Film-Tech Forum ARCHIVE: Lens size calculation

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

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Posted by John Wilson (Member # 269) on 12-16-2003, 07:01 PM:

Does anyone have an easy way to do this? What formula do you use?

Posted by Lindsay Morris (Member # 1430) on 12-16-2003, 07:29 PM:

John,
One that I came across in a publication somewhere is (and works very well):
Multiply Frame width (in mm's) by Throw (in Metres) and Divide by Lens focal length (in mm's) and you get the Image width.
Using those variables you can transpose any to get any other dimension you need.
The one quoted I use a lot for portable shows so it has stuck in my brain and makes it real easy when I am trying to position a machine for a client so they know what image they will get from what spot.
I use 21mm width for 35mm stuff and 9.1mm for 16mm stuff as the frame width.
If you want to use inches just substitute inches for mm's and feet for metres and it works just the same.
Off the top of my head if you want to know lens size for a certain throw try 21mm X Throw (in Metres) Divided by the Image size (in Metres).
So at 50m throw producing a 15m image the lens size will be 70mm.
(50 X 21 Div by 15) = 70mm.

Lindsay

Posted by Gordon McLeod (Member # 33) on 12-16-2003, 07:31 PM:

Use the simple lens calc software and for the mroe accurate use the schnieder design pro all are in the manual section of this site

Posted by Randy Stankey (Member # 64) on 12-16-2003, 09:26 PM:

I agree to use Schneider Theater Design Pro. Not only will it calculate the lens(es) you need but it will print out the specs of your theater right on the dot.

If you want to do it the hard way OR if you just want to know for academic purposes it's all simple proportions:

Any dimension of the film divided by the focal length of the lens is directly proportional to the matching dimension of the screen image divided by the length of the throw, assuming you keep your units the same. The caveat is for anamorphic lenses. You have to multiply by the expansion factor of the anamorphic. (Usually 2.)

[code]

F (Film Dim.) S (Screen Image Dim.)
_______________ = _______________________

f (Focal Len.) T (Throw Len.)
[/code]

Use Means-Extremes Product Property to solve for one unknown when three are known:

Dimension of the screen: S = T * F / f
Throw: T = S * f / F
Film of the film: F = f * S / T
Focal length of lens: f = F * T / S

Remember if you calculate the HORIZONTAL dimension of the sceen in SCOPE you have to multiply the answer by TWO to get the right answer. The anamorphic lens expands the picture horizontally by a factor of two. If you are calculating the horiz. dim. of the film in SCOPE divide by two.
This formula works for virtually all lens/image size calculations, be it movie projectors, slide projectors, video projectors or even cameras.

If you don't know the actual throw from screen to the nodal point of the projector you can calculate that, even. Measure the height of the screen and get a theodolite (surveyor's transit) to measure the down angle to the screen. Use trig. to figure out the distance from the screen to the prjector. If you can't get a real theodolite you can make one from a compass and a weight attached to a piece of string.

If you aren't going to use the Theater Design Pro software you can make a spreadsheet in Appleworks or Excel. Once made, you simply plug in the known dimensions/distances and the computer does the rest for you. I calculated my lenses by making a spreadsheet first and then using the software. I compared my answers between the two. Since I got the same answer both times I KNEW I had the right lenses.

Posted by John Wilson (Member # 269) on 12-16-2003, 09:33 PM:

Thanks guys.