View Full Version : How To: Ground Glass alignment

30-May-2001, 15:37
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

Kevin Crisp
30-May-2001, 16:33
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.

Doug Paramore
30-May-2001, 18:32
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.

Tim Klein
30-May-2001, 18:43
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.

andrea milano
30-May-2001, 19:01
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?

30-May-2001, 20:04
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.

Robert A. Zeichner
30-May-2001, 20:28
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.

30-May-2001, 20:56
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

30-May-2001, 21:00
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.


Pete Andrews
31-May-2001, 06:30
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.

Pete Andrews
31-May-2001, 08:33
I've just re-read Robert's post where he states that the 5x4 filmholder ANSI register is 0.197" or 5 mm. This is completely at odds with what I've read elsewhere, and also doesn't tally with all the measurements of filmholders that I've taken.I've recently measured every modern Toyo, Linhof and Fidelity darkslide that I could get my hands on (quite a few dozen), and they all measure ~4.76 mm from the mating face to the film surface. The distance to the septum is certainly well under 5 mm in all cases, and the Fidelity and Toyo holders need at least 0.15 mm subtracting to allow for an average film thicknes.A brand new Toyo holder I've just checked with a micrometer depth gauge measures 4.9 mm from mating face to septum, both sides.

I'm now thoroughly confused why all those filmholders seem not to conform to the stated ANSI standard.Anyone got a Sinar precision holder they could measure?

Sal Santamaura
31-May-2001, 11:05
Pete, the ANSI standard really is 0.197" from seating surface to septum ("T") for 4x5 holders. It's larger for 5x7 and deeper yet for 8x10. Most camera makers assume film base thickness of 0.007" which converts to 0.178mm. Yes, I understand the concept of significant digits. Round off at your pleasure.

My measurements of at least two dozen Fidelity/Lisco holders produced data scattered widely above and below ANSI's nominal "T" specification, frequently outside the stated tolerances. I don't claim any specific degree of accuracy, but stand by my high precision.

It could be that your measurements have thoroughly confused you because of those factors influencing the task that Robert enumerated. In my opinion, you're on the right track with Toyo holders. Don't worry about the standard, however. Instead, just make sure your ground glass imaging surface corresponds to your holders, taking into account the focal plane shift of any poorly placed (i.e. on the lens side) frensel. Once you achieve this *system* calibration, all will be as sharp as possible in your work.

Sal Santamaura
31-May-2001, 11:07
Of course, in my last paragraph, I meant that your ground glass should correspond to your holders with film loaded...

Pete Andrews
31-May-2001, 12:46
Sal. Is there an ANSI / ISO spec for where the film surface ought to be? The septum distance is rather a dumb thing to specify.

Sal Santamaura
31-May-2001, 14:22
I don't know of any, Pete, and expect that variations in film thickness from one manufacturer to another, as well as between different products from the same manufacturer, would make such a number less than useful with the type of holders we're discussing. Sinar claims its single-sheet holder places emulsions in the same place regardless of base thickness, but I don't remember offhand what distance that is, and haven't been moved to spend the cash necessary to buy one! It also seems that, lacking a vacuum arrangement, the Sinar holder could only insure registration at its edges. Humidity changes might cause a sheet's center to bulge towards the lens. There has been so little interest in vacuum systems that Schneider recently discontinued its High End Back.

All things considered, I believe the most practical approach is to use good holders like Toyos, calibrate your camera back, and "don't worry, be happy."

andrea milano
1-Jun-2001, 03:12
Harry, as I said there are many aspects to this problem, however to stay on the realistic side, focus ten times any objectin what you think is the same spot (close ups show this even better....) and measure the flange distance, you wil, very probably, unless you are superhuman, find out that you will get at least 8 different mesurements, all different with a difference within the 1/10 of a mm, now this is caused by different bodily inaccuracies like your eyes being more or less tired or your hands on the focus knob, in any case comparing an ojective mesurement to a subjective "impression" of sharpness introduces many more variants than the construction faults or norms. What I mean to say is that applying this sort of scientifical method to a piece of machinery (our body) which in the end is in charge of making the choice of where to focus is nothing more than a theoretical exercice, it is nice to know that you use instruments which by far exceed the human capability of mechanically stay within those limits. In the middle ages it was attempted several times, by many illustruos and learned scholars, to determin the gender of angels, the problem seem to interest the humans a lot, a conclusive response was reached (male) by the force of general agreement rather than the ultimate conclusive proof, observation.

Robert A. Zeichner
1-Jun-2001, 08:19
At the risk of droning on a bit more, the fact that there are human elements involved in actually focusing, loading and inserting film holders, etc. might be an argument for making certain that the gg is in the "perfect plane"! That way, even if you err a bit in your focusing, even if your holders are off a bit, even if your particular film is thicker or thinner than what might be considered the norm, at least there's a chance the errors won't all add up in one direction. If your gg is .007" too deep, you have cut out all the headroom for error on that side of the published tolerance!

andrea milano
1-Jun-2001, 14:02
Yes I agree that the camera has to be within reasonable limits in good working order but my objection is more directed to those who continuosly look for the "minutia" and loose sight of the great picture, I mean if you are convinced that the GG is reasonably allined to film (Holder and sheet film) does it matter that your next film will be less deep and your film less thick(by how much!) when tightening your focus knob takes you already out from what your eye might or might not percieve as the right place to focus. the circle of confusion works in different ways when you talk of a slide and judge the focus on the original or a print and judge the focus on the print (resolving power is much less and enlarging chanes the parameters so the circle of confusion can be bigger), next to this points I like to say that if you print anything with the offset system the "raster" will eat up much of your unsharp bits (up until a point), the best cure for this sophisms was to visit the exhibitions of Dorothea lange and Henri Cartier-Bresson, many of the pictures were soft focussed or bluntly not sharp, this took nothing away from this extraordinary pictures(mostof these pictures appear sharp on printed matter, the story is quite different when examining the "real" prints. Gettig maximum results at f64 by shooting landscapes or shooting still-lifes in the American desert sun is an easy task(I am not sayng that this will make nice pictures) but even the holy Weston wasn't immune from the unsharp image disease. Anyway, life is a huge road and takes all sorts to travel on it, some care about "minutia" some don't, the important thing is that you take nice pictures.

1-Jun-2001, 16:11

I concur that time should be spent making photographs, not endlessly worring about equipment, but since I have noticed a marked difference with the two backs ( one is a meter back the other isn't ), I simply wanted to do some corrective maintenance so I am working from a consistant baseline. One reason to spend the money for a high quality piece of equipment is the peace of mind that WYSIWYG, well, within reason anyhow.

Your arguments have helped me realize that I don't have to be accurate down to .001", but the other viewpoints have pointed out the value of consistancy between the backs I use. I do want them to be close enough that I can feel a level of comfort that the image on my film will be close to what I see when I look at the ground glass.

Thanks again to all who participated in this discussion,

andrea milano
1-Jun-2001, 18:03
Harry, I'd like to add another few lines to this discussion. Metric backs are very precise due to their photogrammetry (measurements done by use of a special camera where, given the focal length ,precisely measured)capabilities, if you measure the picture you will know the distance or any other dimention of a given object. In this case the precision of a back isn't so much used to keep the focus rather than keeping the shape. If a camera points down or up, modifications of the film position traslate into changements of the shape and therefore the dimensions will be altered. Measuring on this bases will be unreliable. Any other photographic application than Astronomic photography and photogrammetry have no real use for such backs, but I might mistake there. Greetings to all!

1-Jun-2001, 20:36

Sorry I was not clear, the one back in questions is not a metric back, but rather is the Sinar metering back which has the capability for film plain metering with a probe and Gossen meter. I use this a lot for macro photography in the studio where I am using a flash and don't have room to get a meter close to the subject. It also saves me from having to do bellows extension calculations :-).

In the field I prefer the lighter and smaller non-metering back (which I just purchased) since I can use a hand held meter to setup the shot and a tape measure and table for calculating bellows extension factors.

Best regards,