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Author Topic: How to increase the focal length of a lens Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 05-09-2003 09:32 AM                        The museum has a fairly upscale LCD projector that they want to use for video presentation when film festivals come in with video tape. Problem is, the existing lens on this Sony vidprojector is designed with a short throw lens and produces a image that fills the screen at about 25 feet. Short of seeing if Sony makes a longer focal length lens (which they tend not to do for this type of projector), is it possible to put a reverse magnacom in front of this thing to reduce the image size? I want to get the unit into the booth, which is about 70ft from the screen. The image is quite bright with the existing lens at the shorter 25ft throw; would the magnacom suck up a lot more light at the longer distance? My gut says it shouldn't, at least not a whole lot since I am not blowing the picture up any more than with the short FL lens and the closer distance -- the light is still being spread out over the same area. The only light reduction would be what the extra lens element would cause. Not much, I am hoping. Any guesses if this could be done with existing magnacoms?  |  IP: Logged Bill Gabel Film God Posts: 3873 From: Technicolor / Postworks NY, USA Registered: Jan 2002 posted 05-09-2003 09:52 AM                     Our NEC projector had four lens options. My throw is 43 feet from booth to screen. 1) Short focal lens .084 2) Zoom lens 1.5-2.5:1 3) Zoom lens 2.5-4.0:1 4) Zoom lens 4.0-7.0:1 We went with the lens option #3. It works great with all the video formats we use here in Post-Production.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-09-2003 02:14 PM                     We use the NEC LCD machines for adverts in the rainbow group and we purchased lens for them from Buhl Optical to get the needed focal length  |  IP: Logged Jon Miller Jedi Master Film Handler Posts: 973 From: San Diego, CA, USA Registered: Sep 1999 posted 05-09-2003 06:44 PM                     Frank, I'd worry more about the Magnacom causing serious vignetting of the image. I had a similar situation involving a more modest Sony projector (a dinky VPL-CX11, for those keeping score) and to reduce the image size I thought about trying a Sankor 0.7x/1.2x "conversion lens" I had on hand. To hold the lens in front of the projector, I built a support using 2x4s and a lens bracket from a Simplex E-7. Turned out the idea was not so great after all as the conversion lens vignetted badly due to the wide angle of the beam at the projector lens, even when zoomed to its minimum. Ironically, the image suffered little loss of brightness. If I had access to a 4"-barrel Magnacom (don't know if such a critter exists), the idea probably would have worked. Navitar markets Buhl Optical retrofit lenses. If you see your projector in the list, a lens retrofit may be worth investigating. Be aware these lenses cost in the neighborhood of $1000 and, if you retrofit, your projector will become useless for short-throw applications.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 05-09-2003 10:25 PM                        As usual, thanks, guys. And the idea that converting to a long throw will make the projector useless for short throw situation in the museum is just fine with me. The fact that I am going to bolt the projector to a plate that is welded onto the side of the lamphouse pedestal will make it even less of a temptation for the museum video types to "borrow it"....you know how that works. I know Buhl can make anything you ask them too (for a price), but before I go that more expensive route, I am going to try various incarnations of magnacoms, maybe even trying to ram a slide projector lens in there to see if it will work -- you can get slide projector lenses in all manner of long focal lengths (I've got a 12 incher for our big theatre -- I used to project date slides on the bottom of the coming attractions: STARTS SUNDAY, etc). Then there's Edmund Scientific Optical -- surely there is a piece of glass that will do what I need it to do. And what fun it is to play, even if it doesn't work. Frank  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-09-2003 10:47 PM                     Be aware that NEC voids the warrenty if any lens is insalled othr than theirs. Also I have yet to get one of those UHP lamps to last close to the claimed warranty as they have a clause of 6months from manufacture or 2000hrs which ever is less Be carefull as the optical block with the prism and pannels is very delicate  |  IP: Logged Jeff Taylor Jedi Master Film Handler Posts: 601 From: Chatham, NJ/East Hampton, NY Registered: Apr 2000 posted 05-12-2003 09:23 AM                     A follow up to Gordon's statement on the lifespan of the UHP lamps...I just got a 1500 lumen Viewsonic video projector and noticed that it's got one of those UHP's. What exactly is the technology of those lamps and their power supplies. Mine seems to require a minute or so warm up for the power supply before the lamp will strike / light. Are they actual arcs or more akin to a metal halide? Any info appreciated.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-12-2003 09:45 AM                     UHP stand for Ultra High Pressure and they are a metal hallide lamp conatining different metals which is why they have a colour warm up and don't hot restrike well Some of the powersupplys take time to build up sufficient no load voltage to make them strike  |  IP: Logged Jeff Taylor Jedi Master Film Handler Posts: 601 From: Chatham, NJ/East Hampton, NY Registered: Apr 2000 posted 05-12-2003 09:56 AM                     As always, thanks Gord. Should I assume that like a xenon they don't take kindly to frequent strikes and short operating cycles?  |  IP: Logged

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