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Discussion Starter · #1 ·
Some guy experimented and turned his 50" LCD Rear projection tv into a 100" rear projector simply by removing the screen and building an enclosure with the new larger screen a foot or two further than the original screen.


When he first took off the screen to find out if it would work he said the picture filled his entire walls from 6 feet back and was fully in focus, clear and bright. So, he went forward with the project.


Ok, this surprised me that it stayed in focus even from 6 feet away. The size had be nearly 200" from that far. When I did this with my CRT HDTV I had to refocus the lenses and redo convergence, but they use a totally different kind of lenses and projector.


So, does any knowlegeable person know if a 1080p DLP RPTV would work the same and stay in focus, at least a couple feet from the screen?


I have Sharp ET20U super short throw projector that can vary pretty greatly in size and still be in good focus and it's not as short throw as a Rear projection tv.


So, do you guys think maybe a super short throw lens allows for focus to be retained more at much bigger sizes?


Anyone with a DLP (or a newer LCD rear projection set, not flat panel) RPTV willing to simply remove the screen and test it out?
Please? lol.
 

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Frankly i doubt his claim that it stayed in focus when the throw was increased. Rear projection tv's are usually quite similar to front projection but with a first surface mirror to bounce the image off of and onto the screen within the small given space.

But as far as i'm aware lcd, dlp, and crt rear projections still have focus elements that must be adjusted if the throw is changed. But i don't doubt the rest of his claims. From what i've found, lcd and dlp rear projection tv's use the exact same lamps are front projection, so their brightness is comparable. But realistically it is cheaper to just get a front projector.
 

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Discussion Starter · #3 ·

Quote:
Originally Posted by jarrod1937 /forum/post/16927944


Frankly i doubt his claim that it stayed in focus when the throw was increased. Rear projection tv's are usually quite similar to front projection but with a first surface mirror to bounce the image off of and onto the screen within the small given space.

I know how they work already, however, rear projection sets use lenses that are extremely short throw compared to even the standard "short throw" projectors some of us have. That may change things up a bit as far as where and how it focuses and at what size.


Even without the mirror it probably throws a 100" picture from 3 feet away; something our front projectors do no come close to doing.

Quote:
But as far as i'm aware lcd, dlp, and crt rear projections still have focus elements that must be adjusted if the throw is changed. But i don't doubt the rest of his claims.

But at what degree of the change in distance before it "must be changed"? If if you had a 1.5:1 projector and you move it two feet back the focus difference is probably minimal and the size increase is only 1ft. 4inches, but with the setup they have in the RP set if you moved the screen 2 feet further you'd gain most likely 50-70 inches. So, the focus difference may be extremely minimal with the lens since the distance needed for a noticeable and huge gain in size increase is relatively small.


It got me to think of videography/photography and how a smaller aperture extends depth of field (ie; keeping virtually everything, even at long distances in focus) while a larger aperture helps achieve more shallow focus (ie; blur out the background from the foreground and vice versa). Maybe this relates here in why the focus didn't noticeably change, or change much with a couple of feet added.


Quote:
From what i've found, lcd and dlp rear projection tv's use the exact same lamps are front projection, so their brightness is comparable. But realistically it is cheaper to just get a front projector.

Where and how can I get a 100-120" 1080p DLP HDTV that can do 120hz and 3D for $1099 with higher contrast, lamps under $200 and the ability to project with a dark gray screen for dramatic ambient light contrast/black enhancement for under $50,000 (or higher) if this does work?



I believe the lamps are rated for about 6,000-8,000 hours also; much more than my lamp at half the price. Also, if I wanted a wide angle lens that was even close to the short throw of those in these set it would cost about $2,000 extra. So, cheaper....better deal? Not if this works, which is why I wanna find out, plus having superior ambient light performance.

Here are some of the posts;


# Joejubee Says:

September 30th, 2008 at 7:32 am


Can you comment on the focus-factor? I'd anticipate that the focus would get all out of whack, and be too off to be corrected using the tv's focus controls. Good hack though!


[Reply]

# BonMul Says:

September 30th, 2008 at 8:04 am


Focus is perfect. When I opened up and put the projection on the wall, I was surprised. The focus is crisp, as it was with the small screen.

[Reply]


Mr Bob Says:

October 30th, 2008 at 11:51 am


Just realized this is not a CRT RPTV, it's a single lens one. Must be LCD or something.


Modifying a lot of what I said above


This CAN be done with CRT tech, but a lot more things have to be taken into consideration. This DOES explain why the brightness is still usable, and why the focusing stays roughly the same, with such a small, single aperture


Mr Bob
 

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Discussion Starter · #5 ·

Quote:
Originally Posted by btokars /forum/post/16928720


I've seen this done. At the Optoma headquarters in Milpitas California, the have a wall set-up in their lobby. It works very well.

Awesome. BTW, I was coming back to post that I think I may have answered my own question. lol. I own the VERY short throw "Toshiba ET20U" Projector which throws a 100" image from 4.3 feet away. I kept the text menu up while playing an HD-DVD (it's a 480p projector though) and moved the projector back from about 90" wide until it reached about 16 feet wide (It's filling my entire wall) and I didn't have to touch the focus controls. This projectors isn't as short of throw as the rear projection sets either.


For some reason I'm a bit worried about plunking down the cash still, but if it works the same way then that is AWESOME. A cheap 1080p rear projector with 120hz capable of 3D.


With this set I wouldn't have to build a an enclosure using a GIANT mirror and it would project straight from the middle with no need for keystone correction since it has no offset.


Part of the image is on my 9'2" high gain silver screen and the rest of it on my white wall. My silver torus screen owns. lol. Even at nearly 16' feet wide it's very bright.....even with a little bit of light. With the right type of lighting it looks like a plasma tv. I need to building me about a 16' wide version of this screen.



If I can find or buy a new firewire card I will post pics. Does anyone know if the Samsung HL61A750 is as bright as a typical DLP set since it uses LEDs? It might be a potential great one to convert and avoid ever having to buy another lamp.
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by btokars /forum/post/16928720


I've seen this done. At the Optoma headquarters in Milpitas California, the have a wall set-up in their lobby. It works very well.

BTW, you are referring to them removing the screen of an existing fixed size tv, correct?
 

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Quote:
Originally Posted by btokars /forum/post/16928720


I've seen this done. At the Optoma headquarters in Milpitas California, the have a wall set-up in their lobby. It works very well.

Kind of funny... since I don't think that Optoma makes many RPTV's... :) and the question further down is probably very appropriate ... more than likely their designers can probably do what ever it takes to make a "large rear display system".
 

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Discussion Starter · #8 ·

Quote:
Originally Posted by budwich /forum/post/16929116


Kind of funny... since I don't think that Optoma makes many RPTV's... :) and the question further down is probably very appropriate ... more than likely their designers can probably do what ever it takes to make a "large rear display system".

I'm wondering exactly what they did as well. I'm thinking they have a short throw projector in behind a huge wall screen; possibly short throw, I dunno. However, my recent test with my short throw projector shows very promising results with focus at different distances that vary a HUGE amount in size difference.


The focus was fine from under 50" all the way up to past 210" without touching the focus again. I focused on the smaller screen then brought it back several feet and then even closer too. Focus was good.


The possibilities of this discovery are a bit exciting. I've seen a 1080p 3D Capable 120hz DLP with an LED light system go for just a bit over a grand on ebay. Now, if it can retain good focus from its original size upto or past 110" like my Toshiba ET20U with a short throw lens one could have a giant 1080p 120hz DLP rear projection system that's 3D capable, has no lamp costs and is much better in a little ambient light than front projection....for around $1,400 or less. Of course, I'm open to going with a lamp based one as well since their lamps are cheaper and I believe have a bigger lifespan on average.
 
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