I thought I'd start a new thread dedicated to DIY lenses built from trophy prisms. Others have posted some really great info but a lot of it was buried down in the 'Prism Sources' thread. I encourage others to post info about their DIY lenses here as well.

I'm using 4 x 6 prisms from Precision Crystal. I mocked up the prisms in front of my projector on a temporary stand just to see how they worked out. I've got to admit I'm pretty impressed with the results. For $58 bucks including shipping it's a bargain. But they're not perfect, there are trade offs. There are very faint 'ghost' reflections visible at times on the screen. Most often when there is a very bright object against a dark background. You can especially see it the during credits and some title sequences. (I've subsequently masked off the ends of the prisms and these ghost images have gone away.) CA has not been noticeable on normal projected moving images and I've not yet projected a grid.

I have a fairly long throw ratio and the 4x6 prisms are big enough but only just barely. My projector is a Panasonic PT-AX200U on a curved DIY acoustically transparent screen.

I've taken a slightly different approach to mounting the prisms, rather than glue a triangular plate with a bolt sticking out of it to each side of the prisms, I've opted instead to epoxy a 3 inch diameter, 1/2 inch thick plexiglass disc to only one side. The disc has a 1/4-20 threaded hole tapped in the center so the mounting bolt will come in through the outside of the housing, rather than protrude from the inside. The wide disc should form a fairly stable platform and keep the prism aligned perpendicular to the housing. An added advantage will be that I can remove one side of the housing to clean the prisms without disturbing their position, plus I don't have to worry about precisely aligning either the bolts on each side of the prism, or the holes in the top and bottom of the housing.

Here is a picture of a rough setup now that the discs have been epoxied to the prisms. I blacked out the edges of the prisms with an alcohol based opaquing fluid.
http://i157.photobucket.com/albums/t63/BungalowFilms/home%20theater/AL_setup.jpg

Here's the bottom of the disc with the threaded hole in the center.
http://i157.photobucket.com/albums/t63/BungalowFilms/home%20theater/AL_disc_bottom.jpg

The discs were epoxied using JB Weld epoxy, which sticks really well to glass.
http://i157.photobucket.com/albums/t63/BungalowFilms/home%20theater/AL_epoxy.jpg

Very nice work. So far, this is really smooth. I am pretty compulsive and this is looking like something I might consider doing (althought I usually just read these threads vicariously). I am looking forward to the rest of the build.

Where do you get alcohol based opaquing fluid?

Eroc

Where do you get alcohol based opaquing fluid?

Eroc

It use to be sold by companies that handled printing supplies. It was used to opaque areas in printing negatives. But it's been more than a decade since I've bought any so I'm afraid I can't point you in the right direction. (It does have a pretty good shelf life though. The fluid I have was made by Deitzgen, but I'd heard they discontinued it (that's why I stocked up so many years ago). Another brand that I was aware of was "Black Satin Negative Opaque". You might do a search.

The thing that makes this stuff so good is it had a very high carbon content and is very opaque, even in a thin film. Most paints aren't really that opaque because the base binders are transparent or translucent. Plus if you make a mistake it comes right off with a little alcohol, but not with water based lens cleaners.

Most often when there is a very bright object against a dark background. You can especially see it the during credits and some title sequences. (I've subsequently masked off the ends of the prisms and these ghost images have gone away.)

Completely eliminated or "mostly" eliminated?

Do you plan to use them in a rotating "stretch-pass" config or a movable sled config? If sled config have you checked for vertical image shift introduced by the lens?

Thanks!

Completely eliminated or "mostly" eliminated?

Do you plan to use them in a rotating "stretch-pass" config or a movable sled config? If sled config have you checked for vertical image shift introduced by the lens?

Thanks!

The major ghost relections that came out of the ends of the prisms are complety gone. I've not notice any others yet, but I've not had the chance to look at things like credits or titles where they would be most obvious. I'll report back when I do.

The lens will be mounted on a movable sled. Until I get the projector mounted back up onto the ceiling and off the table I don't know how bad the vertical shift will be or what the solution will be. How have you others out there dealt with it?

I have just knocked my lens up in a box and it looks awsome :eek:

I can't believe something like this is available to the average joe with under £50 to spare.

That price includes the screen btw. I bought some timber, metal mounts, corner fixings, BOC and black masking material for about £30. I've not put the screen together yet but holding it up it's far better than even the InFocus screen I'm using now! The picture looks at least 20% brighter :eek:

Oh, and it.. is... HUGE :D

-Joel

Welcome to the club! Yes, there are pretty expensive alternatives that have correction features and optical coatings, etc. But the price difference for what I believe are pretty incremental advantages is simply amazing.

Welcome to the club! Yes, there are pretty expensive alternatives that have correction features and optical coatings, etc. But the price difference for what I believe are pretty incremental advantages is simply amazing.

I don't think I'd go that far. I'd call the difference between the top of the line Prismasonic and the ISCO III incremental. The differences between either of those and a DIY or low cost 2 prism solution are night and day.

True, it is satisfying to many and that is great, but to call the advantages of the very good lenses "incremental" is a gross misrepresentation.

to call the advantages of the very good lenses "incremental" is a gross misrepresentation.

I actually think you are possibly over stating that a bit. I can't see any flaws in my setup at all (maybe it could look a bit tidier?) but now that I have my screen up I can honestly say I have no desire to swap it out for a commercial lens and see no real reason to.

I had just one reflection on the screen caused by light reflecting back into the projector lens but one small strip of masking tape later and the screen is reflection free.

I also think the focus is pretty sharp too which I was expecting to go right out.

I'm by no means a video projection engineer but I have been in this game for long enough to know this simple solution, if executed properly, is a great alternative to commercial products and can produce fantastic results.

-Joel

Any progress BungalowFilms?

I have been in this game for long enough to know this simple solution, if executed properly, is a great alternative to commercial products and can produce fantastic results.

I completely agree. This is certainly true in many areas.

I don't know what your anamorphic solutions is (PC trophies or what), but have someone bring over an ISCO III and compare them. There is NO way you won't see a difference.

I didn't say that a DIY or simple 2 prisms is not decent or not good enough for the vast majority of people, I just said that the difference between an ISCO III and a pair of trophy prisms is not "incremental". If you really believe that, you have not really compared them.

BTW, here is my simple rule: if my wife can't see an improvement that I can see--it is incremental. If she can see a difference--it is larger than incremental. ;)

Any progress BungalowFilms?

Right now I'm in the process of building a new DIY projector mount as the commercial 'universal' mount that I had was universally wobbly it was impossible to fine adjust the alignment. Hopefully I'll get it done this weekend then I can get back to working on the lens.

Dave

Right now I'm in the process of building a new DIY projector mount as the commercial 'universal' mount that I had was universally wobbly it was impossible to fine adjust the alignment. Hopefully I'll get it done this weekend then I can get back to working on the lens.

Dave

Definitely curious to know what you find. Also, are you still using the Panny 720p LCD or a higher CR machine? I think with the low CR of the 720p LCDs, reflections are harder to see. Definatley try some bright white credits on a black background.

I'm very curious to hear what you find.

BTW, here is my simple rule: if my wife can't see an improvement that I can see--it is incremental. If she can see a difference--it is larger than incremental. ;)

Well that I can agree with ;)

-Joel

Dave,

Also, I will reiterate my request here again, which I already posted in the "Prism Sources" thread:

Could you please comment on picture sharpness while using the prisms? with panny's "smoothscreen" technology, the picture is somewhat soft compared to pj's without it, and now with added prisms, the projected image would be softer than before. A few screenshots with and without the prisms would be helpful. Thanks.

BTW, here is my simple rule: if my wife can't see an improvement that I can see--it is incremental. If she can see a difference--it is larger than incremental.

I use the same criteria, unfortunately my wife seldom notices an improvement:( - or else she won't admit to seeing a difference so I won't spend any more money:)

I use the same criteria, unfortunately my wife seldom notices an improvement:( - or else she won't admit to seeing a difference so I won't spend any more money:)

When my wife does she a difference, she always caveats it with, "but not enough to justify the expense."

So I've put together some A-B comparison screen shots using the trophy prisms. The following are the relevant pieces of equipment involved:

Panasonic PT-AX200U Projectors (720p)
directly connected via HDMI to
Panasonic DMP-BD35 Blu-Ray Player (Blu-Ray discs used)
DIY Shearweave Projection Screen, 2.37:1, 120 inches wide - curved
4x6 Prisms from PrecisionCrystal.com

For each comparison the playback was paused on a frame where the image was in focus and free from motion blur. The camera was setup on a tripod with the same manually set exposure used for all shots. The exposures were made in RAW format with no compression or post processing until the final save as a jpeg. Each freeze frame was shot first using the Zoom and Image Shift method, then for the anamorphic lens the projector was reframed back to 16:9, set to vertical stretch and refocused using the built in focus chart.

Each comparison shows a section cropped from the approximate center of the screen shot.

The results are, I'm afraid, not that good for the prisms! In each case the image projected through the prisms is not as sharp, or as bright, as the zoomed image. What's more the prisms introduce a very slight overall blue tint which is visible in the skin tone. There is also a fairly noticeable blue chromatic abberation visible on the right side of the white-on-black title. (To be honest, there is also to a lesser extent, a little blue CA visible in the zoomed image, which may be what you can expect from a lens on a projector that only costs $1,300).

http://i157.photobucket.com/albums/t63/BungalowFilms/Screen%20Shots/CARS_02_COMP.jpg

http://i157.photobucket.com/albums/t63/BungalowFilms/Screen%20Shots/CARS_03_COMP.jpg

http://i157.photobucket.com/albums/t63/BungalowFilms/Screen%20Shots/PIRATES_02_COMP.jpg

http://i157.photobucket.com/albums/t63/BungalowFilms/Screen%20Shots/PIRATES_01_COMP.jpg

http://i157.photobucket.com/albums/t63/BungalowFilms/Screen%20Shots/PIRATES_03_COMP.jpg

The results are, I'm afraid, not that good for the prisms!

This just only reinforces a point I've recently started to talk about on AVS - optical surface finish. Even though these prisms used "appear" optical grade, the thuth is they are not as they were never designed to be optical components, but trophies.

The glass type may very well be BK7 and finshed to an optical finish, but with out knowing the actual specs, the claim is loose at best and if your seeing noticeable change, then sorry they are not optical grade.

CA is of course another issue and only correctable with the addition of extra elements - and no you can not just use 4 prisms together as you need to use not only a different glass type and bond them correctly, but also have to know the precise angles to make it work. This then of course takes this well away from the DIY sector.

Your comparison is very good. Now I would like to see this done with a propper lens...

Mark

Dave, different prisms have different amounts of CA. Also, the CA is dependant upon careful alignment, if the PJ's lens is not perfectly center and using no horizontal shift and the prisms are not aligned perfectly, you will get worse CA.

The CA on the right side of your image is excessive even for trophy prisms. I suspect you could get it better--but it won't be great. As an aside, with VERY careful alignment and set up, I've got an HTB lens set up so that the worst CA at the worst side of the screen is about as bad as your "S" in "Pirates". Nothing even close to being as bad as what it is on your "pearl".

Lastly, I assume you are using your PJ to do the vertical stretch. I've not looked at the capabilities of the Panny there to see how good the scaling algorithm is. A 2.35:1 movie vertically stretched on your 720p without the lens should be more detailed than it shown in letterbox on the 720p. If it is not, it is because they are not doing the scaling correct--you should look at that.

The reason being, most scope movies have a vertical resolution of the active image area of 800-817 pixels. Scaling that down to 720 should result in very little loss of detail. If however, they are downscaling to 500-something then upscaling for the stretch, you'll loose quality.

I'm currently using a 720p projector. The scaling is being performed by a Lumagen which takes a 1080p/24 signal, and scales to 720p/72 (locked) which is then sent to the projector with 1:1 pixel mapping. Given that horizontal resolution is less important to human perception than vertical, and downscaling 800 to 720 results in about the same loss of detail as upscaling 800 to 1080, my 720p actually does scope content pretty impressively (many of those in my neighborhood think it looks better on scope content than one of the other neighbors Panny AE2000).

I would say the best thing to do is use an external processor for the high quality scaling (and because it can also correct for any picture differences introduced by the lens), and try some other solutions. Solutions like the HTB or Mark's are not going to give you ISCO III performance, but for a reasonable price, they can give you much better performance that your screenshots especially if paired with quality scaling.

True, this was still a rough setup so it's not a fair demo of the CA inherent in trophy prisms. As I did watch sections of Pirates the CA, as bad as it was, really isn't that distracting, mostly visible when something dark is silhouetted against a light background.

I was really looking at sharpness issues (brightness loss and color shift can be compensated for) and there I find these particular prisms unsastisfactory and will shelve them and search for a better solution. I am not all that surprised, afterall, they only cost $50.

I am sure an external scaler would improve the quality of the image. As nice as the Panny is, it is still a budget projector and neither it's lens nor processor is going to be as good as those in some of the more expensive machines.

Only this past week have I upgraded from standard def to Blu-ray and that by far has been the most substantial increase in image quality yet.

I was really looking at sharpness issues (brightness loss and color shift can be compensated for) and there I find these particular prisms unsastisfactory and will shelve them and search for a better solution.

I have found the ghosting with the "French" prisms to be bad, but their focus preservation was pretty good--since the native panel CR of your current PJ is low, they may work for you.

The current HTB prisms actually have pretty good focus preservation. They have a 15 day return policy if you try and don't like.

Once properly masked, i found the ghosting with the trophy prisms to be almost non-existent. I only saw a little bit on the front side wall of my theater, well off the screen. Not really an issue.

Are the 'French' prisms still available?

On another issue. There was a fairly bad vertical shift (downwards) with the prisms in place. How do people deal with that?

Which trophy prisms are you using? I've got the 4"x6" versions from Precision Crystal and I'm not seeing any vertical shift at all. With my setup I do get some pretty bad ghosting, though, even after having masked the edges of my prisms with black electrical tape. I also painted the inside of my lens casing black, and went so far as to try sticking black velvet on the inside walls of the thing, too, just to see if that worked any better than the black paint. It didn't -- at least not that I could tell. I plan to keep monkeying with it to see if I can make things any better, but for me, at least, it's still nowhere near bad enough to just scrap the whole project.

Are the 'French' prisms still available?

Yes.

On another issue. There was a fairly bad vertical shift (downwards) with the prisms in place. How do people deal with that?

very common with prisms. Basically, you need to try them in every combination of orientation with regard to each other. Swap the front and back. Turn one over 180 degrees, etc. If you are lucky, you'll find a combination with little to no vertical shift. Although more often then not, there will still be a little, so you will have to use your PJs vertical shift to fine tune if you are using them with a movable lens.

Which trophy prisms are you using? I've got the 4"x6" versions from Precision Crystal and I'm not seeing any vertical shift at all. With my setup I do get some pretty bad ghosting, though, even after having masked the edges of my prisms with black electrical tape. I also painted the inside of my lens casing black, and went so far as to try sticking black velvet on the inside walls of the thing, too, just to see if that worked any better than the black paint. It didn't -- at least not that I could tell. I plan to keep monkeying with it to see if I can make things any better, but for me, at least, it's still nowhere near bad enough to just scrap the whole project.

Part of the problem with ghosting is that we are not all comparing apples to apples. On a 720p LCD (which has a poor native panel CR), the black is so elevated, ghosting does not look nearly as bad as it does on a PJ with a native CR of even 4000:1 to say nothing of something like an RS2. Elevated blacks hide or diminish the impact of ghosting.

I've only got a calibrated 3800:1 on my current PJ and I can't stand any ghosting. I did scrap the DIY prisms due to the issues.

To BungalowFilms - Great comparison! I really appreciate that you to the time and effort to make a well controlled and objective comparison.

Now I would like to see this done with a propper lens...

Mark

When can you start? ;)

I did these tests with my projector mounted back on the ceiling and the prisms mocked up on a rig held in front of the projector by a very sturdy tripod. I found it interesting that tilting the prism assembly didn't change the vertical shift but rather distorted the shape of the projected image. I'm sure I could have monkeyed around with the whole rig a bit more to make the shift less noticeable. But neither the vertical shift nor the ghosting are much of an issue (okay, I'm not seeing any ghosting to speak of so there's no issue there). For me, it all comes down to image sharpness, and since the image projected through the prisms is not as sharp as the zoomed image, I'm going to scrap this particular set of prisms. But that's not to say I won't tinker around with something else.

As to my setup, the rotational centers were 4 3/4 inches apart with the projector shooting into the square face of the prisms. In these tests the prisms were only mounted to a base, there was no housing around them (yes, there were major reflected images on the side walls back by the projector but I knew these could be easily masked later).

As to my setup, the rotational centers were 4 3/4 inches apart with the projector shooting into the square face of the prisms.

What do you mean by this? Was one of the faces of the prism parallel to your PJ's lens? If so, it should have been at an angle to the PJ lens.

Yes, sharpness is an issue with the trophy prisms. The "French" prisms are better at preserving sharpness.

To BungalowFilms - Great comparison! I really appreciate that you to the time and effort to make a well controlled and objective comparison.



When can you start? ;)

I need to work out how to use the Raw setting on my camera first ;)

My new lens is not too far away (hoping). I can't zoom with the BenQ. It simply does not allow this, so I could not really make the comparison a fair one...

Mark

Sorry Person99 for the confusing post. Hopefully this drawing clears it up. I meant that the square corners of the prisms were closest to the projector lens. BTW, the rotated angles of the prisms shown in the drawing are only approximate.
http://i157.photobucket.com/albums/t63/BungalowFilms/home%20theater/Prism_layout2.jpg

To BungalowFilms - Great comparison! I really appreciate that you took the time and effort to make a well controlled and objective comparison.

Yeah ditto! Thanks Dave. Although trophy prisms have this in-built flaw of transmitting somewhat an "un-focused" image, It will amplify this depending from what source it's being fed, here, namely the Panny with "smoothscreen" thechnology. That's why I wanted you to comment on it, and I'm sorry to hear that the trophy route didn't workout for you. I've heard this same thing from a couple of other Panny owners. I'm not sure why the French prisms work "better" at focusing than trophy ones but I'm guessing it's because of their angle is 20 degrees vs usually about 14 degrees on the trophy wedges. This heigher angle makes for a less displacement of the prism leading to less bending of light and in turn less "de-focus"???? someone correct me if I'm wrong.

.... My new lens is not too far away (hoping).

Mark

Glad to know that Mark, Hope it comes out soon. Now you have them corrected for CA but what about the "Corrector"? is it anywhere in the equation yet or maybe sometime down the road? :-) you also mentioned a cylindrical lens as well, what's all that about?... Horizon?...

Sorry Person99 for the confusing post. Hopefully this drawing clears it up. I meant that the square corners of the prisms were closest to the projector lens. BTW, the rotated angles of the prisms shown in the drawing are only approximate.
http://i157.photobucket.com/albums/t63/BungalowFilms/home%20theater/Prism_layout2.jpg

Dave, couldn't help noticing but in your drawing, the prism nearer to the pj has the broadest face surface, facing away from the pj. Try it the other way around where you let the broadest surface face towards the pj and the right angle becomes inwards. The other prism's orientation looks ok. By doing this, I'm sure it will lessen the CA that you're seeing to quite an extent. As Person99 pointed out, do play around with them some more before completely shelving them away.