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The Good, The Bad, and The Keystone!!

9074 Views 12 Replies 7 Participants Last post by  Prof.
I've had my projector ceiling mounted now for about 400Hrs....

Prior to buying the projector and setting up the screen, I had read on numerous forums not to use Keystone adjustment with Digital projectors, that it would degrade the image...
Keeping this is mind, I carefully measured everything so that all was square and the offset was correct for the distance I was projecting from..

As usual for me,it wasn't spot-on and so when the first image was projected, I found that I needed to tilt the projector down a fraction..
I couldn't raise the screen that little bit because it's set in a shadow box that is screwed to the screenwall frame..

Anyway, being awestruck by the first image I was seeing on this screen, I forgot all about any keystone issues, and started watching all the DVD's I could find from my small library..

Over the weeks and with the initial excitement starting to wear off, I started to notice that the image had a little bit of a keystone problem..It had never bothered me, nor had any visitors ever said "your picture isn't square".

So the other night I wondered, "should I try the dreaded keystone adjustment"!!!...

I had always adjusted focus using a close up image of a face in the particular movie, but the other night I happened to get an image of a face with graphics right at the top of the screen and also at the very bottom...
I found that getting both sets of graphics razor sharp was impossible..and here it was, I did have a keystone problem !!

Well, there's nothing left then but to try a KEYSTONE ADJUSTMENT !!!...
I dubiously pushed the button 1 step positive .....miraculously the image popped up perfectly square..so I thought "what was all this fuss about making keystone adjustment?"...the image hadn't disappeared into a dark abyss, never to be seen again or some other strange thing..
AHA!! it was then that I noticed that my graphics image at the top of the screen was even more out of focus than it had been before...

What had I done?..Did I push the wrong button or something?...How could it be WORSE!!
So I reasoned that something else must be wrong..Maybe I should have pushed button for one negative step...but then if I did that, the keystone shape would be worse..

So as they say,:"nothing ventured nothing gained" and changed the keystone to 1 step negative.......

My eyes opened wide, and my jaw dropped...I couldn't believe what I was seeing!!!
The graphics at both the top and bottom were razor sharp, but so also was everything else...but then I got the real surprise..Distant images had become much clearer than I had ever seen before, giving a much more 3D look to the overall picture.
I had found the "Golden Challis"...or at least that's what I thought until I noticed my ugly keystone image shape...Yikes!!..it looked like a WEDGE..

I then realized that this was my mounting problem...That little bit of down tilting had caused this monster, and the keystoning was just showing me how far out I was in my perfectly calculated plans....

So it's back to the construction site to try and make that screen tilt a little bit so I never have to go through that nightmarish experience ever again !!!

Keystone adjustment can be your friend.....
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Fortunately our Z4 has lens shift... so we don't have to use the keystone. I've always heard similar to what you mentioned as the downfall of using keystone... although it bothers some worse than others.
I know a lot of people look down on keystoning, and I for one feel it degrades the image, however,

I recently read a forum post that talked about some cases where keystoning makes sense.

basically what it comes down to is this -
Keystone affects image sharpness and clarity, but nothing else - so if you love the colors of an entry level DLP like the mitsubishi HD1000U or the Optoma HD70, and are going to primarily watch movies, keystone probably won't harm anything.

It's also been said that a small amount of tilt leads to a small amount of keystone - might not even be noticable...
And lastly, if you have a fixed screen, you may be able to tilt it to match the angle of the projector...

So in summary, while my viewing habits dictate a very sharp picture, I think there are many people who would enjoy watching movies on a big screen - and hope they join us all in takeing the projector plung, and don't get too scared off by keystone...
It's also been said that a small amount of tilt leads to a small amount of keystone - might not even be noticable...
And lastly, if you have a fixed screen, you may be able to tilt it to match the angle of the projector...

That's what I've now done....I managed to loosen the screen mounting from the shadow box unit, and swing the bottom of the screen out 20mm.
This allowed me to reset the keystone to 0...
Cool Prof!

What Projector are you using?
It's an entry level Benq W100...Lovely pic. for the price..It just doesn't have lens shift..
I take it you didn't have a velvet trim border to absorb any spillage? Or was it just not filling the screen?

No..there wasn't any problem with the border or the image not filling the screen..
The problem was that my offset wasn't correct, due to the fact that I have 10' ceilngs, and the telescopic arm for the projector mount doesn't extend quite far enough, so I had to tilt the projector down a little..This brought about the keystone problem, which I eventually corrected by tilting the screen slightly to compensate..
The improvement in image sharpness really surprised me...
I've got a couple clicks of keystone in and the only thing it does is make an anti-alias'd edge. Then again, I also have lens shift on my PLV-HT60 and it doesn't take much keystone to make it perfect.
I've seen a few different projectors with keystone, and one thing I really believe is that the quality of the projector's internal scaler makes a huge difference in what a keystoned image looks like.

I think this is amplified or reduced if the scaler is needed for the source image.

for example, 480p doesn't natively fit most projectors, so it's already being scaled. whereas a 720p source on a 720p projector can display its source 1:1

The other factor is source material - movies tend to have a lot of movement, close ups etc.. so I dont think most people would notice keystone when watching a movie, even if it's 720p (But note that most are 480p and hence are going to be scaled anyways)

so that leaves static images - ie a computer screen. This is where most people would notice keystone - for a few reasons - first - you usually use computers resolution to match the screen so there's no scaling at all going on in the projector, Second, because computer images are 'static' your eyes have a lot more time to examine the image and be critical of it. This is the scenario where the quality of the scaler can make or break keystoning.

If we took a poll, of 100 people with projectors who've tried keystoning, and talied the results, I think you'd get split results some would claim it didn't effect the image quality, and some would claim it did.

That would actually be a cool thread - Keystone on/off screen shots for various projectors. I think I'll have to start something like that!
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You can also lower the pj instead of tilting it. I don't know if your mount allows that without some changes. Tilting the screen out at the bottom makes the screen perpendicular to the pj axis, so the picture on the screen is square, and in focus across the screen. However, you still have keystoning. This time it's between the screen and your eyes. It's about 1 degree. You probably won't see that.
When I put up my pj I mounted it very close to the ceiling and my screen sits on top of my towers so the top is very close to the ceiling. All I did was tilt the screen forward at the top, until keystone was correct, so it faces slightly down, this worked out perfectly because my eye level at my seat is below the center of the screen and this helps by facing the screen towards me.

I think this is the better way to do it...with the screen angled downward..
Angling it upwards, puts the wrong angle to the screen, making it further out of alignment to your seated position..
In my present situation with high ceilings there's not a lot I can do to correct the angle of the projector..
When I move to my new house, I will have the standard 8' ceilings so it shouldn't be a problem..The telescopic arm will give me enough adjustment to keep the projector level..
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