...
-- Bill Glickman (Bglick@pclv.com), December 26, 1998
When I bought a used Horseman I shot a roll to test the lenses. Much to my surprise the shots taken wide open (of a newspaper page) were horrible. I wondered if the ground glass/film plane were the same- they were not. the previous owner had the ground glass in reversed! A depth gauge in thousanths revealed the major discrepancy. I don't think film thickness has too much to so with it .003"-.004".
-- George Nedleman (GNLN@THEGRID.NET), December 26, 1998.
What means "eradict"?
1.) Ideally, the film plane and the front of the groundglass and or groundglass intensifier combination are the same. I say ideally because film in virtually all holders, except the very exotic and expensive Sinar and vacuum platen holders, curves very slightly, especially if you are shooting at a severe downward angle with large (8x10 and up) size film.
2.) On the lens side, where ground grain of the glass is.
This is one of those questions that proves that theorectically at least, photography is impossible. If you are having sharpness problems and are using good equipment more than likely the problem lies elsewhere. Possibly in your technique, possibly in the way your camera or lens are put together, but more than likely the problem is not with groundglass to film plane alignment. Recently Bill you posted about prontor shutters. If you switched to prontor shutters did you do the switching or did you have a qualified repair man like Steve grimes do it. Have you taken your lenses apart to clean them? Have you had your lens tested on a collimeter? How powerful a loupe are you using? You don't need to be using more than a 4x. You are shooting landscapes correct? Could wind be vibrating your tripod? There is literally a near infinity of questions to be answered. If you bought your lenses from an overseas source and they are dogs, are is the dealer going to make them right? etc., etc.
-- Ellis (firstname.lastname@example.org), December 27, 1998.
One last thing. The reason micrometer kits are not sold with LF cameras is because they aren't generally (99.9% of the time) needed, especially with high end cameras where precision is what you are paying for. if you are having this headache with both your new 4x5 and 8x10 cameras and their respective lenses then I think that the error must lie with the operator.
-- Ellis (email@example.com), December 27, 1998.
Just to prove how right Ellis's statement is, after all the hours I spent once in calibrating, checking, rechecking, emailing, etc., it turned out the previous owner had simply installed the screen wrong. A quick call to the screen's mfg would have saved me hours. It was educational, however.
There's a simple test. Place a yardstick on the wall and photograph it, focusing on, say, 18". Do this reasonably close and from a 45 degree angle. If your lens is sharp wide open, use that aperture or close to it. Focus with a good loupe, develop the film and use a high power loupe to confirm 18" is the sharpest point or within the sharpest area.
-- Mike Long (firstname.lastname@example.org), December 27, 1998.
Thank you for the responses... a few comments on the above, the camera equipment is all new Toyo, VX125 with the same film holders. The lenses are all new Nikkor and Fuji purchased from B&H and Badger. There are no prontor lenses installed. The ground glass / fressnel is installed properly. As for eradict focussing issues, I was refering to shots that were taken many times with the same camera position, yet some were not as sharp, there was no wind at all and I never changed the f stop or the focus position, I only altered some filters in my Lee holder. This led me to beleive there was film alignment problem in some of the film holders? I agree with a lot of the issues you all raised, maybe it makes sense to shoot some test targets with the same film holders I used on the above trip, this would isolate any film holder problems without trying to measure them with a depth micrometer? Does this make sense as a "peace of mind check?" I also will try the yardstick test, it seems to confirm the lens though, correct?
-- Bill Glickman (Bglick@pclv.com), December 27, 1998.
It's, of course, not totally impossible for an mfg to have something wrong or a lens to be out of whack (or is that "in whack" if it's wrong?) However, I would certainly suspect something other than your equipment based on the VX and the new lenses.
I've made a pact with myself: I bought all new film holders and I intend to do so on a periodic basis, replacing the last set. When they are new or like new they are very smooth, no warpage and tight. Sure makes life easier.
The focus test on the yardstick will test your system, not just one component.
-- Mike Long (email@example.com), December 27, 1998.
A few years ago, I took a workshop taught by Joe Englander. At the first session, using a micrometer system, he measured the groundglass placement of all the camera being used by everybody in the workshop. Amazingly, almost every one was off. The moral of the story seems to be that just because we pay lots of money in the name of precision doesn't necessarily mean that we get it.
-- Rob Rothman (firstname.lastname@example.org), December 28, 1998.
Thank you Rob for that response. Now what were instructed to do about this? What did you measure the micrometer to? What were the tolerances acceptable? Did you fix your camera?
-- Bill Glickman (Bglick@pclv.com), December 29, 1998.
Maybe it is just because I use crummy equipment manufactured well before the war (that would be WWI, not the war between the states) but I can't imagine focusing to the tolerances these two threads are discussing. Can anyone really focus to the 1/100th of an inch? But people here are talking about 1/1000ths . . ..
-- Erik Ryberg (email@example.com), December 29, 1998.
It's been a long time, so I really don't remember just what tolerances are considered "acceptable." I've just been using my camera the way it is; frankly, if a photograph is not razor-sharp, I feel that it's just as likely to be my own fault (sloppy focusing) as an out-of-alignment ground glass. Most of my photos are sharp enough for my purposes (I generally don't print larger than 16x20). The point of the exercise, at least as I interpret it, was not to demonstrate that all of our cameras are junk; rather, its that there are a lot of different factors which can affect sharpness; one of them, which should not be taken for granted, is GG alignment. If you're not getting pictures which are sharp enough to satisfy you, then maybe that's one of the things you should look into.
As for the actual measurement procedure, my recollection is that Joe used a machinist's dial gauge which was mounted in a flat metal bar so that the shaft of the dial gauge was precisely perpendicular to the bar. First, the gauge is zeroed by holding it against a flat surface such as a pane of glass. Then, after removing the camera back, the bar is held against a flat surface on the back, and the plunger of the gauge is depressed until it strikes the inside of the groundglass. The measurement (which represents the distance from the reference surface to the GG) is noted. Then, the same measurement is taken with a film holder inserted. In theory, the difference between the two measurments should be no more or less than the thickness of a sheet of film.
-- Rob Rothman (firstname.lastname@example.org), December 29, 1998.
A pane of glass 4x5" in size can be held against the flat surface of the back if you cannot attach a metal plate to the dial caliper. (It must be something that does not flex.) In this case you must deduct the thickness of the glass from your measurement. It's probably easier to check quickly in this manner than to fashion a flat metal bar for the gauge.
Reference the previous post, while .007 is a very small variance, which is probably compensated for if you normally use F22 or so, you will see a difference in the sharpness of your work if the back is proper.
It gets dicey with a fresnel. One mfg. claims (or at least did 3 years ago) it is not possible to keep the tolerance when a fresnel is installed due to bending of the light from the concentric rings. Other mfg's don't sell their cameras without a fresnel installed and results are sharp. The mfg that says you can't use one for most critical results now offers one. My guess is that he is correct in theory but in the field, it works just fine.
-- Mike Long (email@example.com), December 29, 1998.
Head to the address above at the related Large Format Photography site. It's worth checking out the collection of writings on this issue there. In particular, there is a copy of Joe Englander's article from Camera & Darkroom magazine on the effects of different holders on sharpness.
-- Greg Lawhon (firstname.lastname@example.org), December 30, 1998.