# Thread: Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

1. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

There is a very good article in May/June 99, Photo Techniques magazine about thi s subject. The author spent six years using a new 4x5 view camera not realizing the film and ground glass were mis aligned by the thickness equal to 3 sheets o f film. He thought enlargements were suppose to get blurry becasue he had nothin g to compare them to. I have been hearing this from many LF photographers, many of them using new equipment!

The method of testing this seems quite easy (with the proper measuring device of course) - with the 4x5 back removed, measure the distance from the ba ck to the ground glass and then insert a film holder loaded with film, and measu re from the same back position to the closest part of film. With any luck they should be close to equal. I am unsure what tolerance should be considered accep table? Does anyone know? However, what none of these articles ever mention... where do you measure to if there is a factory installed fresnel lens in front o f the ground glass. Should you still take your measurement from the back to the fresnel, or do you remove the fresnel lens and measure through to the ground gl ass? Any input would be appreciated. Thank you. .

2. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

The tolerance is 0.007 inches, which happens to be the typical film thickness. Take the fresnel out before measuring.

Another good test is to put a newspaper flat on the ground, mark the middle, focus on the mark, and photograph it. The resulting photograph will show if the ggs is out of position.

4. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

Alan, .007 is what I was afraid to hear. On a brand new 8x10 metal field camera with all new film holders many of my measurements are .025" apart from ground glass to film! This is equal to the thickness of 4 sheets of film! It appears the film holders and ground glass are not consistently off in any one direction, therefore shims would not help!

Milandro, your input was very helpful, and now I don't feel alone like everyone makes you feel. The point is here, if the equpment is off, it just adds to the errors that us humans are prone to, eyesight, camera shake, camera movement when loading film holders, etc. So if I am off the equivalent of 4 pieces of film to start and then add my field errors, its amazing even the chromes looked focussed. Of course I first noticed this problem when small enlargements were made. I figured if I bought all new equipment and paid the extra dollars, I would not have to deal with issues like this, but how wrong I was! I imagine for older used cameras things could be much worse! I was amazed that Wista lacked knowledge or quality controls in this area, the whole purpose of a camera is to block light and maintain ground glass and film alignment. Whats even worse is, I bought the same brand film holders as the camera maker to assure all components are speced by the same manufacturer, and it did not help at all! Very frustrating, but better off spending the time to fix the problem now vs. the tremendous traveling expenses to shoot on location and film / processing cost to notice it after the fact!

5. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

I bought a used Horseman 980 with a 105 and 180 Topcon lens. Testing them with Newspaper target, I was horrified to see the 8x10 enlargements of negs taken wide open- FUZZY. I measured the distance from the G Glass to the bosses that meet the camera body and then the film distance to the same bosses on the film back. The measure was off the thickness of the glass!! The PO had the Glass in upside down.

6. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

The ANSI standard for the depth of a standard 4 x 5 inch film holder is 0.197" plus minus 0.007" . Most film has a base of 0.007" . When film is loaded in the film holder, the depth is 0.190" . This is the measurement used by Sinar cameras. Wisner cameras use a compromise of 0.192" to allow for wear on the wood and because Tech Pan film, used by some photographers to achieve ultra-sharp images, has a base of 0.004".

The ANSI standard for 5 x 7 and 8 x 10 inch film holders are as follows (film thickness has not been deducted):

5 x 7" 0.228" + - 0.010

8 x 10" 0.260" + - 0.016

See my View Camera Construction FAQ at: http://home.sol.no/~gjon/lffaq.htm

7. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

To clarify the various issues presented, and introduce a new one, the problem -- of getting accurate ground glass and film plane alignment -- has three components. First, is the ground glass and/or fresnel correctly installed (i.e. is the fresnel behind the ground glass, as it almost always should be)? That is the easiest to solve. Second, did the manufacturer, in designing and constructing the camera, ensure that the ground glass and film plane would be within acceptable tolerances. Third (here's the new problem), film, whether sheet film or roll film is not flat. Short of using vacuum backs (there are probably other problems there), even if the other two problem issues are resolved, there will always be problems with critical focus. Therefore, conducting empirical tests (i.e. shooting newsprint) by shooting your lens wide open will most likely lead to disconcerting results.

8. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

Bill says "It appears the film holders and ground glass are not consistently off in any one direction..." It could be that the holder or GGS is twisted (in which case it should be trashed). Or do you mean that the difference varies over time, possibly due to temperature effects?

As Jon points out, the tolerance for 10x8 is 0.016", so 0.025" isn't too bad. If the ggs and film are not in the same plane, they might still be parallel, or they may be non-parallel (e.g. correct in the centre, but the ggs may be too far forward at the top, and too far back at the bottom). Non-parallel is a harder condition to identify.

If the manufacturer isn't aware of these issues, I can find no excuse.

However, my view is that we needn't be too paranoid, because we have depth of focus, i.e. a tolerance at the film plane due to the small aperture of the lens.

[pre] Depth of Focus = 2.C.N' [/pre]

where C = permissable circle of confusion, and N' = effective aperture. For C = 0.1mm and N'=16, Depth of Focus = 3.2mm or 0.126", and this is eighteeen times the official 5x4 manufacturing tolerance. (Of couse, it is much less wide-open.) If your subject is essentially a plane, such as a landscape (or newspaper), you will be using movements to attempt to keep it all in focus, and the Depth of Focus will easily take care of the manufacturing tolerance. The film may not be flat, but this will also be easily taken care of by the Depth of Focus.

How accurately can we focus? It will depend on the camera, GGS, and loupe, but I think my focusing tolerance is slightly worse than the manufacturing tolerance.

If your subject is not a plane (e.g. railway tracks leading to a building, and you want everything in focus) then you are relying on some Depth of Field, and this will reduce the Depth of Focus available to you.

9. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

Alan - Your response really puts things in perspective! You have applied the simple depth of focus formula to this issue, something I admittedly overlooked to see what the potential effects mis alignment would be. Sometimes we can not see the forest through the trees, and this was a case in point. It's so applicable, I suggest the main page have a link to explaining this, because at one time or another someone with focussing problems will always need this information, and there seems to be not one all encompassing explanation of this. This responses in this forum is about as good as it gets!

Also I would like to point out, that measuring these problems with dial depth guages and micormeters is not an easy task. The problem is getting a completely level reference source to measure from that has no flex in it, specially for 8x10. The the reference source can not be moved between the different measurements, i.e. glass measurement vs. film holder measurement. I was told by a manufacture that even their testing methods proved to be inaccurate, although much more accurate than our home methods, until they purchased a \$40,000 laser device that does all their checking. So my point is here, get a good reference piece of material such as a milled flat piece of metal with holes drilled in appropriate places, than a method to assure the depth gauge is hitting the glass or film holder almost perfectly perpendicular, then still treat your results cautiously. If you are off say 50%, as Alan points out, it seems depth of focus will correct any error that small. How far off until one is concerned would need to be evaluated taking all varialbes into consideration. If the test shots prove their is an alignment problem, then maybe you should send the equpment to be aligned with a laser machine and good repair man. I thank everyone for there input, it has been a great learning experience for sure!

10. ## Ground Glass and film plane alignment - how is fresnel lens thickness taken into consideration?

Bill, Let me add my (long) 2 cents worth for you and for the benefit of others followi ng this thread that might have the same problem. I own a Wista with the frenel/p lastic one piece unit that you describe and am aware of the focussing problems y ou describe. Depending on the focal length of the lens, the light leaving the gr ound glass is more or less perpendicular to it. Less when the lens focal length is further from the length that the frenel is optimized for (I assume a normal f ocal length is optimal). Therefore, extreme wide-angle and long lenses will resu lt in light leaving the gg at an angle, making it difficult to use a high-power loupe resting flat on the glass surface. The solution to this problem is to use your loupe perpendicular to the rays and not parallel to the ground glass. This is done by (in my case) turning the loupe over (or removing the barrell) so that there is nothing between the actual lens in the loupe and the gg. Then one can easily find the proper angle and distance from the gg for proper focussing, even in the corners. I use an 8x loupe and have no problems with this method. If the concentric circles of the frenel lens or the texture of the ground glass are/is in focus then you are at the right distance. The proper angle is found by searc hing off-axis until you find the brightest image. Really a lot easier to do than to describe. As for the proper alignment and relationship of frenel and gg I quote Ron Wisner from an article I snatched from the Veiw Camera Magazine site:

"One subject which comes up often is the correct position of the fresnel lens. I n years past, several arrangements have been used, including placing the fresnel behind the ground glass, in front of the ground glass, and incorporated into a ground glass made of plastic. However, there is only one correct arrangement. Th e ground glass surface should face the camera lens, and the fresnel is placed be hind it, on the outside of the camera, facing the photographer. The textured sur face of the fresnel should be placed down, against the ground glass.

There is one particular reason for this arrangement. In manufacturing cameras an d film holders, one overriding concern is the correct position of the focal surf aces of the respective parts. In the film holder this would be the position of t he septum, against which the film rests, and in the camera this would be the pos ition of the diffusion surface, or ground side of the ground glass. Nothing is m ore important than the proper registration of these two elements.

If the fresnel were placed in front of the ground glass, interposed between the lens and the ground glass surface, a lack of registration can occur. This can be explained by considering the effect of a parallel sided glass plate such as a g lass filter on a beam of light. Rays passing through such a plate are displaced by about one third the thickness of the plate, and depending upon the angle when passing through the plate, will be displaced laterally as well.

Furthermore, the greater the angle, such as in a wide angle lens, the greater th e effect, resulting in an apparent curvature of the image, when no such real cur vature exists. The result will be erroneous focusing of the edges of the image."

Testing to make sure your gg and film are in approximately the same plane and pr operly aligned, coupled with the correct frenel/gg arrangement should get get on e close enough for anything but the most critical work. Also, measure ALL of your film holder, especially if you have a motley collectio n of old and new, wooden and plastic such as I do and discard those that are off . Hope this all helps. Regards ;^D>

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