Ground Glass- which way round?

Discussion in 'Large Format Cameras and Accessories' started by Adrian D, May 12, 2008.

  1. Adrian D

    Adrian D Member

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    I've recently acquired a Century 5x7 view camera, and in the process of getting it ready for use, decided to take out the ground glass screen to clean off some dust and accumulated grime.

    I noticed that the screen had been mounted with the ground surface facing toward the photographer, and the smooth side of the glass facing inside the camera.

    After a successful clean (a rubber eraser gets rid of unwanted pencil-drawn grid marks- I prefer a clear uncluttered image on the ground glass) and a gentle wash in warm soapy water, I am now unsure as to which way to re-install it in the camera!

    All previous LF cameras I have used have had the ground surface facing the photographer, so I'm wondering:

    a) does it really matter?

    b) perhaps the previous user of the camera mounted it the wrong way round by mistake?

    c) as the camera has been modified with two aluminium strips along the length of the springback (to shift focus back a couple of mm to account for the thickness of modern darkslides, according to the previous user) perhaps reversing the GG was part of this modification?

    Your experienced thoughts would be much appreciated!
     
  2. matt miller

    matt miller Subscriber

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    Usually the ground side faces the lens. You might have an answer in item C though. You should definitely take some measurements and see what's right.
     
  3. resummerfield

    resummerfield Subscriber

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    Normally the “ground” side of the ground glass is toward the lens, as you mention. But in any event, the “ground” side of the ground glass must be at the same location as the film in the holder. Since this camera has been modified, it is possible that the previous owner has reversed the ground glass.

    I would compare some measurements. Remove the camera back. Put a sheet of film in your holder, and insert the holder in the camera back and measure the distance from some fixed point or plane to the film. Then remove the film holder and measure from the same point to the ground glass. The measurement to the ground side should be the same.

    There is a good example of how to do this in the book “Way Beyond Monochrome” by Ralph Lambrecht and Chris Woodhouse.

    Finally, check everything with some test shots.
     
  4. Adrian D

    Adrian D Member

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    Many thanks, next free moment I'll approach it with some precision measuring tools!

    Appreciate your guidance
    Adrian
     
  5. Ray Heath

    Ray Heath Member

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    g'day Adrian

    i would imagine the ground surface is toward the lens so that the light rays hit the glass and, well i don't actually know, do whatever it is they do to create an image on that surface

    as for a modification needing the glass to be reversed so as to be at the same distance as the film plane, wouldn't that only be a matter of a couple of millimeters? would it even matter? wouldn't depth of focus keep the image sharp?

    Ray
     
  6. Jim Jones

    Jim Jones Subscriber

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    Ray, it would matter. A focus error of 2mm with an aperture of f/16 results in a blur of .125mm, which would be visible on the negative. A decent lens on a 5x7 should resolve detail several times finer than that. Enlarging the negative would only make it worse.
     
  7. keithwms

    keithwms Member

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    Test registry with polaroid or Fuji instant on a good, wide open, fast lens at close focus (as close to full bellows extension as you can arrange)....
     
  8. Ian Grant

    Ian Grant Subscriber

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    Something sounds wrong, the ground glass reversed and spacers on the spring back.

    You definitely need to check the film plane / glass register.

    Ian
     
  9. Ray Heath

    Ray Heath Member

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    thnx Jim

    could you please expand on the .125mm?

    also, relating to my previous question, could you explain depth of focus? isn't it somewhat like depth of field?

    Ray
     
  10. RobC

    RobC Member

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    firstly, advice from keithwms is wrong. The depth of focus is smallest at infinity. At close focus such as 1:1 the depth of focus in creases and is therefore less critical than at infinity.
    Secondly, if the back has been modified so that the ground side is facing the photographer, then measuring using the ususal depth micrometer won't work because the GG will defract the the light before it hits the ground surface. i.e. the GG acts as a lens with focus shift which will be different in the centre than at the edges. And since a lens can be quite close to the GG the effect can be quite marked which is why the ground surface should be facing the lens.

    What I would do is to first measure the depth of your films holders with film in them. That is the depth you want the ground side of GG to be at. replace the GG with ground side facing the lens. Then measure its depth. if it is deeper than your film holders, then you need to remove some of that aluminium strip shimming. If it is shallower then you need to add to that aluminium shimming. But either way, the ground surface should be facing the lens.
    And when you do film test, focus on inifinity as that will show focus error far more than close focus will.
     
  11. keithwms

    keithwms Member

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    Ah I was thinking of depth of field, sorry, yes, do the test at infinity focus. Note to self: never offer advice before the morning coffee break.

    Anyway a wide open lens should be fast enough for a good test.
     
  12. Murray@uptowngallery

    Murray@uptowngallery Member

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    REfract , not defract, (rab knows, but R is really close to D on the keyboard) between the non-ground side facing lens and the GG imaging surface.

    The thickness of the glass blank between the lens and the groung glass image shifts the focal point further (longer) by the an amount = glass thickness/glass index of refraction.
     
  13. Steve Smith

    Steve Smith Subscriber

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    My method of checking is to palce a scrap piece of film into a film holder and place it in the camera. With a lens board but no lens fitted, measure the distance from the emulsion of the film to the front of the lens board using a steel rule.
    Then remove the film holder and take the same measurement - this time it's from the inside face of the ground glass to the front of the lens board. They should be the same.
    If you're worried about scratching the ground glass, put a few strips of tape over the end of the rule (before you take the first measurement).


    Steve.
     
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  15. RobC

    RobC Member

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    Yes but there is no refraction in the center where the light rays are perpendicular and there is increasing refraction towards the edges where the light rays are at an increasing angle to the GG. So if you wanted to critical focus a wall, then if the center was infocus the corners wouldn't be and visa versa.

    (Murray knows this but I just want to make it clear for the OP so he goes with the ground side facing the lens)
     
  16. RobC

    RobC Member

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    Yes its just like depth depth of field except it happens either side of the film plane. If you imagine an X where the left side of X is the light rays from a point in the subject. The center of the X is where the film should be and the right side is where the light rays would project behind the film.
    Depth of focus is the distance from the left edge of X to right side of the X where the height of the X is the maximum acceptable circle of confusion (coc) or airy disc as some like to call it. If the film is shifted left or right in relation to where the Ground glass surface is then it shifts the focus plane of the subject which is also shifting the near and far depth of field points.
    I'm not good on the maths for this but getting your GG surface distance correct removes any doubt about it being the cause of focus problems.
     
  17. Ray Heath

    Ray Heath Member

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    this is all very interestingt guys, but does it matter in everyday use

    can it be accurately measured?

    Steve suggests a piece of tape over the end of the ruler, wouldn't that throw off the measurement if hundredths of a mm is the kind of effect you are allowing for?

    actually, it's not even possible to measure to that accuracy with a steel rule

    so, if the film plane and gg plane were different by up to say 3mm, would it matter, would it be noticable on the resulting print?

    what about the effect of the film bowing out in it's holder?

    Ray
     
  18. RobC

    RobC Member

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    Yes it can be measured with a simple depth micrometer. 3mm is a very large amount. If you imagine a 35mm slr which has film under tension with a pressure plate, the margin of error in film plane placement is minimal. The same is true for medium format cameras. You will often hear large format photographers tell you the jump in quality from 35mm to medium format is much bigger than the jump from medium format to large format. Yet the 35m format size is 864mm², 6x6 is 3136mm² and 4x5 is 10800mm² or thereabouts. So 6x6 is approx 3.6 times the area of 35mm and 4x5 is approx 3.4 times the area of 6x6. The increase in size to 4x5 is similar to the increase in size to 6x6 from 35mm so why is the jump to medium format bigger than jump to 4x5?
    The answer is because obtaining film flatness and correct film plane registration on 4x5 or larger cameras is very difficult to achieve and if you don't achieve it, then you are throwing away the benfit of 4x5.
    Applying tilts and swings makes it even more difficult and requires extreme precise focussing and film plane registration otherwise you are better off with medium format or larger than 4x5.
    So to answer your question, 3mm is a massive amount, especially if you have for example, applied some tilt to obatain near to far sharpness. If the film plane is out by 3mm, what you thought was going to be sharp near to far will all be unsharp. View camera movements also narrow the depth of field so there is less margin for error. How much error is acceptable? You aim for zero error. If you don't aim for zero then you are wasting your time with largeformat.
     
  19. Jim Jones

    Jim Jones Subscriber

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    Sorry, I'm not good at explaining what I know, and often not good at knowing what I try to explain. The cone of light at f/16 is 16x longer than the diameter of the cone. 1/16 of 2mm is .125mm.
     
  20. infest

    infest Inactive

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    Nothing is more dangerous than Americans trying to think in the metric system... :-> Wasn't a Mars lander lost because of a wrongly applied conversion?? Yes, 3mm is an awful distance for wrongly put gg. Definitely not a correct way to put the gg as you found it. The correct registration is always tricky and must be solved with the camera construction in mind. Which probably explains why you get different advice (some of it very good).
     
  21. Mark Sawyer

    Mark Sawyer Member

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    Steve's method works accurately, at least, as accurately as your eyes allow you to measure, and the tape won't matter if you have it on the ruler for both measurements. I'll add the following caveats:

    Use a long ruler, and have the other end resting on a reference point in front of the camera. A slight change in angle will through off your measurements.

    It doesn't matter what the distance from film plam/gg to lensboard is, since all you're checking is that the distance matches exactly. When I check my cameras, I usually adjust the extension to get an even reading, like 10 inches.

    And yes, the ground side should always face the film, as it is very susceptible to temporary marks (like oily fingerprints) and permanent marks (like scratches from loupes and any other contact with hard objects). Having the ground surface on the outside would insure a damaged surface that just gets worse and worse.

    Oh, and if the image is upside down, take the ground glass off and rotate it 180 degrees...
     
  22. infest

    infest Inactive

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    The last time I tried it, it didn't help.:sad: So I took the whole camera and put it on the tripod upside down - it works like charm!
     
  23. Ray Heath

    Ray Heath Member

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  24. infest

    infest Inactive

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  25. Adrian D

    Adrian D Member

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    Thanks all for your instructive, interesting and funny comments! I have ended up doing exactly what Steve suggests, and found that, somewhat ironically, when the GG is placed the correct way round, there is a mismatch of 3mm, yes, 3mm, between the GG surface and the actual film plane in a standard Fidelity holder.

    Its off to the hobby shop for me now, to find some brass or hardwood panel in exactly 3mm thickness... wish me luck!

    Thanks again and keep up the good work... this is a great website.

    Cheers
    Adrian
     
  26. df cardwell

    df cardwell Subscriber

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    Please post a picture of your camera
    An early Century might be made for different film holder specs than we use today.