Film-Tech Forum ARCHIVE: Scrims for lamphouse to change f stop

This is topic Scrims for lamphouse to change f stop 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=009875

Posted by Bernie Anderson Jr (Member # 410) on 05-07-2009, 07:10 PM:

I was presented with an issue today. I'm updating a booth for a lab in NYC , they asked me today about scrims for the lamphouse/projectors to cut back the f stop. They are doing something with a picture a 1/4 of the normal 1.37 aperture size which would increase the brightness (I have no idea what that's about). They do not want to defocuse the bulb or use filters, but they want to be able to swap out this "scrim" to change the stops by half or by 1. Is this a manufactured item? They're using a variable iris Schneider lens for their normal projection, but this job will require a different lens, for which they don't want to spend the money. Understandable. I know there was one post about f stops, but not specifically using scrims. Any thoghts?

Posted by Mark Gulbrandsen (Member # 72) on 05-07-2009, 10:03 PM:

I'm not sure other than through experimentation how you would calibrate a series of scrims. Stainless steel screen could be used but it may eventually deterioate causing the light level to wander. If this is a smnall lamp... say 1kw or under then they'd probably last a long time... but larger lamps would take theor toll on the screen material. Iris's are really the best way and can also provide a bit of extra depth of focus at the same time. Can be a big advantage.

Mark

Posted by Gordon McLeod (Member # 33) on 05-07-2009, 10:50 PM:

I usually make them from stanless steel window screen material and mount them in the front of the lamphouse

Posted by Bernie Anderson Jr (Member # 410) on 05-08-2009, 04:29 PM:

So nothing is really manufactured for such a job then? Just trial and error.

Thanks

Bernie

Posted by Gordon McLeod (Member # 33) on 05-08-2009, 04:40 PM:

I made up many different ones over the year for various labs and shouts to match what they want and it was a trial and error process