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Discussion Starter #1
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.



Here's the bottom of the disc with the threaded hole in the center.



The discs were epoxied using JB Weld epoxy, which sticks really well to glass.
 

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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.
 

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Discussion Starter #4

Quote:
Originally Posted by Eroc_FX /forum/post/15251364


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.
 

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Quote:
Originally Posted by BungalowFilms /forum/post/15247655


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!
 

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Discussion Starter #6

Quote:
Originally Posted by Person99 /forum/post/15265155


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?
 

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



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



Oh, and it.. is... HUGE



-Joel
 

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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.
 

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Quote:
Originally Posted by Steve Scherrer /forum/post/15273599


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.
 

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Quote:
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
 

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Any progress BungalowFilms?
 

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Quote:
Originally Posted by xenepp /forum/post/15281098


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.
 

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Discussion Starter #13

Quote:
Originally Posted by oman321 /forum/post/15281413


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
 

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Quote:
Originally Posted by BungalowFilms /forum/post/15281674


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.
 

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Quote:
Originally Posted by Person99 /forum/post/15281488


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
 

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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.
 

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Quote:
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
 

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Quote:
Originally Posted by phisch /forum/post/15287913


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."
 

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Discussion Starter #19
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).









 

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Quote:
Originally Posted by BungalowFilms /forum/post/0


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
 
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