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This topic comprises 2 pages: 1 2
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Topic: Curved Screen & Focus
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Steve Guttag
We forgot the crackers Gromit!!!
![](http://www.film-tech.com/ubbmembers/268e.gif)
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-13-2002 08:15 PM
Now I know I've covered this before...
Curved screens take many forms....
The shallowest curve that had/has wide use is the 1/20 curve that the Schneider program will have you build. It offers some improvement in focus but mostly allows a theatre to advertise "curved screens."
The next one would be the "scope screen" which had the curve radius equal the throw. This was done to help focus back when lenses were not as good...particularly when dealing with CinemaScope projection. Alot of those theatres were uses fast f/1.7 lenses to get as much light as possible (as compared to the f/2.0 lenses of today). Also the older lenses were not coated so you have lens flare adding into the apparant focus mix.
The next level of curved screens is the optimized light curve. This curve is designed using "ray-tracing" techniques to direct the reflected light over as much of the audience as possible. This screen is ALWAYS a gain screen since the gain screen's scatter is taken into account. As the screen gain goes up, the curve gets deeper. As the screen gets deeper the sweet spot for the audience gets narrower. As the theatre gets wider (ie the theatre is shaped like a square), the shallower the screen becomes and the sweet spot moves towards the rear of the theatre. Typically, the screen gain for such screens are 1.5 - 2.0. One can often drop an entire lamp size by using such a curved screen. Thus reducing heat and associated focus problems...furthermore one's electricity and lamp costs also are reduced.
The next category are the deep-curves. These are done for effect. Examples are the Cinerama, Cinemiricle, D-150 and Imax-Dome. In all instances, special lenses are required to project a "satisfactory" picture on one. If a lens designer knows what the geometry is of the screen and it's relationship to the projection aperture, they can design a lens optimized for the set up. ISCO makes "off-the-shelf" deep curve lenses and stand alone deep-curve adapters. They work well on Cinerama type screens. Another little tid-bit is to use a Magnacom when dealing with deep curves for 35mm "FLAT" films...it will match a Cinerama type screen VERY well.
The above mentioned deep-curve lenses and the Magna-com will show a notably distorted picture on flat screens and not focus as well as they do on deep-curves.
Modern prime lenses are typically f/2.0 now (give or take a tenth). They are also much superior at the efficientcy with light and depth of focus. As such, they will focus on a pretty wide range of depths.
As someone already noted, you can stop down a lens to improve it's depth of focus (just like a taking lens' depth of field can be increased by slowing it down to a higher "F" number). ISCO offers f/2.4 and f/2.7 "studio" stop down rings for their Ultra-Star PLUS line of lenses (the red ones). Schneider is working on an adjustable "f-stop" lens as well. Schneider will stop down their lenses if you ask but it is a factory performed operation. For the current Schneider lenses the optimum stop down point requires lens disassembly. ISCO has managed to have the stop-down rings acceptable at the front of the lens only...funny their older style Ultra-Star Studio (gold with a blue band and states "studio") had 3 internal stop down rings and one had to order the lens as an f/2.7 "studio" lens or the standard f/2.4 lens.
Steve
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Steve Guttag
We forgot the crackers Gromit!!!
![](http://www.film-tech.com/ubbmembers/268e.gif)
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-14-2002 09:31 AM
First things first....
Yes, I MUCH prefer a curved gain screen to a lackluster flat matte screen. So there! However you will find there are many, like yourself that prefer a flat matte screen. So clearly there is an esthetics issue here on the curved versus flat.
Now as to gain screens...just because you've seen poor examples or clearly poorly implimented gained screens doesn't make them, in and of themselves bad. I'll take it one further, to state, generally, that gain screens are worse or inferior to matte screens is flat out wrong. You have to judge something when it is being used correctly .
The sweet spot of a gain screen that is properly curved is often the ENTIRE audience since a properly designed screen system takes the audience seating area into consideration. If you come to the States some day, I'll show you a couple installations to prove my point.
Now with a FLAT screen or a shallow screen, then yes Matte White is the only way to go. With a curved screen (proper) then matte-white screens are prohibitive since they will self-illuminate (some call it cross-reflection).
Don't be too proud of this technological terror you've created with your matte screens...I've noticed that many screen manufacturers are cheating their matte-whites and are having gains approaching 1.2 so their's is brighter! That also helps with the shallow curves too.
As to the speed of the lens...Fast denotes more light...slow denotes less light...you'd have to check with someone on the history of lenses to come up with the origins of the terms. The speed, or "f-stop" relates the exiting diameter of the lens to it's focal length. Thus a lens with a larger exiting diameter will be a "faster" lens with a smaller "f" number. In the Drive-In heyday, where focal lengths were quite long...a 70.6mm lens barrel effectively limited you to slower than f/2.0 once you crossed the 141.2mm focal length. By going up to 101.6mm barrels you could once again speed up the lens to f/1.7 to get more light on those screen towers.
Steve
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