View Full Version : Testing for Correct Position of Ground Glass

Brian Ellis
15-Feb-2005, 13:35
I just installed a Maxwell screen to replace the Linhof ground glass in my Master Technika (which in turn replaced the Beattie screen that was on the camera when I bought it). Since this is the third screen that's been on the camera I'd like to to make sure the position of the screen is correct. I remember an article in View Camera magazine some time ago that described a testing methodology using playing cards and I vaguely recall another method mentioned here of focusing on a point on a ruler. However, I don't remember the details of either method. Would someone please describe a method they've used to test the correct positioning of the ground glass or cite me to a place where a detailed description can be found? I searched the archives but unfortunately the word "test" or "focus" appears in hundreds of messages. and there doesn't seem to be an "exact phrase" method of searching the archives. Thanks.

Calamity Jane
15-Feb-2005, 13:42
I can't help you with the proceedure you mention but I can tell you how I do it on the 4x5 cameras I build.

The distance from the face of the film holder to the surface of the film is 0.197" (5.00mm) on a 4x5 holder. I check the position of my ground glass by measuring from the face of the GG frame to the surface of the glass and make sure it is exactly 0.197" If the distances are the same, they will both focus the same. (Dial caliper or depth micrometer will both work.)

I do not know if the distance is the same with other film formats.

Michael S. Briggs
15-Feb-2005, 13:47
Here are two previous discussions from the archives: Ground Glass focusing errors at http://www.largeformatphotography.info/lfforum/topic/118997.html (http://www.largeformatphotography.info/lfforum/topic/118997.html) and
Is my ground glass mis-seated? at http://www.largeformatphotography.info/lfforum/topic/497848.html (http://www.largeformatphotography.info/lfforum/topic/497848.html).

15-Feb-2005, 13:50
Hi Brian,

Put a ruler a foot or so in front of your lens, at an angle to it (exact angle doesn't matter). Focus with the ground glass on one specific line on the ruler, and take a shot with the lens wide open. Negative should be sharp at that line, or if not you can tell from which line is sharp as to whether the GG is too near or far from the focus plane. I have found that a Polaroid is good enough for this test.

Bob Salomon
15-Feb-2005, 13:56
When you replace the gg on a Master Technika you simply place it on top of the shims. You do NOT adjust the shims. Therefore there should be nothing necessary for you to check. Unless someone has adjusted the shim needlessly.

Brian Ellis
15-Feb-2005, 18:15
Bob, my concern was that someone before me did something to the shims or did something else that might create a problem. The camera had a Beattie screen on it when I bought it, which means someone before me did at least one replacement of the original screen, and I don't know what they did or how many different screens have been on the camera. I suspect everything is fine but out of what might be an excess of caution I thought it would be best to test to be sure. Thanks for your respoonse and those of the others who responded.

Calamity, I couldn't measure something to a distance of 0.197 inches if my life depended on it. I have trouble figuring out which line is a fourth of an inch and which is an eighth of an inch on a ruler.

Jeffrey Sipress
15-Feb-2005, 20:51
If a camera is built to precision standards, why would shims be neccessary?

John Berry ( Roadkill )
15-Feb-2005, 21:35
Reply to Jeffery: I don't know about the camera in question, I used to rebuild heavy duty transmissions. In most trannys there was a bore for a bearing. On the german trannys, it was bored a little deaper so it could be shimmed to perfect specs. When doing production machining there will be a tolarence. I suspect it was machined with shimming to spec in mind. Just my guess, but bob would be the one to know. John

Jorge Gasteazoro
15-Feb-2005, 22:24
Brian, the late Barry Thornton had avery good chapter on testing in his book "elements of darkness". Basically you can pick something like a picket fence, identify a single slat and focus on that slat, take a pic (polaroid is great for this) and then look for the slat in focus. If the one you chose is the one you picked then your GG is aligned, if the one in focus is closer to you and away from the "central" slat, then your gg is too far out, if the one in focus is farther from you and the "central" slat then the gg is too close to the frame.

Of course in his book he built a special target for this, but you can do it as I describe, all you have to do is pick something that has a row of uniform things in sequence. I guess any fence will do.

Good luck.

Leonard Evens
16-Feb-2005, 02:14

A few comments.

It is best to have the ruler or other target making a 45 degree angle with the lens axis. I find a sheet of newsprint taped to a wall and photographed about 5 feet from the lens is a good choice of target.

You are never going to get it exactly right because there are inevitable focusing errors. So you should do it several times to be sure.

There are some simple formulas which can be used to determine the amount of the shift from gg to film plane. Suppose the position in the subject seems to shift by a certain distance along the target. Take 70 percent of that shift to account for the 45 degree angle. Then the actual shift on the film side of the lens is that amount multiplied by the square of the magnification. The magnification can be obtained in a variety of ways. One is to take the subject to lens distance, divide by the focal length, subtract one, and then take the reciprocal of the result. For example, suppose the camera is focused on the target 1.5 meters or 1500 mm from a 150 mm lens. 1500/150 = 10. Subtract 1 to get 9. The magnification is 1/9. Suppose there is an apparent shift along the target of 2 cm or 20 mm. Multiply that by .7 to get 14. The shift in the film plane is 14 x (1/9)^2 = 14/81 ~ 0.17 mm.

Jeffrey Sipress
16-Feb-2005, 11:29
Thanks, John. I understand all about manufacturing, tolerancing, engineering, and precision machining. I do it for a living. Not automotive, however. Of course, in an engine or transmission, there are many places where the stack height of numerous components needs to be held quite tightly, and in those cases, shims are usually the way to dial it in such an assembly of many parts. With film holders, there is apparently a 5mm standard distance from the front of the holder to the film plane. Just one part. Using common precision measuring instruments such as depth micrometers or calipers, the distance from the camera's mating surface for the holder, to the ground side of the glass should be easily attained. I'm sure this all sounds foreign to many folks whose lives are not rooted in machine shops or high precision manufacturing, but the point here is that there is one proper physical location for the film and the glass. It can be measured, and made correctly by a camera maker without requiring shims. If the GG and film occupy the same plane, then focus is assured. All kinds of tests are not necessary. Just the measurement of one dimension.

Calamity Jane
16-Feb-2005, 12:09
Jeffrey, you are absolutely correct!

In my second camera, I redesigned the GG holder so the glass goes in from the rear (eliminating the need for any spacers, shims, etc.) and the thickness of the GG frame step determines the glass position.

After making the frame close to (but larger than) the required dimension, I simply measure the depth of the glass at each of the 4 corners and sand on a flat metal plate until all four corners are exactly 0.197" Although it requires a little "hand finishing", it is the only critical dimension in a LF camera and is well worth the effort.

With photographic methods of checking the plan of the GG plate, one would have to take multiple photos of the measuring stick, one in each corner of the plate, to determine if the glass is square to the centreline of the lens. If tests showed it is not in focus at all four corners, one STILL has to resort to measuring the glass position with a depth gauge to determine what corrections are needed.

If I didn't own one, I'd go out and spend the $35 for a dial caliper and just MEASURE it. At least that would tell me what shims are required and where they need to be :-)

16-Feb-2005, 14:21
Yes, Bob:

Why ARE shims necessary in a piece which could be machined to the exact dimensions well within acceptable tolerance?

Bob Salomon
16-Feb-2005, 14:45

Everything has tolerances. So the shims are adjustable. If they were not adjustable and the screen was a fraction of a mm off you would then be asking why the shims aren't adjustable.

Jeffrey Sipress
16-Feb-2005, 16:43
"Everything has tolerances".

True, and everything must, in order to be made and assembled. But exactly what tolerances are we talking about here? I make hundreds of parts a day, holding +/- .00025" on four dimensions of an aluminum part that is only 1.5" long. Of course, this is way beyond any tolerance necessary to construct the finest cameras. And, it is beyond what most machine shops do. It's the niche market that I'm serving. I would imagine that the 2mm (.1968") dimension we have been discussing could easily have a tolerance of +/- .oo2" without any noticeable deviations in focusing ability. And, this tolerance is easily maintainable by just about every machining or manufacturing facility. Just make it right at first, and skip those silly shims.

Bob Salomon
16-Feb-2005, 16:51
"The particular dealer you purchased "

And if you prefer the fresnel to be under the gg rather then on top of it? Or you want to install a mask under the gg rather then on top of it?

The Master Technika has been available for 29 years. The GG Frame that holds the gg is the same (except for the change in the springs in the arms) as the one for the Technika V and the Technika IV. The IV was introduced in 1956. Over that time the fresnel position has changed from under to over the gg. The frenel itself and the gg itself has also changed over that time.

The 001615 gg frame must adjust to work properly on a 1956 camera as well as a 2005 camera. That 1956 camera did not have a 0 gg. Each model IV had the gg 0 position that was unique to the camera. Only with the V did the gg position have a 0 position. However the current 001615 will easily replace the one on a 49 year old camera should the owner need a new one.

That is another reason why the shims are in the back.

For the current user of a Master or a V there is no reason for them to adjust shims unless the fresnel position changes. So all of this is moot for a V or Master owner. It is of extreme importance to the owner of a IV.

Michael S. Briggs
16-Feb-2005, 19:14
Is the ground glass positioned the same way on the Technikas as on the Technikardan? On the Technikardan 45S, at first glance the ground glass position appears to be set by shims, but a closer look shows that the position is set with adjustment screws.

If you take the ground glass off, you will see four metal strips that appear to be shims. These strips have small holes through which a black projection fits, preventing the strips from slipping out of place. If you remove one of the metal strips, you will find that the black projection is the tip of a screw. The screw has a reduced diameter tip that fits into the hole in the metal strip. The metal strip rests on the shoulder of the screw, where the reduced diameter ends. So the position of the shoulder of the screw tip sets the position of the metal strip, and thus of the ground glass. The front of the screw is accessible from the front of the frame that holds the ground glass, so the position of the ground glass can be adjusted by turning the screw. (I haven't tried this, and I don't advise it unless you have proved that the focus is wrong on your camera.) There appears to some type of cement applied to the front of the screw to prevent the screw from turning accidentally.

This design has some advantages. Compared to shims, it is easier for a repair person to adjust -- they don't have to have sets of shims of various thicknesses, they merely turn four screws. For the factory, the screws are probably set in some sort of fixture. Perhaps the frame is mounted in a fixture and the screws turned until pre-set dial indicators read correctly, or until the screws hit stops. Compared to precisely machining the seat for the ground glass, this method allows adjustment, e.g., if someone wanted to use a Fresnel lens in front of the ground glass. This might be the reason for the design, since as Bob explains Linhof made a design change in the position of the ground glass. Or the reason might be manufactcuring cost.

In any case, both measurements of the ground glass position with a depth micrometer and photographic tests show the focus of my TK45S to be correct.

Bob Salomon
17-Feb-2005, 02:45
"Is the ground glass positioned the same way on the Technikas as on the Technikardan? On the Technikardan 45S, at first glance the ground glass position appears to be set by shims, but a closer look shows that the position is set with adjustment screws."

All Linhof 45 cameras use the same GG Frame 001615 so all have the shims.

Dan Neilson
17-Feb-2005, 15:31
Bob, are you still watching this thread?

If I remove the fresnel from my Color Kardan (Tecknika V rear), will it affect the focus on the ground glass? Do I need to adjust the shims?

If I put the fresnel on to a Bi-Kardan, do I need to shim the ground glass?

I quess that what I am asking in general Is, does a fresnel either in front or behind the ground glass affect the focus on the ground glass?

Michael S. Briggs
17-Feb-2005, 15:39
Dan, it depends on the location of the Fresnel lens. A Fresnel lens in front of the ground glass (i.e., the lens side of the ground glass), shifts the focus. So changing between no Fresnel and a Fresnel in front of the ground glass, either adding or removing the Fresnel, means that the position needs to be adjusted. Adding or removing a Fresnel behind the ground glass doesn't change the focus.

Previous discussions: Ground glass focusing plane at http://largeformatphotography.info/lfforum/topic/497868.html and
Shim thickness to replace fresnel at http://largeformatphotography.info/lfforum/topic/499244.html