View Full Version : optically calibrating focus

30-Apr-2016, 19:35
You also need to carefully and OPTICALLY calibrate the alignment between your groundglass (gg) and the film plane. You do this optically using the technique described by Robert Zeichner in View Camera a few years back. This is the only way to properly take fresnel screens into account.*

I found the above at http://www.largeformatphotography.info/roundup2x3.html.

I always calibrate ground glass by physically measuring the depth of film holders and making sure the ground glass is the same depth in it's frame. I admit I don't know how to deal with fresnels except test shots which can be expensive

Is the method mentioned above different or special? Anyone have a copy of the article referenced?

Doremus Scudder
1-May-2016, 01:36
First of all, as far as I know, installing a Fresnel lens behind the ground glass, that is, between ground glass and the viewers eye, does not shift focus at all, just redirects and brightens the image. In this case, it is projected onto the frosted surface of the ground glass, which should be in the same position as the film.

Some cameras were designed for the Fresnel screen to be installed between ground glass and lens. In most cases, the manufacturer has taken the focus shift introduced by the Fresnel into account when designing the position for the focusing screen. As with anything positioned between lens and image-forming surface, a Fresnel here will shift the focus about 1/3 of its thickness. (Often the problem in this case is when the original Fresnel/gg sandwich has been replaced with a simple ground glass and the viewing screen is hence in the wrong position and must be shimmed out to compensate.)

If you're ground glass is correctly positioned and your negatives are sharp, then I wouldn't worry about needing to reposition anything. If you add a Fresnel behind your ground glass, all should be well.

If you intend to install a Fresnel screen between ground glass and lens, be aware that unless you have one of the cameras that was designed to accept this option (i.e., it would have a recess for the Fresnel, and you would have to make sure you had a factory Fresnel or at least one that met the factory specs regarding thickness and focal length) you will have to make modifications to your camera back to allow the Fresnel screen to sit closer to the lens. A Fresnel screen between lens and ground glass needs to be about 1/3 of its thickness closer to the lens than a correctly-positioned ground glass does due to focus shift. Most cameras don't have allowance for this.

If you're not sure your ground glass is positioned properly, the optical test is easy (and should work for positioning a Fresnel/gg sandwich as well): set up a shot with a ruler at an oblique angle to the camera back. Focus wide open with a longish lens on a particular marking on the ruler; everything else should be out of focus. Shoot wide open and see if your focus on the film corresponds to that on the ground glass. If not, shim or remove shims as needed. Check this with a wide-open shot of the horizon with lots of detail (I like pine trees on mountaintops) to make sure your infinity focus is correct. If you need to reposition a ground glass for some reason, it may take a few iterations of this to get it exactly right.

If you are good with machinist's tools, measuring works just fine for positioning the ground glass. The problem for amateurs is that they often don't know how to properly use a micrometer or calipers and end up with too much error in their measurements. Since you are an experienced camera builder, I assume you know how to measure properly, so you won't need the optical test for much except maybe as a control. For positioning a Fresnel, you would have to calculate the amount of focus shift and allow for that in your measurements. I don't know how to do this, but someone here likely does.

FWIW, I do the optical check on all my cameras periodically when I have my darkroom set up for printing. I shoot a sheet of film for each camera, then just toss the negs into the print developer for 3-4 minutes, stop, fix, rinse and then check the focus on the still-wet neg. Much faster than film developing.