GG brightness with rear tilts

[cowanw]

I picked up a copy of a neat little book, this summer, in one of the many used books stores on Vancouver Island. It is Corrective Photography by Lewis Kellsey, put out by Deardorff (and signed by Merle S. Deardorff to John Palmer, who may be John E. Palmer, a photographer of Afro-Americans (principally) in Galveston, Texas in the 1940-1960 peiod).
Anyway, it got me working systematically to understand camera movements, and my question here is the degree to which the GG darkens as I tilt or swing the back.
It is a darkening that starts immediately with tilt and progresses as the degree of tilt progresses. This is not the question of compensation for differing reproduction ratios seen with extreme tilt/swing.
The lens is not obstructed, nor limited in movements. I use a Cambo (4x5 or 8x10 with 4x5 reducing back). Removong the back I can see the round lens in all 4 corners.
I understand I may be over tilting from usual practicality, but I have not read any reference to the back GG getting so dark with tilt/swing.
Is it normal for GG brightness to diminish progressively with rear tilt/swing?
Thanks
Bill

[CP Goerz]

Do you have a fresnel in place?

[Bob Salomon]

Is your eye/loupe staying on the optical axis as you do the movements? And make sure you have a fresnel to get the mosy even coverage.

[cowanw]

yes and yes to both. Does every body else's GG stay just as bright with tilt/swing as at the zero position?
Thanks
Bill

[Nathan Potter]

yes and yes to both. Does every body else's GG stay just as bright with tilt/swing as at the zero position?
Thanks
Bill

No, with rear standard tilt the screen brightness diminishes very slightly when looking at the optical axis thru a loupe. The more you tilt the rear standard the more the brightness diminishes. It is analogous to the diminution of intensity you see as you move a loupe away from the optical axis toward the corners of the film while keeping said loupe orthogonal to the screen.

The phenomena simply has to do with the angular scattering of light from the screen and is quite dependent on the structure of the screen grind. At small normal tilts, say 5 degrees or less, the effect should be hardly noticeable with a good bright screen.

Nate Potter, Austin TX.

[cowanw]

Your suggestion of 5 degrees is probably my issue. I put the exposure to the test with a Horseman meter and at 25 degrees of swing I lost 1/3 of a stop. At 5 degrees nothing happened to the needle reading.
I was trying to get a book placed at 45 degrees to the lens axis to come around parallel using the back swing; of course I was using a lot of swing to do that. Clearly the commercial photographers don't put the product at 45 degrees.
Still its amazing to see such a darkening to the eye with no real change in exposure. What you see may not be what you get.
Thanks for your help, all
Bill

[Bob Salomon]

Your suggestion of 5 degrees is probably my issue. I put the exposure to the test with a Horseman meter and at 25 degrees of swing I lost 1/3 of a stop. At 5 degrees nothing happened to the needle reading.
I was trying to get a book placed at 45 degrees to the lens axis to come around parallel using the back swing; of course I was using a lot of swing to do that. Clearly the commercial photographers don't put the product at 45 degrees.
Still its amazing to see such a darkening to the eye with no real change in exposure. What you see may not be what you get.
Thanks for your help, all
Bill

Was the meter cell flat on the gg for both readings?