Critical height placement of ground glass

[mooresean68]

Hi All,

Looking for some help/advice in determining how critical the height of the ground glass is on the holder.

I know, I know it is critical, I get that seeing how it is where the film plane should be but here's what I'm getting at:

I have 2 5x7 agfa ansco universals now that are of similar age and on both the frame for the gg have sloped corners. The red outline is where the current gg sits and the teal outline I believe is where original gg sat matching more to the placement in the first agfa.

204829

So the first question is: Is this 2.5mm difference -that- critical, particularly when significantly stopped down?

And I'm assuming that the first image below is proper for placement on these agfa's where the glass flat on the frame vs raised like in the 2nd image.

Then my second question is if the plane of the gg is of critical placement why would they design sloped corners like this anyway? My first thought was to cut the gg so it would be of precise placement for that holder/camera combination but that seems a little over the top considering the time and age.


On the first of the two agfa's the ground glass sits flat in the frame (sorry it's a bit hard to see)

204831

The second the gg sits raised about 2.5mm higher as in this image

204833



Thanks
-Sean

[Jon Shiu]

yes, critical. just cut off the ground glass corners so the ground glass sits correctly.

[Oslolens]

I'm thinking reasons for sloped corner could be two-fold: easier or faster milling and stronger corner.

Sent fra min SM-G975F via Tapatalk

[Kiwi7475]

2.5 mm is quite a bit but it all depends on what you want to achieve. Depth of focus is given by the nearest and furthest points of apparent sharp focus in front of the lens. This depends on focal length, aperture used and the circle of confusion. In its most simplistic form, without significant magnification, the depth of focus can be approximated by: 2 * stop_factor * circle_of_confusion. If you accept a circle_of_confusion of say 0.1mm (as an example, everyone has an opinion about this), then at f11 you get ~ 2.2 mm. If you considered your error +/- then you would need to get to f22 to get 4.4 mm (+/- 2.2 mm), but this is not probably what you have. In other words, 2.5 mm could probably be acceptable if you want to shoot at f22 and smaller, if you accepted a 0.1mm circle of confusion. YMMV.

[Bob Salomon]

2.5 mm is quite a bit but it all depends on what you want to achieve. Depth of focus is given by the nearest and furthest points of apparent sharp focus in front of the lens. This depends on focal length, aperture used and the circle of confusion. In its most simplistic form, without significant magnification, the depth of focus can be approximated by: 2 * stop_factor * circle_of_confusion. If you accept a circle_of_confusion of say 0.1mm (as an example, everyone has an opinion about this), then at f11 you get ~ 2.2 mm. If you considered your error +/- then you would need to get to f22 to get 4.4 mm (+/- 2.2 mm), but this is not probably what you have. In other words, 2.5 mm could probably be acceptable if you want to shoot at f22 and smaller, if you accepted a 0.1mm circle of confusion. YMMV.

No.
Depth of focus is the area behind the lens where the image plane has to be located. The shorter the lens the narrower the depth of focus.
What you were describing is depth of field which is the area in apparently sharp focus in front and behind the point focused on in front of the lens. The wider the lens the more depth of field at a given aperture.

[Kiwi7475]

No. I am aware of the difference between depth of field and depth of focus. Am I and wikipedia both wrong??

https://en.wikipedia.org/wiki/Depth_of_focus

<<
Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera and is therefore sometimes referred to as "lens-to-film tolerance".
If the depth of focus relates to a single plane in object space, it can be calculated from

t=2 N c v/f
where t is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length. In most cases, the image distance (not to be confused with subject distance) is not easily determined; the depth of focus can also be given in terms of magnification m:
t=2 N c ( 1 + m)

The magnification depends on the focal length and the subject distance, and sometimes it can be difficult to estimate. When the magnification is small, the formula simplifies to t ≈ 2 N c.

The simple formula is often used as a guideline, as it is much easier to calculate, and in many cases, the difference from the exact formula is insignificant. Moreover, the simple formula will always err on the conservative side (i.e., depth of focus will always be greater than calculated).
>>

[Bob Salomon]

No. I am aware of the difference between depth of field and depth of focus. Am I and wikipedia both wrong??

https://en.wikipedia.org/wiki/Depth_of_focus

<<
Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera and is therefore sometimes referred to as "lens-to-film tolerance".
If the depth of focus relates to a single plane in object space, it can be calculated from

t=2 N c v/f
where t is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length. In most cases, the image distance (not to be confused with subject distance) is not easily determined; the depth of focus can also be given in terms of magnification m:
t=2 N c ( 1 + m)

The magnification depends on the focal length and the subject distance, and sometimes it can be difficult to estimate. When the magnification is small, the formula simplifies to t ≈ 2 N c.

The simple formula is often used as a guideline, as it is much easier to calculate, and in many cases, the difference from the exact formula is insignificant. Moreover, the simple formula will always err on the conservative side (i.e., depth of focus will always be greater than calculated).
>>

This is what you said:

“. . . Depth of focus is given by the nearest and furthest points of apparent sharp focus in front of the lens.. . .”

[grat]

Pardon my ignorance, but isn't the critical factor the location of the film plane? The film, and the ground glass, need to be on exactly the same plane for focus to remain consistent when you insert the film holder(s). How thick is the glass on the one that's raised versus the one that isn't? Does it have consistent focus when you take pictures? If so, that's what's relevant.

As for the sloped corners, it would allow precise control over the depth of the ground glass by trimming the corners, if you couldn't get glass at a consistent thickness, and would eliminate the need for any type of shim.

[Bob Salomon]

Pardon my ignorance, but isn't the critical factor the location of the film plane? The film, and the ground glass, need to be on exactly the same plane for focus to remain consistent when you insert the film holder(s). How thick is the glass on the one that's raised versus the one that isn't? Does it have consistent focus when you take pictures? If so, that's what's relevant.

As for the sloped corners, it would allow precise control over the depth of the ground glass by trimming the corners, if you couldn't get glass at a consistent thickness, and would eliminate the need for any type of shim.

The image plane, film or digital, lies within the depth of focus.

[ic-racer]

Red Box: Yes, looks like someone had the wrong ground glass on there at one time.
Blue Box: Still does not look correct. The glass should be laying flat and not making any marks in the wood. So, you might need to trim the corners of the glass even bigger like this one in the picture

204849

[Kiwi7475]

Pardon my ignorance, but isn't the critical factor the location of the film plane? The film, and the ground glass, need to be on exactly the same plane for focus to remain consistent when you insert the film holder(s). How thick is the glass on the one that's raised versus the one that isn't? Does it have consistent focus when you take pictures? If so, that's what's relevant.

As for the sloped corners, it would allow precise control over the depth of the ground glass by trimming the corners, if you couldn't get glass at a consistent thickness, and would eliminate the need for any type of shim.

Yes they should match, ideally. You focus to the ground glass and then put the film at that location. If you focus wrong because the ground glass is not aligned (e.g. you put a fresnel between the ground glass and the lens) or because the holders have issues (OP's problem) then you will turn a sharp dot into a circle, which is the issue of depth of focus.

[Kiwi7475]

This is what you said:

“. . . Depth of focus is given by the nearest and furthest points of apparent sharp focus in front of the lens.. . .”

And you're right that I wrote it wrong. My apologies. The math is still correct for OP's case, but that sentence explains depth of field, not focus, which is what I was trying to explain :-)

[PRJ]

The ground glass needs to sit at the same distance as the film from the back of the camera. Measure the depth of your holders with film, then the thickness of the frame where the ground glass sits. In other words, the frame thickness has to equal the depth of your holders. That will tell you if it is correct or not. You will obviously need calipers to do this. If the rail on the frame is correct then figure out where the rail gets thicker and clip the ground glass before it hits that point.

While you are at it, you might at well check to see if all your holders are the same...

[Bernice Loui]

If one is interested in producing in focus images on film based on the focused image on the ground glass, the diffusion side of the ground glass MUST be at the same mechanical dimension as the film in the film holder. This means the tolerances between diffusion side of the ground glass to film in film holder within thousands of an inch if properly done. The belief stopping down will fix it is not a good assumption as that is not the proper cure to achieve precise focus register to ground glass diffusion side to film sheet.

This requirement becomes more demanding with large aperture lenses and increasing sheet film size, a design requirement that is difficult for lightweight wood folding cameras to meet.


Bernice

[Len Middleton]

When I requested Richard Ritter to make replacement dark slides for my two 8x20 holders, he requested that I sent the camera back with them, as well.

As it turned out, Richard needed to shim the ground glass, as it was in the wrong position. His explanation was that when my camera was built (late 1920s, early 1930s), it was common practice to put the ground side of the GG on the outside of the camera, rather than on the inside (i.e. between the back and the lens).

Than could account for a significant difference, if that is the situation in your case.

Just hope I correctly interpreted Richard;s explanation...

[mooresean68]

Thanks for all the great responses everyone! Looks like I have a little bit of discovery and work to do.

[Nodda Duma]

Gonna skip all the math and just say you should place the ground glass within +/- .005” of the film plane in a loaded holder relative to a common reference surface such as the lens board.