Film-Tech Forum ARCHIVE: screen size

This is topic screen size in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=012330

Posted by Bill Hall (Member # 10520) on 07-12-2018, 08:12 PM:

HI I am working at restoring a projection booth at my house. I would like to know how to figure out a screen size before I start to make one My scope aperture plate is 11/16 by 13/16 of an inch. My throw is one hundred feet from the lense to the intended place to construct the screen I would appreciate any input on how to figure out the size I need . Thank you
Bill Hall

Posted by Mike Croaro (Member # 3123) on 07-13-2018, 01:40 PM:

Hello Bill,

It would be helpful to know what is the farthest distance anyone will sit from the screen?

Thanks,

Mike

Posted by Mike Blakesley (Member # 26) on 07-13-2018, 02:27 PM:

Our throw here is about 95 feet and our screen is 30 feet wide, which is a nice size, if that helps.

There are a few other things to consider too, mainly the room dimensions (ceiling height) and width of usable area where the screen will go.

Are you using film or digital equipment or some other kind of video setup? What kind of light source?

Posted by Monte L Fullmer (Member # 2797) on 07-13-2018, 04:21 PM:

Schneider has a nifty lens size calculator to get the screen to match the throw.

If you already have lenses, then it would be a modification of calculation of screen size and focal length.

quote: Bill Hall
My scope aperture plate is 11/16 by 13/16 of an inch

Or, .690 x.8125. This comes out to 1.178/1 ratio.

Times two would come barely over 2.35/1.

That's a full open plate.

I've seen 3.75" primes shooting close to 100 feet on a 35 ft screen.

Posted by Monte L Fullmer (Member # 2797) on 07-13-2018, 06:11 PM:

To add: this is the magic number we need to help you with if you have anamorphic lenses in your possession, is the focal length of the prime lens, which is the rear lens.

It would be the series of numbers beginning with the "f" stop.

For example: f1.9 3.25 inch (or if schneider, or the similar, the measurements would be in mm, like 82.55 mm).

Posted by Martin Brooks (Member # 1269) on 07-14-2018, 12:26 AM:

w = da / f where:
w=width of the screen in feet
d = distance from the screen in feet
a = aperture width in inches
f= focal length in inches

Posted by Phillip Grace (Member # 2262) on 07-14-2018, 12:26 AM:

Hi Bill
You should download Schneider's Theatre Design Pro software from the Schneider Optic website. This is the most accurate software of its type, and will give you reliable results under all sorts of conditions. Note that there are international standard dimensions for projector apertures in all formats. T.D.Pro works to those standard dimensions. You can also add non-standard formats if you like, (The size of a used aperture plate may not be correct in any respect. It was probably cut to to suit a specific situation). You will be able to work the software to calculate any variable for the equation - ie. screen height, screen width, throw distance, lens focal length. It will also indicate the degree of distortion, with image cropping required due to projector rake and offset from centre line.

Posted by Steve Kraus (Member # 476) on 07-14-2018, 10:29 AM:

Add a 2 to the top of the fraction in Martin's formula for Scope.

I don't memorize the formulae. I just remember that it's a fraction and know that throw and aperture dimensions have a positive effect on image size so go on top; focal length has the opposite effect and goes on the bottom. And I remember the aperture dimensions. Just re-arrange to solve for what you need.

The units can be feet or meters (for screen size and throw) and inches or millimeters for aperture and focal length. So long as you have both throw and screen size in one unit and aperture and focal length in another unit, it will be fine.

25.4 mm = 1 inch is the conversion if, like me, you remember aperture dimensions in inches but modern lenses are measured in millimeters.

Posted by Steve Guttag (Member # 268) on 07-14-2018, 11:23 AM:

I tend to rewrite the "formula" as:

(ah/sh)*pd*25.4=EF

ah = aperture height in inches (e.g. .446 for flat or .690 for scope)
sh = screen height in feet
pd = screen height in feet
EF = Effective Focal length in millimeters.

In this form. you need not worry about the anamorphic lens since it only affects the width.

So, if you had a screen that 12' x 28'-4" with a projection distance of 65'

Flat would be (.446/12)*65*25.4 = 60.4mm (use a 60mm)
Scope would be (.690/12)*65*25.4 = 94.9mm (90mm, always round down).

That said, I would suggest for this theatre to reduce the screen height to 11.75' so that a 95mm lens could be used with less cropping. The Flat lens could bump up to 62.5mm

Posted by Bill Hall (Member # 10520) on 07-14-2018, 08:49 PM:

HI With the throw at 100 feet I would think people as in friends would sit about 80 feet to sit back from the screen. I have a super lumex lamp house and it has I think either a 2000 watt bulb or a 1600 watt bulb and the picture would be outdoors. I have looked on the lens to see if there are any numbers or f numbers but I did not find any so at a little bit of a loss there.
I appreciate the replies and the help to soon make a movie experience in the back yard to share with others
Thank you

Posted by Monte L Fullmer (Member # 2797) on 07-17-2018, 03:11 PM:

I'd use a 2K bulb if shooting outside being of all of the ambient light around, esp with a 100 ft throw.

You want the screen to be lit up.

Posted by Bill Hall (Member # 10520) on 08-04-2018, 10:49 AM:

Hi I had the lamp house operational last night and marked the outline of the lighted area of the screen I got the width at about 20 feet for under 100 foot throw. The height is about 12 or so feet high. I tried to run a loop of film but it got caught and broke a number of times so will try tonight. I did adjust the projector to center the width and height so will try again tonight . My platter quit on me a speco lP 270 as it blows the breaker as soon as I turn it on so one more item to fix before I can watch my trailers and hopefully soon watch a movie on the screen .
Thanks for any input one the above topics.
Bill Hall in Ontario

Posted by Ed Inman (Member # 2583) on 08-07-2018, 04:31 PM:

12' x 20' would be about right for a 1:1.66 plate but about 10.8' x 20' would be closer to the more common 1:1.85 flat ratio. And either one will require significant masking on the top and/or bottom for Scope prints. It's an issue I am quite familiar with since my back yard outdoor plywood screen is of a fixed size with only limited masking capabilities.

Posted by Scott Norwood (Member # 30) on 08-07-2018, 05:29 PM:

The amount of light needed on the screen is affected by the screen size, lens speed, and screen gain. Throw distance has nothing to do with it, except at very large (hundreds of feet or more) throw distances.

I'm still not sure that I understand the original question (from a month ago). The ideal seating location is 2-4x the screen height (e.g. 20-40' back from a 10' high screen). From that, you can get the size of the screen that you need, and from that you can figure the focal lengths and lamp size.

Posted by Bill Hall (Member # 10520) on 08-07-2018, 09:14 PM:

Hi many thanks for the replies . I think what I would do is make the screen width 22 feet and the heigth 12 feet to accomodate Scope movies. I plan to use white canvass material using the flat side of the material for a crisp picture . That is the plan for now and I could make a curtain to be in front of the whiter screen for masking so My hope now is to repair the Speco 720 platter as there is a major short somewhere so not able to run film through the projector so I am hoping to find the problem and repair it as it had been working very well for a number of days.
Many Thanks
Bill Hall