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Author Topic: Short Throw Projection Ian Bailey Master Film Handler Posts: 317 From: Nambucca Heads, Australia Registered: Jun 2003 posted 10-17-2005 08:30 AM                     I have been told: That it is more difficult to achieve uniform focus over a short throw. That you may need a slightly higher wattage Xenon,compared to what you would use in a larger theatre,to achieve a bright image over a short throw. Is that correct?  |  IP: Logged Dieter Depypere Master Film Handler Posts: 343 From: Deutsch-Wagram, Lower Austria, Austria Registered: May 2005 posted 10-17-2005 09:15 AM                     Ok from the focus aspect, when you use a lens with a high focal length, there should be no problems (hope this is correct about the f.l.). I can't tell exactly about the brightness - my friend uses a 400W halogen bulb for a 6-9 foot throw (35mm that is).  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-17-2005 09:20 AM                        Very short focal length lenses have less depth of focus, so focus is more critical. The power required to light a given size screen is not affected by the throw, except perhaps the optical efficiencies (f/number, transmission) of the lenses available in the short focal lengths.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 10-17-2005 01:14 PM                        The biggest illumination problem is that light projected into the Center will reflect straight back and therefore be intense. The light send to the corners is sent at an angle so the picture will less bright in the corners. It is certain that light will vary depending on where you sit. Dark corners, bright center is an illusion since equal illumination is sent; it is just not returned from the screen. That is the reason why the front row of a large cinema looks good while, conversely, the same screen with a shorter throw has light "problems." Louis  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-17-2005 01:27 PM                        Generally, the concerns outlined by Louis apply to the use of gain screens. A gain screen in a very short throw situation is likely to have poor illumination uniformity. Even properly curving the screen per SMPTE Recommended Practice RP95 can only provide optimum illumination to the center of the seating area.  |  IP: Logged Amanda Mundin Expert Film Handler Posts: 122 From: Belper, Derbyshire, UK Registered: Sep 2005 posted 10-17-2005 02:06 PM                     John, below what focal lengths would you consider "very short", with focus becoming more of an issue?  |  IP: Logged David Stambaugh Film God Posts: 4021 From: Eugene, Oregon Registered: Jan 2002 posted 10-17-2005 02:14 PM                        The "short-throw" houses around here have always had sub-optimal image quality. Not bright enough, too much keystoning. It doesn't help that they have 40-foot-ish screens and sitting in the back row still seems too close.  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-17-2005 02:20 PM                        It's a judgement call as to where the shallower depth of focus becomes problematic, but extra care is needed with "flat" (1.85:1) lenses shorter than about 45mm, and "scope" lenses shorter than about 70mm. Focus issues do depend on the lens design (ability to focus on the film plane, which is usually somewhat curved due to film curl), lens f/number, and on any film deformation caused by the radiant energy of the lamp.  |  IP: Logged Mike Croaro Master Film Handler Posts: 394 From: Millbrae, CA Registered: Apr 2005 posted 10-17-2005 04:44 PM                     Dave make mention of one of the problems in smaller houses - screen size. Just as large auditoriums have large screens, small ones should have small screens. Installing a King Kong size screen in a small auditorium only exasperates the problems of keystoning, poor illumination, use of extra wide angle lenses etc.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 10-17-2005 08:16 PM                        Mike: not only that, but EVERYONE can see all of the problems (if any) since the image is too large. Louis  |  IP: Logged Amanda Mundin Expert Film Handler Posts: 122 From: Belper, Derbyshire, UK Registered: Sep 2005 posted 10-18-2005 04:00 AM                     Seems a good time to mention screen size, I was planning to have a 30ft scope screen with about a 40ft throw, our auditorium is quite square and is 45ft wide, what are anyones thoughts on whether that is too big, too small, just right etc??  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-18-2005 05:41 AM                        quote: Amanda Mundin ...our auditorium is quite square and is 45ft wide So exactly what is the distance from the screen to the back row of your seats? I would suggest the screen be no higher than 1/4 of that distance for showing 35mm film.  |  IP: Logged Mike Croaro Master Film Handler Posts: 394 From: Millbrae, CA Registered: Apr 2005 posted 10-18-2005 09:57 AM                     Hi Amanda: Many people will disagree with me but assuming the distance from the screen to the last row of seats is 40 feet I would go with a screen no more than 21 feet wide. ie: F: 9 X 16 / S: 9 X 21. The formula used is based on my opinion and those of Gerry Harrah of Harahh's Theatre Supply Company in Hayward, CA. My former theatre (Ukiah 6, Ukiah, CA) had two 60 foot throw auds with screens 12 feet tall X whatever with flat & scope produces. It was Gerry's opinion at the time (1998) that the screen size PERFECTLY matched the size of the aud. Creating a formula based on screen and aud size produces the fgures above. Mike  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-18-2005 10:16 AM                     The other part of the equation is the audiences "precieved" size of the screen if the stge wall is 40x20 then a 28' wide image would be preceived as unacceptably small when the patron enters the theatre even if the theatre is only 40' long  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 10-18-2005 10:59 AM                        quote: Gordon McLeod The other part of the equation is the audiences "precieved" size of the screen if the stge wall is 40x20 then a 28' wide image would be preceived as unacceptably small when the patron enters the theatre even if the theatre is only 40' long That's when what's on each side of the screen becomes important. Curtains, rather than a blank wall, enhances the apparent size of the screen. A 40 foot wide screen in a room with the back seats only 40 feet from the screen would have few seats with a comfortable viewing angle for most people.  |  IP: Logged

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