Film-Tech Forum ARCHIVE: Need help with throw distance

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Posted by John Hegel (Member # 628) on 07-10-2003, 01:42 AM:

Hello,

I am working on an outdoors screening/drive-in movie (one time show)and need some help with figuring out how far to place the "booth" from the screen.

We are showing "Twister" in 35mm/scope.

The lenses that I have to chose from are

Baush & Lomb (scope)E.E. 6.5"
Super Lite E.F. 5.5"
Super Lite E.F. 6.5"

The screen will be 15'x34'

My question is, what combination of lenses will be best to use, and how far will they have to be from the screen.

The rest of the equipment is:

simplex xl
xetron 2,000 watt lamp
strong power supply
cfs "super platter"
home made A.M. sound and 2 D.J. sound systems comming to help out

p.s. the show is tonight at 9:30, so I doing this as a last min. thing.

Thanks for your help,
John

Posted by Aaron Sisemore (Member # 145) on 07-10-2003, 01:51 AM:

For a screen 34x14.5' you will need a throw of:

132' 7" for the 6.5 (163mm) lens

and

112' for the 5.5' (137mm) lens.

At those throws, I doubt that 2000W is enough lamp for the job, especially with those old antique lenses.

-Aaron

Posted by Phil Hill (Member # 371) on 07-10-2003, 04:23 AM:

Remember: Throw distance has NO effect on screen brightness. (UNLESS you have SPFX like fog in the theatre or are projecting to outer space with extremely long throws!)

Things that DO affect screen brightness are: The format of the print. The square-footage of the projected image (magnification). The efficiency of the lamphouse and reflector (f-stop). The wattage and age of the xenon lamp. The efficiency of the projection lens (f & t-stop). The screen gain. And of course, the alignment of the optical system.

>>> Phil

Posted by Bob Maar (Member # 763) on 07-10-2003, 06:55 AM:

Error

Posted by John Hegel (Member # 628) on 07-10-2003, 10:52 AM:

When I put the scope lens on the front of the flat/primary lens, does that effect the focal length (i.e. do I have to be a different distance from the screen)?

Posted by Mitchell Cope (Member # 32) on 07-10-2003, 11:54 AM:

Actually no. The height of the image is the same with or without the anamorphic. Along the horizontal, the image is expanded by a factor of 2.

What I think throws people off most about anamorphics and the change in focal lengths between scope and flat images is they don't account for the larger aperture size for scope motion pictures. It's that larger aperture size that causes you to have a longer focal length lens for your primary. This assumes that you were wanting to keep your image height constant.

Someone check my math with Aaron's:

I say a 6.5" (165.1mm) primary will fill a 15 foot high screen at 142 feet. The width of the 2.39:1 picture will be 35 feet 9 inches (a loss of almost a foot of image on each side). At 136 feet there is no image loss, but you have an extra 7 inches of horizontal screen not being used.

I say a 5.5" (139.7mm) primary will fill a 15 foot high screen at 121 feet. The width of the 2.39:1 picture will be 36 feet (a loss of almost a foot of image on each side). At 114 feet there is no image loss, but you have an extra 7 inches of horizontal screen not being used.

I think we only differ in a couple of mms. Pass them around. [beer]