Measuring depth of groundglass and film planegreenspun.com : LUSENET : Large format photography : One Thread
Can someone please tell me how I would go about checking whether the groundglass and film plane of my camera(s) are on the same level? Do I need a special and expensive tool to measure this?
And if I find out that the two are not along the same plane, how then do I go about to make them even?
Also, do new cameras need to be checked, as well as used ones?
-- Nick Rowan (email@example.com), February 01, 2001
Don't trust even new cameras. Barry Thornton describes a simple procedure in his book "Edge of Darkness"; a book worth reading for those who want to achieve high-definition prints: Take a piece of stiff white cardboard or similar and draw parallel vertical lines about 1 cm (½ in) apart. Draw the line in the middle a bit thicker so you can recognise it. Place the board in a rather small angle in relation to the camera centerline to maximise the distance between the first and last lines and focus on the line in the middle. Print the negative so you can see where the focus falls on teh film. It should also be in the middle but is not necessarily so. The correction to be made depends very much on the result and your type of camera. Good Luck!
-- Martin Glader (firstname.lastname@example.org), February 01, 2001.
An engineers caliper, or a depth gauge, is the tool you need.
If the GG screen is removable, as with an international back, then the procedure is very simple. (an exception to this would be a fresnel screen arrangement, which doesn't have the same optical depth as physical depth)
There should be 3 or 4 small bumps on the GG holder, to locate to the camera film-holder face. Measuring from these to the ground face of the screen should give you 3/16 of an inch or 4.76 mm. There's an ISO standard, which gives the tolerances, I think it's something like plus or minus 0.15 mm. Modern film holders should all fall within this tolerance.
If your GG sits too close, then you should remove the glass and shim it out. In the unlikely event that it sits too far away, first check for dirt underneath, and that the glass is properly 'bedded'. If it's still too far away, the only remedy is to remove some material from the mating surfaces. This is really a job for a skilled machinist with a milling machine.
-- Pete Andrews (email@example.com), February 01, 2001.
With regards to the first technique, photographing the angled ruler : You must shoot a few shots (at least three), refocusing for each one. The chances of a 0.2mm shift of the film plane when inserting a film holder, especially on field cameras, is quite high. I personally found the measuring technique to be more reliable and performed the film test for final verification of the result.
-- Richard Ross (firstname.lastname@example.org), February 01, 2001.
Nick: I had this problem with a B&J view camera I refinished. The device you need is a cheap stainless steel dial caliper. These are $19.99 from most reloading supply companies. (Midwayusa.com is but one. They are accurate to 0.001 inch) (Reloading as in "the other shooting sport.") Anyway, put something really straight across the camera side of the camera back. Use the caliper post to measure how far it is from your straight edge down to the the ground glass. Write that down. Now stick in a film holder with a sheet of film in it (or an old negative you don't cherish) and do the same measurement after pulling the dark slide. The closer the better but you'll never get these to be exactly the same to the 0.001 level of precision, and every film holder is a little different. If your concern is variation across the film plane, then check in several different places for that. I have found the tool useful for all sorts of other things and well worth the investment. For a real life test, I used a long lens and put a row of objects on the top of a fence and shot down the row, focusing on a particular marked object in the series. Expose the film with the lens wide open so you don't mask a mismatch with extra depth of field. (Polaroids are terrific for this.) You usually have to shim the glass away from the lens if there is a problem. Gaffers tape is good for this at it happens to be right about 0.010 inches thick. Good luck.
-- Kevin Crisp (CrispK@hbblaw.com), February 01, 2001.
Nick, The measurements you wish to make are extremely difficult to accomplish without some expensive precision equipment. Not only do you need a high quality depth gauge, but some custom test fixtures that are precision ground to very small tolerances. Without these, you will simply never know what depth anything is. It is for this reason that I developed a technique for testing ground glass/film plane alignment with film. It involves a stepped target with very detailed subject matter (dollar bills) and exposure under conditions that will quickly reveal error. You might check the article I authored in the Nov./Dec. 1996 issue of ViewCamera magazine for the details. The explanation is a bit lengthy to get into in this forum, but many who have read this article have solved their gg/film plane problems once and for all. BTW, John Sexton includes reprints of this article in his workshop handouts.
-- Robert A. Zeichner (email@example.com), February 01, 2001.
About 18 months ago PhotoTechniques published an article showing a very low tech (and easy) way to check the depth of your ground glass and film holders. It involves a piece of metal bar and a toothpick. You use the metal to span across the ground glass frame then tape or glue the toothpick to the metal to register the depth. You can then use this to confirm whether your film holders have the same dimension. Make sure the bar is square and long enough to slide over the frame and film holder so you can check different spots.It sounds hokey but it works.
-- Kevin (firstname.lastname@example.org), February 02, 2001.
I'm very familiar with that article. Unfortunately, it simply doesn't work. You may think it works, but it can't. In fact, this method of measurement is so flawed, it will doubtless lead you to doing something foolish like throwing out perfectly good film holders! It measures nothing! The ANSI spec for 4x5 film holder depth is .197" +/- .007 Taking into consideration that the ground glass and the aluminum plenum in a film holder both exhibit a certain degree of compliance, you can only make meaningful measurements of their depth with a precision depth gauge and specially designed, surface ground, chromed steel test fixtures. The actual depth measurements must be made in such a fashion as to integrate the entire contact surface of the film holder in the camera back. Measurements must also be made under the spring tension of the mechanism in the camera back that holds the film holder against the camera. An alternate way of making these measurements might be with a collimator. I believe the folks at Linhof use some kind of laser type collimator for this purpose. Their gg alignment is without peer! Of course, any way you choose to make these measurements will require taking the film thickness and film curl into account. Bottom line: NONE OF THIS IS EASY. When I used to overhaul professional motion picture cameras for a living, it never ceased to amaze me how the amount of finger pressure I applied to a .200" thick steel backup plate the size of my thumb could influence a depth measurement. And these were tiny parts, much smaller and more rigid than the wood and plastic of view cameras and film holders. Use toothpicks for picking your teeth! Test your camera with film. You won't necessarily be measuring the depth, but you'll confirm performance on the very surface where it counts the most!
-- RobertA. Zeichner (email@example.com), February 02, 2001.
I have done quite a few measurements following the general outline in the last R.Zeichner posting. There is quite a bit of misinformation in this thread but I will consign myself to give you summary points: 1) It does not really matter what the ANSI or ISO standards say. What really counts is the depth of your film holders in relation to that of the ground glass. You can't adjust the holders, but you can adjust your camera GG so that the two depths (holders/Camera) match. (allowing for film thickness 2) Film holders are the $25 pillars that support $1500 lenses. Don't buy cheap holders!! My tests indicate quite a few variations among one of the leading brands. Linhof and Sinar are great, at great cost. Toyo's are a fair compromise. 3) Calipers are totally unsuitable for the kind of measurements you want. Ideally a non-contact device such as a laser gauge is best. Cost is a factor, so use a mechanical depth gauge. The digital depth gauges that exert a constant -low pressure are 2nd best. Mitutoyo makes those but they are very expensive. 3rd choice: a screw type depth gauge. Problem here is that the depth location of the probe will depend on the twisting moment you apply on the gauge. It is not easy to detect the point at which the gauge's probe makes contact. These gauges are relatively cheap. It takes lots of practice and some statistics to assure that your measurements are accurate and consistent. 4) to profile the film plane on a holder or GG you need to take 15 measurements X2 on each side, at least until your statistics give you the green light for 15 measurements each side only. As one poster said it, do not trust camera GG settings. 5) ANSI allows + or - 0.007". Sinar guarantees 1/7th of that for their holders. You get about + or - 0.002 with Toyo's. Not bad considering that one leading brand tested at over 3X that much deviation. On Linhof Technikas adjustments are a breeze. 6) It is not easy when you are dealing with small dimensions. Your depth gauge should have a 6" long foot, but 4" OK. Make sure you calibrate your flat-ground aluminum plate first on a float-glass plate and add or subtract from each measurment accordingly. The aluminum plate is bound to be not quite so flat. Then, zero the gauge against the glass. It is a lot of bother, but considering that you only do it once, it is little pain for bigger gain. There is tons of discussion about lenses in this forum, I fear, by people that have never checked their holders or their cameras. Congratulations, Nick, you have raised a very valid issue and are on the right track! Good luck!
-- Julio Fernandez (firstname.lastname@example.org), February 03, 2001.
Nick, Julio and Rich have excellent points...I tried this myself and spent a lot of money on expesnive instruments and still could not get reliable information. The list of reasons are way to complex to try to mention here. The cost of materials and my time was exhorbitant. For a small fee, send the camera and film holders off to Mamiya USA, assuming you are in the States, and let them do this for you. They have all the lasers and are set up to accomplish this very difficult task...I was upset I did not think of this first.... the slanted ruler test is a great test to find out if you are in the ball park. I did that test with a piece of slanted paper and used very tiny rows of numbers and determined from the gg, the exact rows....they were numbered differently all the way accross the page...that were in focus and slightly out of focus, etc. I left the lens at the same f stop and shot film...it was an excellent indicator...but unfortunately can not be used for adjustments if it's off. Good luck..
-- Bill Glickman (email@example.com), February 05, 2001.