Film-Tech Forum ARCHIVE: A lens question from a new guy.

This is topic A lens question from a new guy. in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Joshua Gravel (Member # 5745) on 08-26-2010, 09:06 PM:

Hello,
My name is Joshua Gravel and I have been a projectionist for a couple of years now but am constantly reminded of how little I know. So on to my question, the theater I work at had the proper lens for flat 1:85 , and for scope 2:35 presentations, but what lens should be used for a film which is 1:35 or 1:37? I want to make sure everything is shown correctly so some info would help. Thanks,
Josh

Posted by Randy Stankey (Member # 64) on 08-26-2010, 09:31 PM:

Each format (aspect ratio) of movie requires its own lens, aperture plate and it requires the screen masking to be moved to match. It's pretty much a custom fit operation for each projector/theater. That's why you can't casually move lenses around from theater to theater without risk of causing problems.

You can calculate the lens focal length required for a given film format, screen size and distance from projector to screen with simple algebra. It's just a proportion.

Simple answer:
We need to know the size of your screen and the distance from the projector to the screen.

Complicated answer:
We can calculate the focal length of the "unknown" lens if we know the focal length of one or more of your other lenses and the size of your screen.

More complicated answer:
You can also download this program which will allow you to calculate which lens you need for a given projector:
http://www.film-tech.com/warehouse/wareview.php?id=786&category=2

Really complicated answer:
If you really want to learn something, just do the math yourself.

Focal Length * Film = Throw * Screen

"Focal Length" = the focal length of the lens.
"Film" = the size of any dimension (height or width) of one frame of your film.
"Throw" = Distance from the screen to the focal point of the lens.
"Screen" = the size of any dimension (height or width) of your screen. (Be sure it's the same dimension as you measured on the film.)

From there, it's just 8th grade algebra. Just remember to keep your units the same.

Posted by Martin McCaffery (Member # 37) on 08-26-2010, 09:32 PM:

Welcome to the gang.

The correct answer depends on screen size and picture throw (how far the projector is from the screen) and, of course, whether you have proper 1:37 aperture plates.

I believe there is a lens calculator in the manuals section. If not, someone should be along soon who has the formulas memorized and can give them to you.

edit: Looks like Randy wandered in while I was typing.

While you're waiting, feel free to browse and see all the wonderful things you'll need to learn [Wink]

Posted by Scott Norwood (Member # 30) on 08-27-2010, 08:25 AM:

Hi, Josh. We met a few years ago at RIIFF.

Is this for the Cable Car Cinema? If so, and if I remember correctly, they have a common-height screen and crop the sides of the scope image slightly. Post the focal lengths of your 1.85 and scope lenses (which should be stamped on the lens barrel somewhere, and will be in either inches or millimeters) and we can figure out the correct 1.37 lens focal length for you. You will still need an aperture plate. These come slightly undersized, and need to be filed out using a loop of RP40 test film. I might be able to dig up a plate that was cut for another theatre that would come close. You have a single Century C, correct? If so, and if you have the small lens holder version, you will also need a lens extension tube and a stop ring.

If this is a one-time screening, you can sort of cheat by using the backup scope lens (unscrew the anamorphic adapter part) and the scope aperture plate. The result won't be correct, but it is better than running an Academy print at 1.85.

Alternatively, send the Academy prints to RISD or the Avon, both of which have the correct lenses and plates for that format. (So does the Columbus, but that has been "temporarily" closed for the last year, as you probably know.)

If you are not in a hurry, and can get the proper lens and an undercut plate, let me know and I'll bring an RP40 loop and cut the plate for you when I am in Providence next.