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Author Topic: What is the largest screen possible with today's lamphous and 35 mm technology? Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 10-05-2003 02:28 AM                     quote: Why do longer lenses need less wattage? I thought wattage was determined by screen size only, not throw. Shorter lenses refract light at wider angles to magnify an image for a large screen/short throw situation (as many new stadium-seated theatres are using), thus require more light (and therefore more wattage) to produce a given level of brightness than the same size screen with a longer throw distance. Example: A 46x19 screen with a 52' throw (and a short focal length lens) will require more lamp wattage to get 16fl than a 46x19 screen with a 150'throw (and longer lens). -Aaron  |  IP: Logged Alan Haigh Film Handler Posts: 45 From: Watford, UK Registered: Jul 2003 posted 10-05-2003 02:47 AM                    Thanks Aaron So long focal length lenses are more efficient than short ones. Presumably the short ones get hotter? Is it a big difference in efficiency? Alan  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 10-05-2003 12:49 PM                        Up to a point. After a certain focal length, long lenses simply can't have their elements be wide enough to keep the f ratio down (that's where 4"-wide lenses come into play). Shorter lenses generally require more glass elements in their design to correct optical abberations that become very noticable at wide projection angles. Those extra elements and the resulting extra air-glass and glass-glass transitions eat up light even when the best optical coatings are used. In the end, the fastest lenses exist in the middle range of focal lengths, with both short and long lenses being slower in terms of f ratios (higher f numbers mean slower lenses and therefore less light). Although a faster lens will put more light on the screen (all other things being equal) faster does not automatically mean better. Depth of focus becomes an issue with fast lenses. From what I've seen and read if one stays within a range of about f2.0 to f2.8 one can expect good depth of focus and reasonable light output. Faster than that with short lenses and/or high keystone angles may result in an inability to get the whole image to focus simultaneously.  |  IP: Logged Alan Haigh Film Handler Posts: 45 From: Watford, UK Registered: Jul 2003 posted 10-05-2003 04:43 PM                    So to some up my disparate thoughts on this thread:- For a big screen you need A big lamp (7kW) A lamphouse with good optics, well aligned. A good lens with a medium focal length, also no keystoning etc. A "high gain" screen port hole glass that isn't fireproof (?!) Cooling! Large amount of money to replace lamps every few hundred hours. And then you're OK up to 50 feet, or 60 with a following wind. Any more and the film starts to buckle with the heat. Not for the faint hearted then!  |  IP: Logged Bevan Wright Expert Film Handler Posts: 176 From: Fountain Valley, CA, USA Registered: Sep 2003 posted 10-05-2003 05:17 PM                        >60ft is definitely possible given the following: 1) 6kW lamp - you can easily reach 35mm's thermal threshold with a 6kW lamp if it is properly set-up, maintained and aligned, 6kW also seems to be a more efficient lamp and lasts longer than a 7kW in most cases - unless you are going to very short arcs. 2) Modern reflector optics (horizontal) 3) ISCO RED lenses or Schneider Cinelux - hopefully someone buy's ISCO out of bankruptcy before they go away. 4) AR coated, both sides (single pane) port glass - double panes in most cases just mean a few more surfaces that don't get cleaned. 5) Proper alignment - light meter must be present when changing lamps and checked every few weeks due to arc burn-back. 6) Good prints - show prints preferably, although I have seen some good high-speed prints but they are few and far between. 7) Perlux II screen - It kills me when people want to do >50ft matte white screens. 8) Cooling - I've seen 6kW lamps go for >1,800 hours before needing to be changed because the booth was kept VERY cool. Watercooling only helps the 'finger burn' factor but may be necessary depending on the projector.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 10-05-2003 08:59 PM                     Water cooling is more neccessary on machines with brass castings such as a Century. Kinoton with - kind to film - Delrin runners also require cooling at high temperatures. Machines with machined aluminum support with matte box stepped light baffle as used in the Kelmar conversion kits and on machines such as the Christie, can dissipate the heat more easily so water cooling option is not usually offered. Glass cold coated reflectors also do a better job of light output versus heat than coated metal units.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-06-2003 09:35 AM                     I have found that the Kelmar aluminum ones are terrible for focus drift with large lamps when they get hot  |  IP: Logged Paul Trimboli Master Film Handler Posts: 274 From: Perth Western Australia Registered: Dec 2002 posted 10-07-2003 08:02 AM                     Just wanted to add that at the IMAX SR cinema I work at we have a Cinemecannica with a Strong X60D lamp house running a 7k lamp on a 22meter screen! Water cooled gate of course! Schnider optics. I know most IMAX SR theatres have the port for 35mm and there are even pictures on film-tech showing them! The light does go out at the corners, but I think it looks good and people love watching movies on a huge screen!  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-07-2003 08:20 AM                        With good engineering design and care in equipment selection and alignment, you certainly can go larger than 25 x 60 feet. But please don't sacrifice image quality for size --- standard SMPTE 196M specifies no less than 12 footlamberts at the center of the screen, with the screen sides between 75% and 90% of the center reading, and no less than 10 footlamberts anywhere on the screen.  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 10-07-2003 07:46 PM                    Design suggestion for Bevan...but I think I should put it in the other thread.  |  IP: Logged Kevin Wale Expert Film Handler Posts: 167 From: Guymon, OK USA Registered: Aug 2003 posted 10-08-2003 01:20 AM                     This has been a very educational thread. Thanks for participating in this thread so much. I have to read and reread to soak it all in. Then I have to really try and grasp all the ideas. It's so great being amongst so many knowledgable people.  |  IP: Logged Jim Leko Film Handler Posts: 90 From: Redondo Beach, CA, USA Registered: May 2002 posted 10-08-2003 02:11 AM                     According to the Arclight Cinerama Dome webpage, the Screen in the Dome is 32x86ft. The screen is curved, so I don't know how much that makes a difference. The picture when I went (Scarface) was plenty bright, and clear.  |  IP: Logged Bevan Wright Expert Film Handler Posts: 176 From: Fountain Valley, CA, USA Registered: Sep 2003 posted 10-08-2003 05:52 AM                        John P., there has been a theory kicked around for sometime that very large screens need less light simply due to the sheer size and amount of light coming back to the viewer. [If I'm not mistaken, IMAX screens tend to get away with less light and have very satisfactory results on their massive sheets.] Several industry experts have discussed doing some studies to prove this theory... have you heard of anyone actually going through with this activity?  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-08-2003 06:04 AM                        I don't know of any study, but I don't subscribe to that theory. It's really the viewing angle that determines how "big" a picture appears. As you know, SMPTE 196M specifies 2 to 4 times the image height as the preferred viewing distance. Most 35mm theatres today put seats in the range of about 1 to 5 image heights. If the theory held, the people close to the screen would feel it's too bright, and the people at the back of the theatre would feel it's too dim. Of course, IMAX and other 70mm special venue theatres have seats much closer than 1X the screen height, so maybe they are a special case.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-08-2003 09:34 AM                     Bill Shaw did tests way way back when regarding the light levels and in the beginning prints like North of Superior as such were timed for light levels around 9FL Often more light shows off strobing more readily  |  IP: Logged

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