Ground Glass- which way round?
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!
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.
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.
Originally Posted by Adrian D
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.
Many thanks, next free moment I'll approach it with some precision measuring tools!
Appreciate your guidance
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?
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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.
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)....
Something sounds wrong, the ground glass reversed and spacers on the spring back.
You definitely need to check the film plane / glass register.
Originally Posted by Jim Jones
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?
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.