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barrel distortion vs keystone correction

post #1 of 13
3/23/07 at 5:34am
Thread Starter 
Lesser of two evils?

My setup...

Panamorph UH380
Optoma H78DC3
120" wide 2.37 screen

With the current projector location (over the first row) I get about 1" of barrel distortion on the top and bottom edges. I can move the projector further back to increase the throw ratio and decrease the barrel distortion, but that would place the projector in the aisle between the first and second rows. Because of my riser, If I were to do this I would have to also raise the projector up so no one bumps into it with their head. My screen is as high as it can go so that would mean tilting the projector down and using the projector's keystone correction. What's worse? The barrel distortion or having to use the keystone correction?
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post #2 of 13
3/23/07 at 5:43am
Keystone is worse. Don't tilt the projector.

I agree that barrel distortion is annoying. I built a new sliding projector mount so I could slide the projector further back and increase my throw distance for the Panamorph. I ripped off the idea from Monkeyman here on the forums. I think he has a gallery here.

1" of barrel isn't that bad for your size screen. The thing that bothered me was the geometry distortion near the edges of the screen. I saw warped skyscrapers, and started building my mount that weekend.
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post #3 of 13
3/23/07 at 7:35am
Keystone is definitely worse than barrel distortion. If the geometry of objects in the frame isn't warped (like Jeff's skyscraper example) and your primary objection is that the edges of the image don't align with the edges of the screen, consider masking a small amount of picture out to make eveything look even. The amount lost will undoubtedly be less than the overscan on a typical consumer TV.
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post #4 of 13
3/25/07 at 1:26pm
Quote:
Originally Posted by richh View Post

Lesser of two evils?

My setup...

Panamorph UH380
Optoma H78DC3
120" wide 2.37 screen

With the current projector location (over the first row) I get about 1" of barrel distortion on the top and bottom edges. I can move the projector further back to increase the throw ratio and decrease the barrel distortion, but that would place the projector in the aisle between the first and second rows. Because of my riser, If I were to do this I would have to also raise the projector up so no one bumps into it with their head. My screen is as high as it can go so that would mean tilting the projector down and using the projector's keystone correction. What's worse? The barrel distortion or having to use the keystone correction?


What is your current throw ratio?

Tom
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post #5 of 13
3/25/07 at 1:28pm
Quote:
Originally Posted by jeffleonard View Post

Keystone is worse. Don't tilt the projector.

I agree that barrel distortion is annoying. I built a new sliding projector mount so I could slide the projector further back and increase my throw distance for the Panamorph. I ripped off the idea from Monkeyman here on the forums. I think he has a gallery here.

1" of barrel isn't that bad for your size screen. The thing that bothered me was the geometry distortion near the edges of the screen. I saw warped skyscrapers, and started building my mount that weekend.

What throw ratio were you first using when you had that bad geometry problem?

Thanks,
Tom
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post #6 of 13
3/25/07 at 6:26pm
Quote:
Originally Posted by Tom Monahan View Post

What throw ratio were you first using when you had that bad geometry problem?

Thanks,
Tom

Not exactly sure. I know I was under the 1.65 magic number...probably 1.3 or so. And it wasn't too bad, just annoying to me. I don't think anyone else really noticed. With home theater, ignorance is bliss. And it's cheaper too!

EDIT: Saw your other post. My max throw in my room was around 1.85. I think you'll be fine with your 1.7 ratio.
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post #7 of 13
3/26/07 at 5:24am
Thread Starter 
Quote:
Originally Posted by Tom Monahan View Post

What is your current throw ratio?

Tom

With the current location, I'm at 1.65.
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post #8 of 13
3/26/07 at 7:29am
richh,

Do you notice barrel distortion when watching program material on your screen? Also, do you have any problem with the sides of your picture also?

Tom
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post #9 of 13
3/26/07 at 1:47pm
Thread Starter 
Quote:
Originally Posted by Tom Monahan View Post

richh,

Do you notice barrel distortion when watching program material on your screen? Also, do you have any problem with the sides of your picture also?

Tom

Really can't say... the projector has been up for awhile, but I just installed my screen . The sides do have some barrel, but it's not as noticeable as the barrel on the top and bottom. I just put up a velvet covered border and it does very well at masking the edges. I do need to do some tweaking though; when I zoom enough to mask the top and bottom barrel, the sides extend beyond the border.
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post #10 of 13
3/26/07 at 5:23pm
I'm using an H-1400M with an upgrade to the front element, and I get a little barrel distortion when in 2.35 stretch mode. The outside bottom edges of the screen bow down about an inch below the middle, and the bottom sides of the screen bow out a little. The top of the screen seems to have very little/if any barrel distortion.

It really doesn't bother me much, and friends and family didn't even notice. However, I will be adding some black velvet masking to my DIY screen soon to take care of it. I also plan on making some adjustments to get my projector as closely lined up with the vertical middle of my screen as possible. BTW, my throw ratio is 1.8 on a 126" diagonal 2.35 screen.

More importantly, I emailed Annsi on this subject and here was his reply.

Quote:


"Hi Mike,

There will exist some geometrical distortion to the image, which is throw dependent. Longer throw gives less distortion. In order to make distortion equal for both upper and lower legs of picture, you have to tilt the lens to the direction of the beam. Doing this it will also be less visible.

Masking the edges or use of curved screen makes the small distortion invisble.

Best Regards,
Anssi Leppanen

Overall I am still very happy with my Prismasonic lens, and think with proper projector adjustment and masking the relatively small amount of distortion will be a non-issue.

Mike
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post #11 of 13
3/27/07 at 5:27am
Do you guys notice any distortions within the image itself (not on the edges)? Items that should be straight, do they appear to be slightly curved?

I'm considering the 1500R with a throw of around 1.5 to 1.6 (I can't move the PJ back any further), but I'm concerned about distortions.
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post #12 of 13
3/27/07 at 4:59pm
I'm also looking at trying that throw out on my Prismasonic. Only problem is I'm on the RS1 wait list and can't find out if it will work until who knows when... If you have any luck with that keep us informed. I'd like to use that throw so I can squeeze a bigger screen into my room.
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post #13 of 13
3/27/07 at 8:17pm
Quote:
Originally Posted by Wet1 View Post

Do you guys notice any distortions within the image itself (not on the edges)? Items that should be straight, do they appear to be slightly curved?

I'm considering the 1500R with a throw of around 1.5 to 1.6 (I can't move the PJ back any further), but I'm concerned about distortions.

No within image distortions at all in my setup.
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