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Author Topic: Curved Screen & Focus Alex Aye Film Handler Posts: 7 From: Kansas City, KS, USA Registered: Dec 2002 posted 12-13-2002 03:00 PM                        I thought...focus has to be on one single plane. Why are the screens curved at the theaters? Images projected on such curved screens still achieve sharp focus technically and practically? What about IMAX domes? How could everything be in focus on such a terribly curved focal plane? What is the trick that I do not know? Hmm...  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 03:27 PM                     A modern len's is designed to cope with flat screens, however logic would suggest that a screen should be curved so that each point on the screen is the same distance from the lens. If you were to take a length of string from the lens to the middle of the screen and hold the string at that point, as you moved from left to right you would move in an arc as the edges would me much further away than the middle. This is especially true of large screens with short throws. The longer the throw the less the difference, and less keystone etc Does anyone have any information or explanation of why this difference does not create havoc with focus. Regards.  |  IP: Logged Alex Aye Film Handler Posts: 7 From: Kansas City, KS, USA Registered: Dec 2002 posted 12-13-2002 04:34 PM                        Ken, I see what you are saying. Any spot on the screen is equi-distant to a spot on the lens that it coresponds to. Since the lens is curve, the bigger replica to it (the screen) has to be curved too. I still don't understand the way focusing works on Dome screens. In projection, is focus a "range" rather than a "point"? Say "depth-of-focus"? Regards, Alex.  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 04:44 PM                     I do know that modern lenses are designed to allow for flat screens. I also know that very large format systems are custom designed so not really comparable to normal projection systems which are intended to be used in a very wide variety of situations. You can't just give your local supplier a call and say I want such and such a lens for my dome projection system... they would have top order it specially. In the days of the magnificent picture palaces where throws of 150 feet were not uncommon the ratio of picture width to throw was a lot less than it is today. Pictures always looked sharp and well lit. Today with thorws of 50 feet and pictures of 30 or 40 feet wide the story is very very different. Lenses have to be of a very high quality to cope with the different ratio of throw and width. They also need to pass much more light etc. Its a very complicated situation and I know of the limitations but not the theory of it all. So I'm looking forward to reading some posts on this. Regards.  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 04:51 PM                     Depth of focus in photograpy is where a distance range is in focus.... Its also a combination of apature and shutter speed to provide the correct exposure. A shallow depth of field will isolate a subject with everything in front of and behind out of focus. This is usually with an wide apature and fast shutter. A deeper depth of field is obtained by a much smaller apature and slower shutter speed, again the combination providing the correct exposure. In projection the situation is similar but again I'm not too sure exactly of the theory. 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 apature over the lens as he claimed it improved the focus. I must admit it seemed to help.... but it reduced the screen illumination considerably. So as in everything there seems to be a trade off. Again anyone out there who can explain some of this?  |  IP: Logged Adam Martin I'm not even gonna point out the irony. Posts: 3686 From: Dallas, TX Registered: Nov 2000 posted 12-13-2002 05:39 PM                     Large format lenses are designed for their specific application. Lenses for flat screen Imax theatres are manufactured by Hughes at a cost of around $50,000 each. Yes, that was fifty thousand. The projector in an Imax Dome theater is elevated to the center point of the dome screen so that the lens is equidistant from all points on the screen. The lens for that application is a super-wide angle lens that projects an image about 180 degrees wide. The theories for an Iwerks Omni theater are pretty much the same. For conventional 35mm projection, Ken's string analogy is accurate. It works that way not only for side-to-side curvature but also top-to-bottom curvature. The non-perforated Torus screens that AMC used for a while were compound-curved screens. Compound-curved screens are great for use in super-huge screens with very short throws, except that what you gain in picture quality you lose in audio quality because the speakers cannot be located behind the screen.  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 05:42 PM                     and in english please..... me.... how many projectionists would that buy in a year...  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 06:19 PM                     Adam you cheated a bit.... you edited your post after I replied.... Must admit I do that a lot.... can never proof read a post and get it right first time. Its a real problem with modern auditoriums trying to stick to the "Bigger is better" philosopy.... on a very persoanl note I can assure you its not true!! The other big problem today is high gain screens.... they look awful. Hot spots, shimmer to the picture etc. As an ordinary member of the puiblic they may not be able to say exactly what is wrong but they leave with an impression that it was 'off'! I don't have high gain screens in my site so I can give my honest opinion of them without fear of.......? They are a case of the right tools for the job. Long narrow auditoriums are fine, but short wide auditoriums are awful. There is a UCI mutiplex near me and the picture's are awful. I've also been to the UGC in Brighton and left within a few minutes and demanded a refund because of it. The modern auditorium DEMANDS the best spec you can afford because if you don't spend up front it will bite you back very hard indeed. Now in the right circumstance a bite can be enjoyable.... but let's not go there  |  IP: Logged Patrick de Groot Expert Film Handler Posts: 161 From: Sprang-Capelle, Netherlands Registered: Apr 2001 posted 12-13-2002 07:02 PM                     The purpose of curved screens is improved light reflection (aimed (as in not diffuse)) Equal brightness accross the auditorium.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 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  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-13-2002 08:32 PM                     errr Steve I'll take all of that in when I'm a little more sober in the morning. It is clear from what you say that there is a len's for every application. High gain screens are easily noticable.... and I personally find them objectionable.... unless you 'happen' to be seated in the sweet spot!! A matt screen gives a better overall performance from the punters point of view in my opinion. Would you ever suggest a high gain screen over a matt screen... as a professional if cost were not an issue? With everything taken into account, short throw, wide screens etc, the only reason to install a high gain screen is operating cost of lamps etc.... not totall performance. or am I missing something here?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 12-13-2002 09:10 PM                     I almost always spec high gain screens. Namely I always curve them They allow 3d presentations They allow on to acually get the 16fl of light on the sheet with out a 10K lamp Audiences are voting with there box office dollars for these very large sheets 55'+ and with out a gain screen it would not be possible to achieve any reasonable light levels. With proper curve design and angling of the frame reasonable falloff can be maintained  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 12-13-2002 09:45 PM                        What exactly is the difference between a "slow" and a "fast" lense? I know that light changes its speed when it enters a different medium, but I never thought it would make any difference since it is so fast.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 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  |  IP: Logged Ken McFall Jedi Master Film Handler Posts: 615 From: Haringey, London. Registered: Apr 2001 posted 12-14-2002 10:19 AM                     Havt to admit that all of the awful high gain screens I've seen have been flat. Maybe someone decided that the extra gain would allow them to make a saving on lamps..... So once again its effectivly the right tools for the job.... Regards.  |  IP: Logged

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