Home of Professional Cinematography since 1996

Camera Ground Glass vs Reality

Published : 8th December 2003


More often than not in cinemas I can see projected image on the black borders of the screen. My question is how accurate are the camera GG markings to what is seen on the cinema screen.

Television GG standard have a TV safe marked where you know picture information enters the twilight zone and may or may not be seen at home.

Should GG marked up for a theatrical release do the same ? I suspect most cinema projection set ups cut off more picture than a good modern TV set does.
Is there a standard for cinemas and is that reflected in camera GG's ?

Many Thanks
Tom Gleeson
www.cinematography.net



Tom Gleeson wrote:

>More often than not in cinemas I can see projected image on the black >borders of the screen.

Note that most theatre aperture plates are hand-filed to deal with things like keystoning. There's also going to likely be some mismatch between screen size/lens focal length/throw.

Jeff Kreines



Tom Gleeson wrote:

>"More often than not in cinemas I can see projected image on the black >borders of the screen."

This is not a camera problem, but problems related to poor projection, specifically :

(1) The projector lens is "off centre" (the eccentric adjustments are off, usually due to slamming the turret shut...


(2) Aperture plate in the projector is out of adjustment...


(3) or the black border (called masking) around the screen is not in its correct position. The accepted procedure is to allow only a hand's width of image shining on the masking. More than that requires the engineer to fix the problem.

>"My question is how accurate are the camera GG markings to what is >seen on the cinema screen."

In the theatres which have been built in the last 10 years, quite good.

Older theatres can have severely cropped images due to not sizing the lens to the auditorium. Contact me off list if you wish more information. Projection set-ups I work with have no more than ten thousands of an inch maximum crop side-to-side and top-to-bottom.

>"I suspect most cinema projection set ups cut off more picture than a >good modern TV set does.

The SMPTE recommended practices suggest that the projector aperture be no wider than .825 of an inch horizontally--for both 'scope and flat (anamorphic or 1:1.85) Allowing no more than a .010 inch crop all the way around the aperture is considered good practice.

Jeff Kreines wrote:

>"Note that most theatre aperture plates are hand-filed to deal with things >like keystoning. There's also going to likely be some mismatch >between screen size/lens focal length/throw."

Very true, and also to compensate for screen curvature, especially in modern auditoriums which are wider than the projection throw. The focal length of the projector lens is quite short, thereby requiring some fancy filing!

>"How about burning a hard matte (can be done easily in the IP stage) >onto the first 100 feet of each reel? Or the whole film? Of course it won't >happen, as there's no money to be made with proper framing!"

Would you believe one distributor (studio) does it for all their films? A few year back they were so apologetic because they cut one shot into the finished IP which did not have the printed-in hard matte. Also, most 1:1.85 trailers have the matte since more than 90 percent of trailers are done on the computer from scanned negative or print material. With anamorphic prints, the lack of a matte is no problem since all of the image is intended to be projected, but it really is needed on 1:1.85 prints.

Projectionist really don't have a sense of correct composition!

Wade I. Ramsey (the other Wade Ramsey)
Dailies Projectionist / Projection Engineer
Formerly Camera Assistant
Formerly Film Editor
Fort Mill, SC