CurtPalme.com Home Theater Forum

mandoloid · Joined: 25 Mar 2009 Posts: 2

Hi folks. I'm a newbie to this site with a sightly off topic question and hope I can benefit from the brain trust here. I've been trying to make a medium format (slide) projector for in-home display. I already have a projector that will handle my 6x9 slides (about a 4" diagonal) but the image it throws from 12' is only about 2'x3'on the screen. Hoping to increase my image size I've been looking at wider CRT and LCD projector lenses to use. Well, I just picked up a lens from a Barco SD2100 LCD projector with a 1.7:1 throw ratio and I expected to get a projected image of about 7' from 12' away. Didn't happen. My focused image is still at 2'x3'. Can somebody explain how I've mis-calculated and/or point me to a lens that might work for this application? Thanks in advance. Confused

perisoft · Joined: 29 Aug 2007 Posts: 2920 Location: Ithaca, NY

Hmmm... it seems like a CRT lens (HD8, HD145 or similar) would give you what you're after. HD145 would be sharper... you won't get 1:1 ratio but it should be better than 3' wide at 12' - probably something around 6'?

ecrabb · Posted: Wed Mar 25, 2009 2:58 pm Post subject:

I'm guessing the lens (and the 1.7:1 throw factor spec) for the Barco was designed for a 1" diagonal or so LCD panel. To fit the image circle, your medium format slide is probably focused MUCH further away from the back of the lens than would be the LCD imager, so you're getting a lot less enlargement. A 35mm slide would probably be about right for the Barco lens, and would be able to be much closer to the back of the lens.

Like Perisoft said, a lens for a CRT projector would probably be more appropriate size-wise... Of course, unless you buy a really expensive lens for a high-end 9" CRT like the HD-10, you'll probably lose a lot of the resolution contained in the big 120 transparency... and isn't that the point?

If you want to experiment with a CRT projector lens, an HD-144 or HD-145 is probably your best bet, as I think they're probably about the best air-coupled lens you'll find. Of course, you're still going to need a hell of throw to get a decent-sized image... On a CRT projector with say, a 7" diagonal 16:9 image on the CRT face, we need about 10 or 11' feet of throw to get an 8' wide image. Your projected image will probably be about half that size unless you double the throw.

SC

**AnalogRocks** · Posted: Wed Mar 25, 2009 3:17 pm Post subject:

I always wanted to try this. I look forard to you results

jkruger · Joined: 24 Oct 2007 Posts: 2435 Location: Carlsbad, CA

Mandoloid, There should be lots of guys with lenses you could borrow. Where are you located?

**AnalogRocks** · Posted: Wed Mar 25, 2009 3:59 pm Post subject:

He's around Kalamazo or Grand Rapids Michigan I belive.

mandoloid · Joined: 25 Mar 2009 Posts: 2

Hey everyone, thanks for your replies. This really helps out. At first I was elated at "discovering" all of these huge, fast lenses from old CRT projectors but the variety and applications are really bewildering to sort out. Thanks Perisoft and Ecrabb, I actually have an HD-8 on the way and a 6' image would be a vast improvement. I thought it might work for a 4x5 slide projector as well with it's 6 1/2" diameter. Do you know the throw ratio for it? I thought it important to match the negative size to the lens' rear element size to get the largest possible image. My Barco has a 4 1/4" rear element which nicely matches the 6x9 tranny but it also has an EF of 13 3/4" so whatever I project with it, 4x5, 2x3 or 35mm has to be 13 3/4" behind the front element. I suspect it would not cover 4x5 and 35mm is pretty tiny. The HD-8 I believe has a CRT tube right up against its rear element ( i.e. very short EF?) so maybe it will work better for me. AnalogRocks, I'll be certain to let you know which set up works well. BTW Jkruger I'm in northern Michigan. Kind of out of the way. What about a lens for an RPTV? Thanks again.