Film-Tech Forum ARCHIVE: Size of backing lens for particular size scope picture

This is topic Size of backing lens for particular size scope picture in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Ian Griggs (Member # 9542) on 11-04-2016, 03:05 AM:

Hi everyone.
Is there a formula somewhere which gives the correct focal length for a backing lens to give a particular image size?
My problem is that my home situation only allows me an image 1600mm wide. This is OK for 4:3 or WS images using an 85mm lens.
However I want to run scope and my longest lens is 90mm which gives a scope picture 2400 wide - way off each side of the screen.
I expect that I need something in the order of 120mm or 130mm, but is there a formula which can give me the answer? Trial and error is not an option as the Schneider Kreuznach long throw lenses are extremely expensive. I expect you will say "get a bigger screen" ha but that's not possible either. The anamorphic I am using is a Schneider Kreuznach 2/85mm and the throw is 7.2m. Many thanks!

Posted by Peter Foyster (Member # 9599) on 11-04-2016, 05:45 AM:

Download LENS CALCULATOR from the warehouse.
You will need an older version of Windows to run it though.

I keep an old laptop with XP on it for that purpose.

Posted by Randy Stankey (Member # 64) on 11-04-2016, 01:49 PM:

It's 8th grade algebra. [Wink]

Focal length / size of image on film = throw / size of image on screen.

Keep your units consistent.
Don't forget to apply your anamorphic squeeze/stretch factor.

You can do this with a pencil and paper.

One time, I was even able to use this formula plus a little trigonometry to look at an old photograph and determine the exact location where the photographer was standing when he took the picture. (For historical research.) [Smile]

Computer programs are nice and convenient but I still suggest that, even when using a program, a person understand the concepts and formulas behind the calculations.

What if your computer doesn't work or, as mentioned above, the computer doesn't have the right software or operating system? You'd be screwed if you didn't have a way to do it without a computer.

Posted by Ed Inman (Member # 2583) on 11-04-2016, 04:56 PM:

I wouldn't spend a fortune on lenses just to fill a 5-foot screen. There's a long old Super-Lite 6 1/4 inch (roughly 160mm) lens from Peru on ebay right now for about 20 bucks and if you are patient you can probably find some cheaper than that. I once got a whole box of old inch-style prime lenses in varying focal lengths for about $15.

Posted by Ian Griggs (Member # 9542) on 11-04-2016, 06:13 PM:

Thank you everyone for the suggestions
Yes Ed, there are two Super-Lites on at the moment. However, trying to mount the Schneider anamorphic on the front might be a problem. Although luckily, the two diameters are not far apart. The anamorphic thread 68mm - the Super-Lite front 71mm. We shall see.