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

[bill_glickman]

There is a very good article in May/June 99, Photo Techniques magazine about this 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 of film. He thought enlargements were suppose to get blurry becasue he had nothing to compare them to. I have been hearing this from many LF photographers, many of them using new equipment!

 

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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 back to the ground glass and then insert a film holder loaded with film, and measure 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 acceptable? 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 of 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 glass? Any input would be appreciated. Thank you. .

[alan_gibson]

The tolerance is 0.007 inches, which happens to be the typical film

thickness. Take the fresnel out before measuring.

 

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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.

[andrea_milano]

Dear Bill and other contributors,

Yes it is a big problem and indeed an underestimated one even for

camera builders. The procedure which is described in the question

would be the standard procedure accepted by most, even though, like

some other threads have been saying earlyer, it is prone to give

erratic results given the poor constuction qualities of most cameras

in terms of micrometric precision. However, when I bought my Wista VX,

I disliked immensely its ground glass. Like in many cameras, critic

focussing would be difficult if not impossible because Wista

integrates a groundglass-fresnel in one (plastic) item covered by a

clear glass with a grid.

When using a loupe with an eccessive magnification (7x) you just

cannot focus.A 4x helps but not much. Let alone the fact that, due to

the fresnel lens you can only view the image on the groung glass with

one eye opened and in a certain position at a certain distance from

the camera. Wista doesn't think that this is a problem because most of

the customers do not complain, I did.

Back to the original problem.

To solve the problem, I ordered a Bosscreen 4"x5"( a kind of

brightscreen wich is made from two pieces of glass with in between a

very thin layer of bee-wax and paraffine, very , very, fine

structure!) for my Wista. Well first I got a ground glass which was

too large for the camera then one which was resting higher than the

focal plane.

When asking if they had ever made any focal plane measurements they

answered that these had been made at some stage in the past from the

firm's previous owner.

I didn't get the impression that the guy knew what I was talking

about, anyway, the second time the glass was resting too far from the

focal plane and to prove it I set off to device an empiric demostation

of this fact( he didn't get the theoretical aspects of the matter!).

You take a ruler which has measures in clearly visible numbers. You

want to use your longest focal length lens and the one with the

largest aperture too.Place the camera on a tripod, fully extend the

bellows and mount your lens, place the ruler in front of

the camera at a suitable distance so that you can read on your ground

glass a certain amount of numbers (obviously your camera, or rather

its standards, front and rear, need to be at a 90 degree angle in

relation to the position of the ruler). Try to make shure that you are

focussing (precisely) say at the number 80 of the ruler (the real

distance is irrelevant) by moving the ruler to and fro the camera.

Once you are sure to see a measure clearly, shoot a Polaroid 55 or a

film or even better both( so you can check if film and polaroid are on

the same focal plane) at the largest aperture of your lens. When

developing the film and or the Polaroid 55 you will see if the

focussing of the film plane corrisponds to the image that you saw on

your ground glass. If numbers which where closer to the camera appear

sharper than the chosen measure, it means that the glass is too far at

the back of the focal plane(!!!!) and the reverse for numbers which

are further away. The solution in this last case might be spacers.

Bosscreen acknowledged my test and modified the glass accordingly,

however, being very critical about focussing amounts to a theoretical

problem, in most cases because, and you can verify this yourself, the

focussing capabilities of your eyes are erratic and are due to

different circumstances. Try focussing several times the same object

set nearby your camera, if you mark the distances of the standards on

your camera, you rarely will come to the same distance (in tenths of

millimiter) twice and your ground glass will never be much more

off-set than this little! It makes sense to go to extremes for extreme

photography (very close-up still life, microphotography,

photogrammetric measurements) but than you would want to buy a

specially deviced back with integrated ground glass and film holders

like Sinar or Linhof produce for exeptionally extravagant prices!

So, at times, it can be a problem but if not an evident cock-up, like

mounting the frenel lens in a inverted position with the groung

glass(!) this small differences shouldn't be a real problem. However,

to answer you question, some builders put fresnel first (seing the

camera from the back) and then ground glass and some do exactly the

opposite! So ask the ones who built your camera or else do my empiric

test.

I am not an expert like some other contributors are, so, if I said any

wrong things please forgive me! I do not intend engaging in one of

those internet fights which all too often appear on this forum! I am

prepared to accept, without further comments, any corrections or

contributions. I'd like to remind all of you of the existence of the

World Field Photographers Association (WFPA) and once again put its

URL to your attention. http://www.johndesq.com have a look an join in

it is fun and it is free!

regards

 

[bill_glickman]

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!

 

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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!

[george_nedleman]

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.

[jon_grepstad1]

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".

 

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The ANSI standard for 5 x 7 and 8 x 10 inch film holders are as

follows (film thickness has

not been deducted):

 

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5 x 7"

0.228" + - 0.010

 

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8 x 10"

0.260" + - 0.016

 

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See my View Camera Construction FAQ at:

http://home.sol.no/~gjon/lffaq.htm

[howard_slavitt]

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.

[alan_gibson]

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?

 

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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.

 

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

 

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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.

 

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<pre>

Depth of Focus = 2.C.N'

</pre>

 

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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.

 

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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.

 

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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 <i>Field</i>, and this will reduce the Depth of

<i>Focus</i> available to you.

[bill_glickman]

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!

 

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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!

[doremus_scudder1]

Bill,

Let me add my (long) 2 cents worth for you and for the benefit of others following this thread that might have the same problem. I own a Wista with the frenel/plastic one piece unit that you describe and am aware of the focussing problems you describe. Depending on the focal length of the lens, the light leaving the ground 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 focal length is optimal). Therefore, extreme wide-angle and long lenses will result 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 searching 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:

 

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"One subject which comes up often is the correct position of the fresnel lens. In 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. The ground glass surface should face the camera lens, and the fresnel is placed behind it, on the outside of the camera, facing the photographer. The textured surface of the fresnel should be placed down, against the ground glass.

 

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

 

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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 glass 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.

 

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

 

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Testing to make sure your gg and film are in approximately the same plane and properly aligned, coupled with the correct frenel/gg arrangement should get get one close enough for anything but the most critical work.

Also, measure ALL of your film holder, especially if you have a motley collection of old and new, wooden and plastic such as I do and discard those that are off. Hope this all helps. Regards ;^D>

[bill_glickman]

After applying the Depth of Focus formula, I came to the

conclusion that my film holders / ground glass alignment was not my

problem. To test this, I devised the following test that worked out

great. Using this test method, it can determine how much out of

alignment your ground glass / film holder is. Here is how I tested

it.

 

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I typed up on a 8./ x 11" piece of paper on landscape

orientation the numbers 1 accross the top line, 2 across the second

line, etc. all the way down to the bottom, line 63. The size font

you use will need to be experimented with based on the lens focal

length and the format size. The goal is to get the paper to take up

the full ground glass left to right. Put the test paper on a tilted

typing stand or lean against a wall using a straight supporting piece

like glass. Change the tilt angle so that only one line is in

focus. (you can use 2 lines, but for the purpose of this discussion

I will use 1) You note that line 32 is in focus, and lines 31 & 33

are partially in focus, and lines 30 & 34 are not in focus. Of

course you need to use at least a 4x loupe on the ground glass to

determine this. Then, take your picture and look and examine the

results. If the lines of exact focus are off, determine exactly how

many lines off they are. Say, one line off. By knowing the tilt

angle of the test paper you can determine with a little bit of

geometry the exact distance apart each line is from the lens. This

will give you a good indication of how far off, and in which

direction your alignment is.

[ellis_vener_photography]

Dear Bill,

Maybe you should go over to Phoenix and measure an Arca Swiss or Sinar P camera and see how those numbers compare to your Toyo cameras.

[bruce_curtis]

<p>Hi, this response may be 9 to 10 years late, but a word of caution in reguards to the quote from Ron Wisner about fresnel placement verses correct and incorrect. If you are an owner/user of a Pacemaker Graphic, they always came from the factory with the fresnel mounted on the forward or lens side of the ground glass, and that is the correct position. If you have one with the fresnel mounted to the rear (outermost position away from the lens) this is not correct. Unfortunately they used the same part # for the panels made in the late 40's and very early 50's that came without fresnels and on the optional (then later standard) panels that came from the factory with fresnels. This can lead to some confusion. Their fresnels were 0.070" +- 0.001". When they milled the portion that the fresnels sat on(the GG above it) they machined it 0.070' deeper than the older ones that had no fresnels. The two panels look identical, except for the fact that the surfaces that the fresnel or just plain gg set on are machined differently. If you have a Crown or Speed Pacemaker that is way off in focus check these distances. If you put a fresnel or have one in a panel designed just to have gg, the the gg will be 0.070" further away than it should be. The reverse problem also is a previous owner may have removed a fresnel from a panel that was designed to have one (from the factory), and you will end up with a gg that is 0.070" closer to the lens that it should be. When in doubt measure with a depth gauge. If you don't have a good quality depth gauge, most good quality vernier calipers (starret, etc) have a square rod that protrudes from the opposite end of the dial as you open the caliper. This is a quick and dirty way to measure depth, as long as you keep the end square with the focus panels surface that touches the back of the camera. I've worked on so many Graphics I can easily tell if they are off 0.070". I always check if I'm unsure. Same with all my other view cameras.</p>

[jon_grepstad1]

<p>Please note that in October 2015 my webpages on building large format cameras move to: jongrepstad.com</p>

<p>Jon Grepstad</p>