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Author Topic: where best to put filters to "stop down" the light Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-06-2008 01:16 PM                        Where is it best to put filters to stop down the light output. We have two lenses on our projector both with markedly different light outputs. One lens is set to 16 FL and on the same etup the other is about 30 FL. Currently I adjust the light output but you can imagine this would be a bit tedious. They are both ISCO lenses, one scope and the other academy. We also need to make filter packs for each to make both lenses hit 5800k. Can these be perpendicular to the film plane or is it recommended they are off axis? Many thanks, Adrian  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-06-2008 02:03 PM                     I used to se a stainless steel screen in the light path between lamphouse and projector. This doesn't affect color twemperature only light level. Any place other than in front of the lens. Some Schneider lenses have room for an adjustable iris inside them. This is also a good idea. Mark  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-07-2008 12:06 AM                     Would it be, per chance, that your scope lens gives a brighter picture than the flat lens?  |  IP: Logged Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-07-2008 03:27 AM                        Yes Randy, The scope lens is the culprit in the brightness stakes. One option is to find a better matched pair of lenses but as always its going to be a trade off between cost of lenses or a cheaper solution. The lenses arent that bad. Could be better but workable none the less. Mark, why do you suggest to not put a filter in front of the lens. As we are a DI grading theatre we need to hit 5800K and so need to adjust our white point ever so slightly. From 6300 to 5800 to be precise. We did have a filter pack sitting parallel to the film plane before I broke it. If this is a bad idea what are my options? Thanks, Adrian Hauser.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-07-2008 06:33 AM                        A friend of mine had a similar problem with going from 35mm to 70mm in a relatively small screen auditorium. He told me he used a simple iris (basically an aluminum gobo material -- same as they use in theatre lighting instruments) that was placed over the lens opening. I can't remember if he said it was at the front or rear of the lens. Said he used a protractor to cut the circle true. I would imagine, if it doesn't cause any vignetting, it would be better than filters in that it would eliminate the question of color temp changes that density filters might introduce. And cheap too.  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 04-07-2008 08:21 AM                       quote: Adrian Hauser why do you suggest to not put a filter in front of the lens Adrian, Mark's referring to a scrim, not a filter. Since it's made out of wire, you can't put a scrim in front of the lens as it will obscure the picture. Color correction gels (and/or ND filters) can go in front, however.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-07-2008 08:45 AM                     This is why Christie came out with the motorized optical bench on the SLC. It adjusts the lamp focus between flat and scope in order to attempt to maintain a more even light level between the formats. IN any grading situation you would never want to place any filter in front of the optical path no matter how high of quality that filter may be. Any filter in front will cause some diffraction of the light leaving the lens. No matter how insignificant it may be you still want the optical train as perfect(and color matched) as possible!! The iris in the lens internally als Schneider is good and I would also vote for the scrim because it won't degrade the optical quality of the image nor change the light color temperature. Filters behind the film plane are ok and won't optically degrade the image... I don't know of a scope lens that has a threaded front to allow screwing on or adapting filters. You always end up taping them on and it makes a mess out of everything. Mark  |  IP: Logged Larry Myers Master Film Handler Posts: 371 From: Herndon, VA, USA Registered: Jan 2001 posted 04-07-2008 09:43 AM                 If in fact the scope lens is brighter then someting is wrong with your flat lens. It should be the other way around. That is, the scope lens should be about 16 FL and the flat should be close to 30 FL. The reason is, the flat lenses light is not being spread out 2x in one direction.  |  IP: Logged Richard P. May Expert Film Handler Posts: 243 From: Los Angeles, CA Registered: Jan 2006 posted 04-07-2008 10:20 AM                     In our laboratory screening room, the "natural" light from 1k lamps is too bright for the screen size. We have placed neutral density gel filters, which are in a home-made frame, directly in front of the lenses. The light intensity can be easily corrected, and we see no effect at all on the projected image. The filters are simply taped to the front of the Norelco FP20 cases, in a position that they do not get in the way of the door. We previously had the filters on the rear of the porthole glass. This wasn't good, as the light bounced back from the glass onto the filter, then to the front again, putting a sort of ghost image on the screen whenever there was a bright part of a scene. Previously, I had made small filters and mounted them in a 35mm still photo slide mount. These were placed in front of the projector gate, behind the lens. On a Simplex XL it worked fine, as the gate assembly almost invites such a unit. The Norelcos aren't built that way, and it was hard to keep it in place. There was never any melting or fading of the filters, regardless of the fact they were right in front of the film. The same technique can be used for color correction filters if the lamps are not putting out evenly colored light. The theory of the scope lens only transmitting half of the light of a flat lens to the screen doesn't work in practice. We only find a difference of about 3 fl drop, and to compensate for this, swing the ND filters out of the light path. Don't forget, the light is being spread double only in the horizontal direction. To make a 50% reduction I think it would need to be both twice as high and twice as wide. RPM  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 04-07-2008 11:45 AM                       quote: Larry Myers If in fact the scope lens is brighter then someting is wrong with your flat lens. It should be the other way around. Bzzzt... (with apologies to Steve Guttag.) Scope is naturally always brighter because the frame area is larger. If flat and scope utitlized the same area on the film, then your theory would be correct. If anything, they should be close to the same, but in practice scope is almost always brighter.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 04-07-2008 01:27 PM                        Larry, Look at both your scope and flat aperature plates. The scope plate will have a larger aperature than the flat... Larger aperature means more light allowed through. That is one of the benefits of scope in my opinion there is more picture "information" and more light. IMO The reverse is actually true if your FLAT lens puts out a brighter picture than your scope lens then something is wrong.  |  IP: Logged David Graham Rose Expert Film Handler Posts: 187 From: Cambridge, UK Registered: Sep 2002 posted 04-07-2008 01:49 PM                     Greetings All Schneider make a filter kit with holders at 10 degrees to the lens for exactly the purpose you describe. Follow the link to see what they have on offer. From a snowy Cambridge David Scneider Optics  |  IP: Logged Richard P. May Expert Film Handler Posts: 243 From: Los Angeles, CA Registered: Jan 2006 posted 04-07-2008 05:40 PM                     Sean, Your theory about the scope lens passing more light is logical when comparing with 1.85 format. My comparison was with 1.37 format, which concentrates more light and thus comes out a bit brighter than scope. The Schneider filter kits do look great where precise modifications are needed, but run around $1000 per set. In a two-projector setup, this would buy lotsa gelatin filters. RPM  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-07-2008 11:04 PM                        And LOTS of aluminum donut stop-down rings. Elegante can be quite expensive. [ 04-08-2008, 07:00 AM: Message edited by: Frank Angel ]  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-08-2008 06:47 AM                     First off...if 1KW is way too bright....then 3-wing shutters are the answer...they reduce the light (by 50%) AND eliminate the flicker...it is a win-win and they wont fade or degrade the image in any way...in fact, most say they improve it. As to the Schneider filters...they are primarily to color match machines. While the filter KIT may be expensive...you only need one of them (and a proper meter). Once you know the corrective factor for your projectors...you only need apply the appropriate filters rather than the whole set. Then again, depending on the lamphouse, you can get the mirrors to be matched, which will again, have a stable color temp over a long time, as opposed to filtration. At Colorlab, we did successfully use filters in front of the projection lens to have precise 5400K and 3300K for so both xenon and tungstun could be properly timed. Steve  |  IP: Logged

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