Strangely, bigger picture looks better - AVS Forum
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post #1 of 8 Old 04-20-2001, 06:22 AM - Thread Starter
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This is really weird. I've had my vt540 for over a month now. It's projecting onto a da-lite tripod mounted, glass-beaded screen. 70x70.
Using a progressive dvd player, by the way.

The weird part is that with the projector in front of me on the coffee table, and zoomed out to wide mode, the picture is about 58 inches wide, but frankly doesn't look that great - crawlies, mpeg artifacts, pixels are pretty obvious.

But when I move the projector behind the sofa, and zoom in, and end up with a BIGGER picture at 70 inches wide, the picture improves dramatically. What is going on here? Is it that the picture fills up my visual field and therefore I can't concentrate on the artifacts? Or is there something going on optically in the lens?


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post #2 of 8 Old 04-20-2001, 08:43 AM
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I was told by a professional home theater builder to NEVER use a glass beaded screen for moving video. THree main issues:
1. Glass beaded, while bright, has very bad hotspotting and varying view widths - off axis viewing not very good.

2. The beads themselves give the screen a "resolution" which can interact negatively with the resolution of the projector, creating shifting Moire patterns (what you saw as crawlies).

3. Also, glass beaded is pretty much impossible to clean.

But...be of good cheer! What this means is your projector is probably projecting a vastly superior image to what you are viewing. In other words, if your not satisfied with the picture, would you ratther replace your $200 screen or your $3000 projector? Try experimenting on a white wall somewhere with good lighting conditions. My guess is, while getting a slighly lower contrast and less bright(or, less hotspotty) image, your image QUALITY will go way up! The VT540 at 1000 lumens is plenty bright enough for a standard dalite cinema matte screen with 1.3 gain.

[This message has been edited by TSO (edited 04-20-2001).]
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post #3 of 8 Old 04-20-2001, 08:47 AM
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The size of the pixels is strictly related to the image dimensions. With an XGA machine, an 80" wide image will always display 12.8 dpi. There must be another explanation.
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post #4 of 8 Old 04-20-2001, 10:02 AM - Thread Starter
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Thanks for all the replies. I believe you are right - it's the screen. Not to worry, the screen was a temporary solution, anyway, so the $160 I spent was well worth it to find this out firsthand, and I'm sure I can sell the screen anyway.

A word about crawlies - the screen is making them stand out, but they are there either on the dvd or produced by the dvd player. My money is on the dvd. I believe this because I have my dvd player connected to the projector thru component and to my TV thru an iscan V1 into the (CRT) TV's VGA port. And the crawlies are displayed on both. You just don't see them from 10 feet away on the 36" TV. Get 3 feet away and they are there. I believe them to be MPEG related. They only seem to occur in scenes where there is a natural occurrence of one color that is really a gradient of colors. As an example, the first chapter of the 5th Element,
after you see the earth, then the spaceship flies by, then the meteor shoots overhead - when the camera pans down thru the blue sky to the pyramid - the blue sky is alive with crawlies. HOWEVER, take a computer generated scene of color, like the beginning of ToyStory 2 when you get the Disney castle before the movie starts - one color but no gradient, no crawlies.

Time to look for a matte white screen. Unfortunately I need a pull down because of my room.

thanks again,

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post #5 of 8 Old 04-20-2001, 10:20 AM
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Zoom lens are compromises from the gitgo. Your lens will not perform identically at all zoom settings. Most have a sweet spot which is a step down from a fixed focal length lens. Then again, it could easily be the screen. If you switch to a good matte screen and the effect persists, then consider the lens. Not that your options are numerous. Cheers. Vince

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post #6 of 8 Old 04-20-2001, 12:55 PM
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It's also possible that the brightness reduction reduces the eyes' ability to discern these annoying details.

------------------
Noah

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post #7 of 8 Old 04-20-2001, 01:07 PM
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Noah,

Perhaps you are correct. I do know for myself that increased brightness causes the pupil to contract which increases the effective f-stop of my eye and increases depth of field. It also seems to improve clarity. This is related to why people "squint" when their eye muscles can't create focus. Or maybe they are just looking into the sun and that causes them to squint. ;-)

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post #8 of 8 Old 04-20-2001, 09:17 PM
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Yes, the pixel size is always the same on the projected image. The screen itself literally uses small glass beads to create a textured yet highly reflective surface. However, the distribution of the beads is random across the cloth, not in rows or ordered. The patterns the beads are in across the screen can interact with the d.p.i. projected on the screen. If the screen and projector were absolutely fixed (impossible), then my guess is the crawlies would occur in only certain areas of the screen where the two "resolutions" were interacting negatively. As both the screen and the projector are moved in relation to one another, the trouble spots will shift.

Believe me, I have seen this time and again - I am on staff at a large church which over the last three years has made the transition from overheads and slide projectors to all video projectors. Initially, the budget was created with the intent of using all the existing tripod mount screens - ten 7-12 year old screens, most of them glass beaded. We soon realized that the video projectors would NOT work well with the glass beaded screens, and began to replace them as well. Only a year ago I had our experiences confirmed at Infocomm by an installer who made the above statements about glass bead/projector resolution interaction. The same goes for old striped "silver lenticular" sceeens - moire all over the place! Anyway, I'm not a visual physicist, and not up to giving the numbers, ratios, and ocular anomolies that cause the problems - I just know they exist. As I said above, be of good cheer! I thought my projector was going bad when it developed some strange noise in the picture. Lived with it for a few weeks while contemplating a new projector purchase before realizing that the outlet had simply lost its ground somehow. After grounding the outlet and conditioning the power ($50), the image was perfect! Always start with the simplest (cheapest) solution. You may be surprised!

[This message has been edited by TSO (edited 04-20-2001).]
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