Checking GG distance

[G Benaim]

Hi all,

I checked my GG to lb distance and compared it to the filmholder, and found the holder was 1mm deeper. I figured my back was built for a fresnel between the gg and lens, but wanted to double check here. You measure from the ground/inside part of the glass, right? Just thought I'd ask, thanks,

GB

[Ernest Purdum]

That's correct. Probably a silly question, but did you have the filmholder loaded?

[resummerfield]

When I started checking mine, I would get wildly erroneous reading because I did not use a precision measuring instrument and exact, repeatable procedures. Once my measuring skills improved, my readings became much more precise.

What method did you use to measure (corners only, center only, combination), and with what measuring instrument? Also, did you measure the film holder while it is inserted in the camera back? And did you account for the film thickness?

[Bob Gentile]

"... What method did you use to measure (corners only, center only, combination), and with what measuring instrument? Also, did you measure the film holder while it is inserted in the camera back? And did you account for the film thickness...?"

Interesting. How, exactly, does one accurately and consistently make this measurement?

[ic-racer]

I have found on my cameras with problems, the ground glass was off by about one or two thicknesses of electrician's tape. To test for this I take pictures of a distant scene (at 'infinity') with the lens wide open and I bracket the focusing (ie take some exposures a little in front and behind correct infinity focus on the ground glass. After processing the negatives, I inspect them with an 8x loupe. If the sharpest negative corresponds to the one where the scene was sharp on the ground glass, everything is OK. If the sharpest negative was obtained when focusing a little in front of the correct plane, then the ground glass needs to be shimmed toward the lens.

[KenM]

I initially measured my GG distance with a depth micrometer, but since it only had a base of 4 inches I could only measure the corners, and not the center (which is important, especially if you use a plastic screen which may be warped).

On the suggestion of a friend, I constructed a jig out of plexiglass, which had a hole drilled in it to hold one of these:

http://www.houseoftools.com/product.htm?pid=13065

Dial indicators are cheap, and accurate to 0.001" of an inch. The jig was about 3/4" thick and about 6 inches long. It was built up in the middle, where a hole was drilled to hold the dial indicator. Insert the dial into the jig, zero it, and then slide it around the GG. You'll be able to quickly determine the depth of the glass. Make sure that the probe doesn't protrude more than 0.2" below the base, else you may deflect the GG with the pressure of the probe.

Just a warning: if you do use a plastic GG, don't slide it around, as the GG is very fragile. I would just gently place the jig down at various points on the screen.

Once you know where the screen is out, you can make alterations.

You can also use the jig to test out your film holders. Remember that the film holder will be 0.007" deeper without the film.

Just an FYI: for 4x5, the GG should be set to 0.190". Film holders should be 0.197" (without film). GG depth is +/- 0.007".

[Bob Gentile]

Duh! And here I thought my dial indicator was only good for brake rotors, cam runout... Ha!

Thanks! Gotta try that.

[KenM]

Duh! And here I thought my dial indicator was only good for brake rotors, cam runout... Ha!

I'll post a picture tonite when I get home....it's a real simple thing to make, but describing a tool is much a kin to describing how to tie shoe laces In other words,a picture is worth a thousand words.

[Bob Gentile]

Ken,

Actually, I can visualize a simple jig from your description, but a pic would be nice.

Thanks!

[evan clarke]

Ken, this sounds brilliant..Evan Clarke