Is a curved screen really neccesary? - AVS Forum
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post #1 of 38 Old 08-22-2012, 12:01 PM - Thread Starter
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I have a Panamorph UH480 coming and have pretty much decided on an Epson 6010 (I don't want to deal with JVC's bulb issues being stationed Overseas). I haven't decided on what screen to get yet (I'll ask for help in that respective forum), but obviously it'll be a scope of 2.35:1 or 2.40:1 and about 60" high by whatever is the matching width for the correlating aspect ratio. Currently, it's a living room with the 3 seat couch about 11' back facing the screen, the love seat off to the left perpendicularly and recliner that swivels to the right. I have flexibility to move the seating distance up or back. Right now, I can only mount the projector on a shelf as I have vaulted ceilings.

Since I move a lot in the military, I think it might be easier to get a drop down screen or even a regular fixed screen that'll be easier to set up and tear down to ship. But would it be really beneficial to do a curved screen? I have a budget of about $3k (not even sure if that's enough to do a curved screen) and my dealer has access to Draper, Vutec and maybe DA-Lite.

I'm also curious to know how exactly it works. If it's either a 2.35 or 2.40:1, wouldn't I have to zoom out and overshoot the frame slightly, having some overspill to get rid of the black bars (above or below, again depending on the aspect ratio) when the A lens projects a 2.37:1 image? So, if it were a fixed frame, wouldn't that take care of any GD on the outer portions of the projected image? But what if it was a curved screen?

I do really well when I can see what it looks like in person, but I think that set up is non existent here on Guam. Anybody know of any links or videos explaining/showing this?

Thanks in advance!

William
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post #2 of 38 Old 08-23-2012, 12:30 PM
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A curved screen is nice to help alleviate the pincushion effect an A lens adds to the image, but many use a flat screen and overscan the image a little to hide the curve. In your case I would get a flat screen and overscan.

I had a 2.35:1 screen and overscanned the pincushion with very little loss of the image.

If you're not using an A lens, then you don't need a curved screen. If you use a curved screen when zooming, you'll have barrel distortion to deal with and will have to overscan the image to hide it.

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post #3 of 38 Old 08-24-2012, 01:43 AM
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What about when you watch 16x9 material on a curve screen? Apart from moving the lens, Is there anything you need to adjust the image?
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post #4 of 38 Old 08-27-2012, 01:28 AM - Thread Starter
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Gary,

Thanks for the help. How much overscan is required for a 60ish" high scope, about an inch or so?
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post #5 of 38 Old 08-27-2012, 02:50 AM
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When you watch 16:9 material on a curved screen and remove the lens to do it (some leave the lens in place all the time and scale for 16:9), you will get a little barrel distortion, but it's usually small because the curve is less. You may have to zoom a little but you'll need to experiment to see how much.

I haven't used a curved screen so I couldn't say how much overscan you'd need.

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post #6 of 38 Old 08-28-2012, 04:01 PM
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Quote:
Originally Posted by biliam1982 View Post

I'm also curious to know how exactly it works. If it's either a 2.35 or 2.40:1, wouldn't I have to zoom out and overshoot the frame slightly, having some overspill to get rid of the black bars (above or below, again depending on the aspect ratio) when the A lens projects a 2.37:1 image? So, if it were a fixed frame, wouldn't that take care of any GD on the outer portions of the projected image? But what if it was a curved screen?
I do really well when I can see what it looks like in person, but I think that set up is non existent here on Guam. Anybody know of any links or videos explaining/showing this?
Thanks in advance!
William

The biggest plus for a curved screen is the geometry correction it gives (assuming it is matched to the throw ratio). I explained the 'why' with an image HERE

Mark Techer

I love my Constant Image Height system!
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post #7 of 38 Old 09-02-2012, 06:27 AM
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Originally Posted by biliam1982 View Post

Gary,
Thanks for the help. How much overscan is required for a 60ish" high scope, about an inch or so?

What is your throw distance? The amount varies, based on this distance. If your throw is around 14' then the pincushion is about an inch. If your throw is 18' it takes it down to a little more than 5/8". At 18' throw, I would use a flat screen. At 14' throw, I would use a curved screen with 36' radius.

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post #8 of 38 Old 09-02-2012, 09:22 AM
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Originally Posted by AV Science Sales 5 View Post

. At 14' throw, I would use a curved screen with 36' radius.

I'm not sure he has the room for a 36' radius. biggrin.gif

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post #9 of 38 Old 09-02-2012, 07:11 PM
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Quote:
Originally Posted by bass addict View Post

I'm not sure he has the room for a 36' radius. biggrin.gif

Why not? I'm not sure you understand. He's not talking screen size but, the radius of which the screen is curved at. smile.gif

Example: You could have a 10ft. wide 2.35:1 ratio curved screen with a 40ft. radius.
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post #10 of 38 Old 09-02-2012, 08:06 PM
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Originally Posted by Tom Bley View Post

Why not? I'm not sure you understand. He's not talking screen size but, the radius of which the screen is curved at. smile.gif
Example: You could have a 10ft. wide 2.35:1 ratio curved screen with a 40ft. radius.

I understand perfectly. I was twisting things around a bit. Apparently it was too early in the morning. smile.gif

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post #11 of 38 Old 09-07-2012, 02:41 AM - Thread Starter
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Thanks for all the input guys. Sorry it took so long to reply, been off Island and on Saipan doing some Scuba diving. Great place for that!

My room is about 17.5' deep. So take into account the depth of the projector and room for cable access, I'd say a throw of about 15.5'. But I have free range to move it up 3-4 feet if I need to.

However, I need to think about where I may be in the future, as I move a lot on the military. So I may be leaning against a curved screen, though I would really like one. After thinking about it, a pull down screen would be so much simpler to pack up, ship, and hide when not in use. And if it's only an inch or so to overscan, I think I can deal with that too.
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post #12 of 38 Old 09-11-2012, 07:06 AM
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Quote:
Originally Posted by biliam1982 View Post

Thanks for all the input guys. Sorry it took so long to reply, been off Island and on Saipan doing some Scuba diving. Great place for that!
My room is about 17.5' deep. So take into account the depth of the projector and room for cable access, I'd say a throw of about 15.5'. But I have free range to move it up 3-4 feet if I need to.
However, I need to think about where I may be in the future, as I move a lot on the military. So I may be leaning against a curved screen, though I would really like one. After thinking about it, a pull down screen would be so much simpler to pack up, ship, and hide when not in use. And if it's only an inch or so to overscan, I think I can deal with that too.

Well, if I can help you, let me know.

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post #13 of 38 Old 11-15-2012, 01:50 PM - Thread Starter
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If the A-lens stretches it to a 2.37:1 aspect ratio, would the pincushion be the same all the way around? Or would it be worse on the wider edges of the screen? BTW, I'll be using a Panamorph 480. Trying to figure out what aspect ratio screen I should get. Thanks.
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post #14 of 38 Old 11-15-2012, 03:29 PM
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FWIW I'm another lens user that has a flat electric screen, so I just overscan the image slightly but have a very long throw (projector is at minimum zoom) so the pincushion is minimal...it all depends on the throw really, but it's 100% not an issue on my setup.

However, I have noticed that the new JVC projectors have a pincushion control for when you use the projector's own vertical stretch mode. I think it could be a bit like keystone, where it introduces more issues than it solves with reduces resolution, but I'm going to try it out when I receive my X35(RS46). However, I found that my Lumagen did a better job of V stretch than my existing JVC HD350(RS10) so that will also have some bearing on the matter.

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post #15 of 38 Old 11-16-2012, 04:43 PM - Thread Starter
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Kelvin,

Does the amount of pincushion remain the same all the way around the image? Or is it worse on the edges on the screen and less at the middle of the screen as you use the center of the lens?
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post #16 of 38 Old 11-16-2012, 05:07 PM
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The pincushion is consistent across the screen I believe, but just that in the middle the image comes inside the screen area and at the outer edges it bends just beyond. It's just a matter of equalising the overscan and careful set up of the lens so that the pincushion is equal top and bottom. It's possible to get the bottom perfectly flat for example, but then the top will be twice as bad, so it's not a good choice. There doesn't seem to be any issue with the vertical edges left and right as they seem straight to me. Hope this helps.

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post #17 of 38 Old 11-16-2012, 09:08 PM - Thread Starter
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I think I'm picking up what you're laying down. Thanks!
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post #18 of 38 Old 11-16-2012, 10:22 PM
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Quote:
Does the amount of pincushion remain the same all the way around the image? Or is it worse on the edges on the screen and less at the middle of the screen as you use the center of the lens?

The amount of overlap increases as you move to the edges of the screen. It will be zero in the middle with no distortion at all, and at its most severe at the edges.

I developed an accurate formula for pincushion, which works right down to about a throw ratio of 1.5 (and even then it isn't out by much).

Throw ratio is entirely a function of Throw Ratio ("Throw" divided by "16:9 width" see below for a formula). People who tell you "at this throw, pincushion is such-and-such" are wrong, if they do not factor screen size into it. Calculating pincushion must ALWAYS take account of BOTH throw and screen size.... in other words "throw RATIO".

This is a quote from a recent post I made at http://www.avsforum.com/t/1436661/calculate-pincushion#post_22545142
Quote:
Try this in Excel:

1. First work out your throw ratio (TR):
Where H = Image height: TR = THROW * 0.5625/H

2. Then calculate your pincushion as a decimal fraction of your screen height (using what you asked for, a formula):
Pincushion Percentage (PP) = 4.21/(100*{TR}^1.910963)

(if you multiply this by 100, you get Pincushion Percentage)

3. Then the result you want, the Pincushion... the actual amount your image corners will be higher (or lower) than the image center:
P = PP x H



What's the formula based on?
Some research I did into pincushion when designing my Xeitoptics cylinder lens.

I found that pincushion was a straightforward function of throw Ratio (TR) for my lens design. The formula was derived from establishing a whole bunch of accurately calculated pincushion results in a spreadsheet table and then performing a statistical regression on them. The formula above in (2) gave a "best fit" regression.

I wondered whether it worked for other designs, even prism systems.

So I then tried it with models of other designs that I had, including prism lenses and old-fashioned designs like Isco. The results from all types of designs conformed pretty well (within 1% of the modelling results).
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post #19 of 38 Old 11-16-2012, 10:26 PM
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Quote:
The pincushion is consistent across the screen I believe, but just that in the middle the image comes inside the screen area and at the outer edges it bends just beyond. It's just a matter of equalising the overscan and careful set up of the lens so that the pincushion is equal top and bottom. It's possible to get the bottom perfectly flat for example, but then the top will be twice as bad, so it's not a good choice. There doesn't seem to be any issue with the vertical edges left and right as they seem straight to me. Hope this helps.

Adjusting the tilt of your lens does indeed change the amount of pincushion between top and bottom of the lens.

You tilt your lens in the direction of the excess you want to reduce.

So, say you have too much pincushion at the bottom of the screen (and the top has virtually none), tilt the lens DOWN i.e. towards the bottom of the screen.

You will see pincushion at the bottom edge reduce, while pincushion at the top increases. Keep adjusting until both are either the same or where you aesthetically want them to be.
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post #20 of 38 Old 11-17-2012, 01:25 AM
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Quote:
Originally Posted by Aussie Bob II View Post

Adjusting the tilt of your lens does indeed change the amount of pincushion between top and bottom of the lens.
You tilt your lens in the direction of the excess you want to reduce.
So, say you have too much pincushion at the bottom of the screen (and the top has virtually none), tilt the lens DOWN i.e. towards the bottom of the screen.
You will see pincushion at the bottom edge reduce, while pincushion at the top increases. Keep adjusting until both are either the same or where you aesthetically want them to be.

I usually put up the JVC's green grid focus or zoom pattern as it has horizontal and vertical lines. I then adjust the tilt so that the pincushion is equal top and bottom as you say. It's quite a harsh test of the lens too because it's more obvious that the lines aren't perfectly straight, yet watching ordinary video I never seem to notice.

The horizontal lines look straightest across the centre of the screen height (for all intents and purposes they are flat), but the lines at the top and bottom of the screen seem to be effectively very large radius semi circles, so again I adjust the height and zoom such that the top and bottom of the image just touches the edge of the screen in the centre, so that the 'peaks' in each corner overspill the least amount. This way the white part of the screen doesn't have any area not covered with the image.

The hardest part for me is adjusting the astigmatism of my Isco II lens: I can't turn the adjustment and be stood by the screen to see the effect to best advantage. I've tried getting my son to stand by the screen to tell me when the lines look the sharpest in both directions, but being 13 I don't think he really seems to be as picky as I am. biggrin.gif I end up turning the adjustment to one extreme and noting the number on the lens, then back the other way so I can split the difference. Or making many small adjustments and walking back and forth trying to tell which looks sharpest. I bought some cheap binoculars to try and they didn't help as they weren't sharp enough. rolleyes.gif

Setting a lens up certainly isn't a 5 minute job I find...though it is worth it once done. cool.gif

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post #21 of 38 Old 11-17-2012, 04:22 AM
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I've set up plenty of lenses and it never gets easier.

It can take 2 minutes or two hours, depending on what you had to drink the night before.

Try getting the rotation of three cylinder lenses perfect to +/- 1/120th of a degree, not only internally to each other, but to an external horizon reference. I use a microscope I put together and it can still take an age.
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post #22 of 38 Old 11-17-2012, 09:18 AM
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Glad it's not only me. biggrin.gif I think I had mine about two weeks before I finally got everything really dialed in. Even then it is on a stand so I have to make sure I put it in the same spot in front of my projector each time I watch a 2.35/2.40:1 film, though I've got that down to a fine art now.

I'm moving my projector slightly to a new shelf higher up. I know this will entail tweaking the lens again, so not looking forward to that. rolleyes.gif I may add some small 'stops' to the top of the shelf to help align the lens stand to help reduce set up time as well.

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post #23 of 38 Old 11-18-2012, 05:39 AM
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Quote:
Originally Posted by Kelvin1965S View Post

I usually put up the JVC's green grid focus or zoom pattern as it has horizontal and vertical lines. I then adjust the tilt so that the pincushion is equal top and bottom as you say. It's quite a harsh test of the lens too because it's more obvious that the lines aren't perfectly straight, yet watching ordinary video I never seem to notice.

That's exactly what I do. smile.gif
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post #24 of 38 Old 11-18-2012, 05:48 AM
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That's exactly what I do. smile.gif

You won't need to once you get that VW1000 though. wink.gif

Zooming: Been there, done that, bought the lens...
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post #25 of 38 Old 11-18-2012, 10:41 AM
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Originally Posted by Kelvin1965S View Post

You won't need to once you get that VW1000 though. wink.gif

Ha ha!

I'm a Yorshire man Kelvin. Say no more....smile.gif
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post #26 of 38 Old 11-18-2012, 08:46 PM - Thread Starter
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Thanks for the help guys! I'll try and work out the formula later. Does the Throw and Height need to be in inches?
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post #27 of 38 Old 11-18-2012, 11:09 PM
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Thanks for the help guys! I'll try and work out the formula later. Does the Throw and Height need to be in inches?

No. They can be in carrot sticks if you like, as long as both use the same units.
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post #28 of 38 Old 11-19-2012, 02:40 AM - Thread Starter
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So tell me if I did this correctly...

Based on your formulas using inches as the unit of measure:

1. First work out your throw ratio (TR): H = 60 TR = 180*0.5625/60
1.6875
2. Then calculate your pincushion as a decimal fraction of your screen height (using what you asked for, a formula):
Pincushion Percentage (PP) = 4.21/(100*1.6875^1.910963)
.015489
3. Then the result you want, the Pincushion... the actual amount your image corners will be higher (or lower) than the image center:
P = .015489x60
.92934

So I should have about an inch of pincushion on the edges of the screen that I need to overscan?
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post #29 of 38 Old 11-19-2012, 02:22 PM
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So I should have about an inch of pincushion on the edges of the screen that I need to overscan?

Correct.

You should also add in some extra overscan for 2.40:1 movies.

2.40:1 movies occupy 800 pixels out of 1080 pixels of image height. When the image is vertically stretched, however, the middle 810 pixels will be stretched to fill the height of the image (810*4/3=1080). So between the 800 pixels of a 2.40:1 movie and the top (and bottom) edges of the image there will be a 10 pixel black band.

To get rid of this thin black band you need to optically zoom the height of the image a little more than you might have thought at first. The ratio is 810/800 = 1.0125, one and a quarter per cent.

So, to fill your 60" high screen height you need to zoom it so that the 16:9 grid pattern (or whatever test pattern you are using) enlarges to 60*1.0125 = 60.75" high, an overlap (dividing the extra 3/4" equally between top and bottom), around 3/8ths of an inch top and bottom, say 1/2 an inch to make sure.

To prove it:

800 pixels represents 800/1080 = 0.740740741 of screen height.

Now multiply 60" by that number and you get 60*0.740740741 = 44.4444" BEFORE vertical stretch of 4/3 times.

Putting it all together 44.4444*1.0125*4/3 = 60". Your 2.40:1 movie now matches your exact screen height. 16:9 material will overshoot by a little.

I always recommend adding a little to this to make sure. Adding some extra also accounts for 1.85:1 Academy Aperture films, which are slightly shorter (at 1040 pixels high) than 2.40:1 scope presentations are after vertical stretch.

I know it sounds complicated, but I've seen some unfortunate installations where everything is bolted into place, only to find that the image doesn't quite fit the screen and, because everything else is maxed out, can never fit the screen. You need to be aware of the issues.

A1920 x 1080 test pattern showing all the common movie apertures is at my web site: http://www.xeitopticscom/test-patterns/ (scroll down).
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post #30 of 38 Old 11-19-2012, 05:28 PM - Thread Starter
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So I think I'm getting it. W/ out the lens in place, overscan by about 1/2" for 1.85:1 films and w/ the lens in place, adjust the lens placement so it overscans by about an inch?

I guess all that's left is to get the projector and screen and mess around to see how it all plays out in practice.
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