Groundglass vs. Image Focus Disparity

[mhermann]

Greetings, all -

I'm shooting with a lovely pre-Anniversary 4x5 Speed Graphic with TXP320, loaded into a Graflex Film Magazine. The camera, of course, is a Graflex back, with removable groundglass. The lip and slider on the back are steady; the groundglass is correctly oriented, ground-side toward the lens, there's no fresnel addition. The sheaths in the film magazine are flat. I was doing some vintage-styled portraiture this weekend, focusing on the groundglass, using a loupe, making sure the eyes were tack-sharp. I was wide-open on the aperture, and using a somewhat long guesstimated exposure (about a second +/- for the most part.) There is a little bit of motion blur, but, more egregious than that is the fact that the eyes are not in focus. Rather, on the images, the focus appears to be several inches forward of the eye-plane, on the model's shoulder seam.

Very discouraging, particularly as I don't know any of the standard go-to troubleshooting tips apply here. Any ideas, or am I just going to have to do the "angled fence post" test shot, and manually compensate when I focus with this arrangement? Is there a possibility that a glass plate negative is going to have a different plane than a sheet of film, all other things being equal?

Thanks!
Marc
225918

[Tin Can]

The left eye is sharper and perhaps closer to the shoulder

Try again on still life

[BrianShaw]

What aperture did you use? With some subjects there is an unconscious tendency to move during the LF process. I’ve never been ableto photograph my wife with either MF or LF.

[Vaughn]

A quick test probably is in order. Focus on a distance up a ruler and photograph it wide-open, and check where the focus is on the negative.
If you have a micrometer, measure the distance in towards the film on your film holder and compare that distance with your plate holder. Then check that distance on your camera. Might need a shim or two to position the GG properly.

The standard distance should be posted somewhere on this site.

[nolindan]

If this is a problem with a new camera and not a suddenly appearing problem on an old camera then may I suggest that the camera is designed for a Fresnel screen. Without the Fresnel the ground glass will be closer to the lens than the film will be. As the film is now farther away than it should be the camera is focused for a spot closer to the camera.

There are two styles of film-back casting - one for cameras with Fresnel screens and one for cameras without Fresnel screens. If you have a Fresnel-style back with no Fresnel screen then you can either purchase an after market Fresnel or shim the ground glass back by the thickness of the Fresnel.

You will have to Google for more and wade through an awful lot of verbiage on the matter.

[maltfalc]

easy enough to measure the depth of your film and the gg to see if they match.


If this is a problem with a new camera and not a suddenly appearing problem on an old camera then may I suggest that the camera is designed for a Fresnel screen. Without the Fresnel the ground glass will be closer to the lens than the film will be. As the film is now farther away than it should be the camera is focused for a spot closer to the camera.

There are two styles of film-back casting - one for cameras with Fresnel screens and one for cameras without Fresnel screens. If you have a Fresnel-style back with no Fresnel screen then you can either purchase an after market Fresnel or shim the ground glass back by the thickness of the Fresnel.

You will have to Google for more and wade through an awful lot of verbiage on the matter.

graflex back, not graflok. too old to have a fresnel.

[Joe O'Hara]

A quick test probably is in order. Focus on a distance up a ruler and photograph it wide-open, and check where the focus is on the negative.
If you have a micrometer, measure the distance in towards the film on your film holder and compare that distance with your plate holder. Then check that distance on your camera. Might need a shim or two to position the GG properly.

The standard distance should be posted somewhere on this site.

If you arrange for the ruler to be at a shallow angle to the lens axis, and make a mark on it in the middle and set the focus there, it will be obvious whether your groundglass and your film holder "disagree", and in which direction (if they do, the groundglass is wrong). The idea is that one end of the ruler is closer to the lens than the other, and you focus at a point in the middle. If the point of best focus is not at your mark, you have a problem.

My old Wista was off by 0.01 inch before Richard Ritter fixed it. People thought I'd bought all new lenses after that.

[ic-racer]

I'd hope any camera technician would be able to fix this for you, it is a very basic repair.

[mhermann]

Thanks, all. Took the micrometer to it. The distance from a flat anchor point on the front element to a sheet of film in the film magazine or a glass plate in a standard slotted holder is 40.6mm, +/- .02mm. However, the distance to the groundglass side of the focusing screen when it's in place is 39.7mm. So, the next thing is to try fashioning a 1mm thick shim to, say, sit around the inner perimeter of the removable focusing screen element? Recommended materials for this? I hope it has enough wiggle room.

[nolindan]

So, the next thing is to try fashioning a 1mm thick shim to, say, sit around the inner perimeter of the removable focusing screen element? Recommended materials for this?

1mm thick cardstock: back of a pad of paper; carton from a box of Wheaties ... I'd just wander around the house with a micrometer.

[Chuck Pere]

Thanks, all. Took the micrometer to it. The distance from a flat anchor point on the front element to a sheet of film in the film magazine or a glass plate in a standard slotted holder is 40.6mm, +/- .02mm. However, the distance to the groundglass side of the focusing screen when it's in place is 39.7mm. So, the next thing is to try fashioning a 1mm thick shim to, say, sit around the inner perimeter of the removable focusing screen element? Recommended materials for this? I hope it has enough wiggle room.

Have you seen this tread: https://www.largeformatphotography.info/forum/showthread.php?168032-Smarter-material-for-ground-glass-shim

[mhermann]

Trying to plan where, exactly, the shim would go. It doesn't seem like there's 1mm of "wiggle room" were I to pad out the entire GG holder element. I don't want to put strain on or deform the rails. And, if I did anything with the actual piece of glass, the side clips and microscrews that hold them in (and grip the original side-curtains of the flip-up hood) wouldn't function—nor would the door be able to close and latch.

[maltfalc]

Have you seen this tread: https://www.largeformatphotography.info/forum/showthread.php?168032-Smarter-material-for-ground-glass-shim

totally different back, so that thread's full of advice that wouldn't work.

[r_a_feldman]

So, the next thing is to try fashioning a 1mm thick shim to, say, sit around the inner perimeter of the removable focusing screen element? Recommended materials for this? I hope it has enough wiggle room.

Go to a hobby shop or Blicks art supply store and get some 0.040”-thick styrene strip of the needed width. Plastruct is one common brand.