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Topic: Curved Screen & Focus

Alex Aye
Film Handler

Posts: 7
From: Kansas City, KS, USA
Registered: Dec 2002

posted 12-14-2002 11:25 AM

Thanks folks for your feedbacks.

I have definitive answer for the history of fast and slow lens.

It did not come from projection profession. It originally came from photography/cinematography. When you take a fast action shot, you need a fast shutter speed like 1/1000 sec. Then the amount of light falls onto the film to cause images is very little. To compensate that effect, you either need a film that is very sensitive to light or you have to open the lens aperture very wide.

Thus, the words -- "fast film" and "fast lens".

Fast film: Film that is very sensitive to light. Numerically, such films are dubbed ISO-1600 ISO-3200. These numbers are also called "film speed", appropriately of course.

Fast lens: A lens that can open up really widely, in terms of aperture hole. Anything wider than f/2.0 is considered "very wide" or "very fast". Fast lenses are very very expensive. As a side-effect, they do have shallow "dept-of-field". That means, focusing is very unforgiving.

CONFUSION POINTS:
(1) Fast lens has nothing to do with speed of light or the speed of any part of the camera or the projector.
(2) Fast film is not a film that moves quickly. It is sensitive film.
(3) Wide lens is not a lens to make or show widescreens or panaromic scenes. A wide lens is one that has wide focal length such as 14-mm.

Regards,
Alex

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Bernard Tonks
Jedi Master Film Handler

Posts: 619
From: Cranleigh, Surrey, England
Registered: Apr 2001

posted 12-14-2002 02:01 PM

Ken wrote:

quote:
In one cinema where I worked, I'm cannot say where it was for fear of falling foul of secrecy, the chief had fitted an aperture over the lens as he claimed it improved the focus...

I did the very same with an aged 1.9 backing lens for scope until I invested in an ISCO 2.0 lens. I got the idea from a film mogul’s private cinema, which also white washed the arc lamp mirrors.

I would agree with Ken, there are far too many cinemas equipped unnecessarily with high gain screens. One of the first things I did at my cinema was to replace the silver Perlux screen with a matte white, giving my slightly curved screen a beautiful picture. The other advantage is that the often damaged screens experienced in multiplex cinemas would be far better off, by being able to remove completely any bad stains. As well known, one of the old major circuits in the UK had any renewals done matte white.

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Ken McFall
Jedi Master Film Handler

Posts: 615
From: Haringey, London.
Registered: Apr 2001

posted 12-14-2002 02:38 PM

My problem is not with high gain screens but where and when they are used. If you follow the rules then I'm sure they can look fantasic.... its just a shame that this does not seem to happen very often.

I was fortunate enough to be able to influence the changing of two screens that were high gain. The hot spot in the smaller of the two was truly awful. It was a flat frame and you could see the spot move if you moved your head side to side... it was that noticable. The larger screen, short throw flat frame, had seating way off the side of the screen.... Wrong, Wrong, Wrong!!

The screens have been replaced with matt and the difference is staggering.

I'd like to see what a properly designed and planned install of a high gain screen can look like.... you never know I may just take up the offer of a tour next time I'm in America.

You have been warned [thumbsup]

Regards.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 12-14-2002 05:24 PM

You are right of course, Alex. It struck me last night when I played around with the manual ISO settings of my new digital camera to take some night shots that the terms "fast" and "slow" come from the exposure time it takes to reach the same level of brightness with different lenses and film materials, not from projection. But sometimes the answer is so near that it is "out of focus".

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 12-17-2002 08:50 AM

In simple terms, a "gain" screen acts somewhat like a mirror. So you need to curve it properly (ideally by ray tracing) to reflect the light back to the prime seating area of the audience. SMPTE Recommended Practice RP95 "Installation of Gain Screens" provides a "rule of thumb" formula for the proper curve radius.

Since matte screens reflect light in all directions, they are most suited to theatres having wide viewing angles, and when getting enough light is not an issue. Matte screens should be installed FLAT, as cross-reflections with a curved matte screen will decrease image contrast.

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