1. Lamp type - 600w metal halide conversion
2. Distance to rear fresnel - 210mm
3. Rear fresnel focal length -220mm
4. Distance between front and rear fresnel -30mm
5. Front fresnel focal length -550
6. Distance from front fresnel to LCD -35 mm (unsplit design)
7. LCD diagonal - 17
8. LCD model - Hyvision MV177
9. Distance from LCD to projection lens - you name it, I've tried it
10. Focal length and diameter of projection lens - 18 inch AO opaque pj lens
11. Throw distance to screen - 11'5"

Why can't I get the center of the screen focused? I do have a folded light path, so I am trying all sorts of tiny adjustments to my fs mirror; it sounds like this has been the problem for other people. I even tried a different projection lens (the lotta lens from sciplus) and that wouldn't allow center focus either. Could my fresnels be causing this even though they are behind the lcd? (relative to projection lens) Using a different portion of the fs mirror dosn't seem to help either. Thanks for any help.

Can the fresnels be moved closer together? I don't know if this effects focus but I'm sure I've seen others state that 15mm is enough gap. Have you checked the setup that others have used with the opaque lenses?

Maybe a pro fresnel (650mm) might be better?

When you say you can't get the center of the screen focused do you really mean you can NEVER get the center of the screen focused? Or do you mean you can have either the periphery OR the center in focus but never the whole screen?

Edit: Err . . . I just noticed the post subtitle but my question still stands. And how is your mirror mounted?

Yep, I really can never get the center focused even when I disreguard the rest of the picture. The mirror is is mounted to a board via modified screen clips. It does seem like the mirror can be the only problem doesn't it? I've gone ahead and ordered another mirror and I'll try to mount it more effectively. In the mean time, more tweaking..

I'll try moving them closer together, but I was hoping the unsplit design with fresnels before the lcd would exempt me from fresnels causing focus issues. I've noticed one odd thing as far as brightness goes; there's about a 2 inch space I can move my fresnels back and forth through and I notice almost no brightness change. It's as if I can put the fresnels anywhere and they work equally effectively (or defectively if that's the case). I wonder if it's because I have quite a long arc and a big lens to match. I might have thought the big arc would cause focus issues, but there are areas of the screen that come into perfect focus (namely the right,top, and bottom) But the middle, and left never come into focus.

There's also always the trick of rotating the triplet whilst viewing the projection to see if the focus issues track with rotation of the triplet. Process of elimination can be painful - not only for kidney stones.

and black snakes inside your realm

Thankfully I only have problems to eliminate
I did try rotating the triplet and had no luck. The lens did fine when I had it in the opaque projector. Here's a pic of the whole projection. It's a pain to move the triplet laterally, so that's why the left corners are so much darker than the right in this picture.

I can't seem to get the middle to have the focus or the contrast that the edges do. Could the ikea reflector cause this? I think I'll try without a reflector at all next time.

pics of my rig

Click to view attachmentfocused by moving whole plywood slab on the threaded rods

My suspicion now lies with the projection lens. I am largely ignorant of the suitability of that particular lens for use in our projectors but I believe some have used them successfully. Have you looked around for some plogs using that same lens? I'm wondering if that lens can handle a 17" LCD???

Edit: Wow! It's a monster! Just in case we're dealing with multiple problems here can you lie the projector on its side and test it in straight-shooter mode?

yeah I'm sure it dosn't have the fov a 17'' needs, but I can live with that no problem. I really can't live with the center not being as focused as the edges. My plan was to just not use the edges of the projection... if you cut a 17" screen from 1280x1024 to 1024x768, it's only about 13" instead of 15"

Aha! I see. So, based on past mysteries I'm left with these two observations:

• Others have found mirrors to be problematic in a fashion similar to what you are experiencing. Try to eliminate that as the problem. Can you temporarily rig it for straight-shooting?
• You do have a large amount of space both between your fresnels and your fresnel and LCD. Not saying that's the problem or may even be the problem - it just is not typical.

Perhaps someone else will show up and decipher this problem.

Edit: It's been presented to me that perfect - and I mean perfect - alignment can alleviate a lot of problems. Make sure everything is as near perfectly aligned as possible.

Edit 2: Very suspicious that you can NEVER focus the middle.

Please verify that the fresnel rings are facing each other. It appears as though the field fresnel rings are facing the LCD, when in fact they should be facing the lamp.

...well, that's my guess anyway...

cheers,
gs

What is the lamp arc length?

That is an odd duck ain't it? 600W MH conversion. Of course, I don't retain that stuff well. Maybe it's not so odd after all?

paladin, what's your suspected cutoff for arc length to start to see the problems to which you allude with your question?

Gut feel is 45 to 50mm. 550/220 is a mag factor of 2.5.

Wow thanks for all of the help everyone!

I quadruple checked; the fresnel rings are indeed facing each other.
My lamp arc is a 61mm I believe. That means about a 1/2 inch of spillage at the triplet assuming the projector gremlins are kind. So if the arc length is the problem, could a lightgate fix it?

At least I have a new to-do list for tinkering thanks to you guys:
1) LCD closer to fresnels
2) check for perfect alignment
3) try straight shooter mode to see if the mirror is the problem
4) try without reflector
5) possibly construct a light gate?

On the light gate subject, I admit I'm not well versed. But, would just having gates on two sides work? (so the fresnels never even get to "know" I have such a long arc)

I suppose a constriction (lightgate, iris) might do it but you'll be sacrificing brightness.

I hope you can turn this problem around. If not you're just a new lamp away from possible better results.

Yep. You could cut off the ends of the arc. It would mean sacrificing lumens, but if you made it adjustable like I did with my Iris, then you could come to a middle gound.

Do the corners of your projection really look that dark ? ... or is it just the picture ?

gs

I am no expert but in looking at your specs and pics what I see is that you have too much distance between your fresnels and LCD. I am also considering a folded build similar to yours using a 17" Westinghouse Widescreen and 18" triplet. I will be instead using a 330/650 fresnel combo in most likely an unsplit design since I don't need keystone.

It's hard to tell, but make sure you don't have any pressure on the FS mirror that would cause it to flex such as being screwed in too tight on the mirror retainers. I think it was darkmeat that had the same issues and was resolved by correcting the mirror mounts.

I have two 18" triplets and plan to experiment with one configured as a varifocal for a higher degree of adjustability. Good luck with the troubleshooting. I am watching with great interest. I have run across other 17" LCD and 18" triplet early in my LL research that have been implemented successfully. I am sure you will get there too!

I hope to start soon. Time for this has been a rare commodity.

David

adjangs,

Check out this link from allinbox. It's a 17" Hyundai LCD with an 18" Beseler and 220/550 fresnel like yours. I am not sure what the difference is between the Beseler and the AO triplets. Should be very close. At least you can have some confidence that your chosen configuration should work.

If I read it right he is projecting from approx 11.5 feet with an image of over 7 feet.

Lolos Box translated from French via Google.

David

You were definitely right about the mirror clip; too much stress for the mirror even with the board behind it. I finally got the center to focus today. I'm by no means done tweaking this thing, but I'm a LOT closer now.

You mentioned you're planning on using a 17" widescreen.. I think that will push the limits of the FOV of these lenses slightly farther even than a standard 17". I'm finding that my 18" lens is adequate for a 17" standard panel but it could be better. I hope I can show you some screens to give you a better idea of what I mean. I'm hoping that'll just take better alignment now. Good luck with your build, and thanks a bunch for your help.

They weren't as bad as the pic showed, but now they're even better with the mirror mounted closer to the lens. I'm now wondering if a bigger mirror would help even more. It's hard to tell since the projection reacts so seemingly goofy to objects being placed in the light path. IE stick a ruler infront of the mirror and the image just gets a little darker instead of seeing a shadow of the ruler. I suppose it has to do with focal planes and such.

I think I might've jumped the gun on the necessity of a light gate; I'm definitely going to keep it in mind though.

The Westinghouse is approx 15x9.5 where a typical 17" 4:3 lcd is ~13x~10. I too am concerned with the width. I figure that I could cheat it by using the phase/clock adjustment on the LCD to help it "fit" along with the experiment with the varifocal technique on the 2nd triplet.

Since the LCDs are free to me, the proof of concept is relatively cheap. From there I'll evaluate whether to go with a smaller LCD or Pro Triplet. I hope not to do either if it costs money. I have gotten at no cost a 19", 18" panels and the 17" ws with bad backlights or PSU. Maybe I'll run across a 17" 4:3...

I am going to order my fresnels this week. I was so excited about your post. I called my neighbor last night to borrow his router/table saw and chop saw to make my frames. I hope to get started on the build this week.

David

best focus yet

widescreen DVD style