Film-Tech Forum ARCHIVE: Short Throw Projection

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Posted by Ian Bailey (Member # 1752) on 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?

Posted by Dieter Depypere (Member # 3184) on 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).

Posted by John Pytlak (Member # 331) on 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.

Posted by Louis Bornwasser (Member # 3063) on 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

Posted by John Pytlak (Member # 331) on 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.

Posted by Amanda Mundin (Member # 3386) on 10-17-2005, 02:06 PM:

John, below what focal lengths would you consider "very short", with focus becoming more of an issue?

Posted by David Stambaugh (Member # 1102) on 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.

Posted by John Pytlak (Member # 331) on 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.

Posted by Mike Croaro (Member # 3123) on 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.

Posted by Louis Bornwasser (Member # 3063) on 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

Posted by Amanda Mundin (Member # 3386) on 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??

Posted by John Pytlak (Member # 331) on 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.

Posted by Mike Croaro (Member # 3123) on 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

Posted by Gordon McLeod (Member # 33) on 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

Posted by John Pytlak (Member # 331) on 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.

Posted by Louis Bornwasser (Member # 3063) on 10-18-2005, 12:18 PM:

Amplifying John's reply: It is the treatment of the area next to the screen that seperates good architects from bad. In the "glory days" the very large screens did not even approach the limits of the wall width. One treatment is a diagonal wall which also had the side-benefit of "bending" the vanishing points of the side walls to point at the screen. Another is drapes. Another is a fancy wall with alcoves and statues, etc.

Largeness on a wall is not showmanship; in fact it can be quite obnoxious. Louis

Posted by Amanda Mundin (Member # 3386) on 10-19-2005, 03:40 AM:

What do people think of the theory that the ideal screen size should subtend to a horizontal angle of 45 degrees at the prime seat (2/3rds back) as mentioned in the Dolby Technical Guidelines?
It says Fox came up with this theory back in 1953 when it introduced Cinemascope, do people still stick to this ideal?

Posted by John Pytlak (Member # 331) on 10-19-2005, 10:05 AM:

For a scope (2.39:1 aspect ratio) picture, that 45-degree angle is in agreement with a seat about 3 picture heights from the screen, which is the same aim distance specified for critical viewing in screening rooms in standard SMPTE 196M-2003. Here is the exact wording of the standard:

quote:
9 Review room viewing conditions
All observers in a review room shall be located within a standard observing area which shall be:
a) within the limits of a 15° angle on either side of a perpendicular to the center of the screen, in both the
horizontal and vertical planes; and
b) at a distance of 3 picture heights ± 1 picture height from the screen.

So your "best" seats are between 2 and 4 picture heights from the screen. Most theatres have additional seating as close as about 1 picture height, and at the back of the auditorium, beyond 4 picture heights.