I have two ground glass backs for my Sinar 4x5 and I recently noticed that there is a difference in the focus between them. I measured and I get a different di stance from the carrier to the ground glass between the two. It is an easy matt er for me to measure the distance from the carrier to the film plane of one of m y holders. My question is, once I have this distance do I then measure to the s ide of the glass closest to the lens, or to the side closest to my eye?
Thanks, -harry
Seems to me you want to measure to the front of the ground glass, assuming that the frosted/ground side of the ground glass is on the inside of the camera. That distance should be as close as you can get it to the distance when you have a film holder inserted with a sheet of film in it. But the point isn't whether one back is different from the other, the important point is whether when you focus with each back individually the ground glass is the same plane of focus as the film in the holder with that back. If one back has the ground glass further in or out, and a corresponding change shows up in the measurement to the film, this isn't a problem so long as the ground glass and the film are in the same plane or pretty darned close to it. Put a row of small objects on a fence, mark one of them, and shoot down the row with the lens wide open and the marked object in focus. Try this with each back. If the camera is taking a picture with a plane of focus that is in front of or behind what you saw on the ground glass, this test will prove that immediately. Polaroids are the quick and easy way to check. A surprising number of view cameras are "off" and this will show up on this test. It is often masked for a long time by the depth of field when you stop down, so people don't know it.
Harry: Kevin is correct that the focus may be different between two backs, due to the thickness of the backs. Measure from the front of the ground glass. An easy way to check is to measure from the lens mount opening to the ground glass, then insert a holder with an old piece of film in it and measure to the surface of the film. The measurement should be the same. On some cameras with large lens boards you may need to put a straight edge across the opening and measure from it. On some cameras, the Fesnel lens is in front of the gound glass and you will have to take this thickness into account. Kevin's suggestion of shooting a couple of Polaroids work fine, but if you don't have a Polaroid back you can measure the distance.
What is generally the accpetable tolerance for this type of measurement? Also, does anyone know what tolerances the manufacturers of film holders use?
It seems to me that these tolerances, coupled with the varying thickness of different film bases could potentially add up to less than sharp images. I'm just curious at what point the ultimate difference could affect image quality.
This matter comes periodically up on the forum, many seem to be concerned about it, however there are a number of old threads on this Topic. One thought on this rises to my mind,this is that unless we are talking of hypercritical focussing or badly faulty cameras, the matter is of no importance at all, nevertheless, the theorethical aspects seem to concern a number of aficionados, who am I to play down their doubts?
There is no need to take any measurements from the lens board. Just measure from the face of the glass frame (camera side) to the glass. It must be the same as the dimension from the face of the film holder to the film. After all, they both sit against the back's frame, and no other dimensions are variable.
Measuring film holder or gg depth is difficult to do without precision dial depth gauges and test fixtures. Many have tried to do this with everything from calipers to tooth picks and rulers! It's a useless waste of time! I am of the strong opinion that it's vitally important to make certain the gg is in the perfect plane for everything to work in a satisfactory way under the widest range of circumstances. The reason is this: The ANSI spec. for film holder depth is .197" +/- .007". That's a tolerance range of .014", which is a pretty wide range. Since most film holders are made of molded plastic and sheet aluminum, materials that can warp or at very least, change dimension with age, temperature, etc., it makes sense that you would want your gg adjusted to be right smack in the middle of that range! If this is the case, you will get satifactory results from the widest selection of film holding devices. If your gg were a few thousandths of an inch too deep (further from the lens than the ideal film plane), and your film holder was on the shallow side, you could exceed the +/- .007" pretty easily and end up with soft negs. Measurement of gg depth is tricky. You really need to make many measurements and use a test fixture that contacts the entire mating surface of the camera back. This is because imperfections in the flatness or thickness of the back will severely influence measurements made referenced to a couple of isolated points across the back. this is all easier to draw than describe in words. The film holder depth would need to be made in the same way, with the holder inserted in the back. There are additionally issues of test fixture compliance, depth gauge accuracy and gg/film holder plenum compliance. All these things can and will influence the readings you get and you will get different ones on different days! A test with film is the best way to prove performance. Check out my article in ViewCamera magazine, Nov./Dec 1996. It describes a do it yourself test target that will, if used as recommended, help you determine whether or not you gg and film lie in coincident planes. Feel free to email me with questions.
Thanks to all!
A couple of notes. I do a lot of macro and floral work with images that are 2 or even 3:1. This leaves me with very little DOF. Swings and tilts help a great deal, but at those levels every little bit of sharpness helps.
I recently went from over 50 film holders down to 10, all the same brand and all measuring the same distance when loaded in the back. My camera has a removable bellows so it is very easy to measure from the front of the rear standard to the actual film or GG plane. I am interested in trying the ready loads ( or is it quick loads ) to see if these are consistant from sheet to sheet.
And I have access to a set of machinest's tools including depth gages which give me an accuracy of .001" so this is not an exercise in theory but should actually improve my images.
Thank you all again, -harry
Sorry signed off before I was finished. I do plan to follow the suggestions of shooting a test target, but I want to start with the gg being set as close as possible before I start burning film.
Best,
From the film holder seat to the GG should be 4.76 mm or 3/16". It's very easy to measure this with an engineers depth gauge, or a decent vernier caliper. There should be some obvious seating points on the spring back, and you should measure from the highest points of these to the surface of the GG.Check that the GG is properly seated in both backs first. If one back is undersize, then you just need to add 'shims' under the GG seating. Small pieces of paper or card will be good enough for the job. An oversize back is more difficult to deal with, and some material will have to be removed from the GG seating.
Many older cameras didn't conform to the modern filmholder register of 3/16", and it's worth checking old wooden filmholders and cameras for this vital dimension.While we're vaguely on the subject. Has anyone got measurements of the register of American made Grafmatic backs? The ones made under license by Wray in England seem all totally non-standard.
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