HINT: LENS SHIFT AFFECTS IMAGE - AVS Forum
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post #1 of 10 Old 10-04-2012, 07:04 AM - Thread Starter
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I got my 8000 a couple of days ago and have just trying to get it tweaked to suit me.

The big thing I learned is that the use of the lens shift has a large affect on convergence. This may be common knowledge by this forum crowd but I didn't understand how much it affected it until I was unhappy with the convergence performance of the 8000 initially. I installed the PJ inverted on ceiling with the PJ essentially level and used the lens shift to center the image vertically. I had my mount in correct position so that hardly any horizontal shift was needed. My layout is with the PJ about 15.5 feet from a 92" screen with the projected image bottom about 42 inches above floor level. With the PJ installed to be level, convergence was out by at least 3mm, which I could notice which made print fuzzy and halo-ed and of course made images less sharp.

So, I moved the lens shift joystick to the approximate middle position and then tilted the PJ until the image hit the vertical level I wanted and the convergence was spot on. This adjustment meant that I had to use -5 of keystone adjustment but that doesn't seem to affect convergence. That places the front of the PJ about 2 inches lower than the back of the PJ in my case. Doesn't look as good as the PJ parallel to the ceiling but it definitely fixed the convergence issue and I am HAPPY!

Hope this helps somebody.

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post #2 of 10 Old 10-04-2012, 07:12 AM
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Dan,

That is very interesting as I will be purchasing my first projector soon and due to the fact that my ceiling is high and I had the carpenters build a valence to hide the electric screen, I will need the lens to be about 10 inches above the top of the screen (and would prefer it to be even higher). The AE8000 has excellent lens shift and I can be even higher than 10 inches above the screen if necessary.

How much higher is the center of your projector lens above the top of your screen?

Also do you notice any geometry issues with the image after you tilted the projector?
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post #3 of 10 Old 10-04-2012, 07:38 AM
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May also be attributed to Chromatic Aberrations

I would also take mis convergence over any level of keystone... Keystone destroys fine resolution detail and you can very very easily tell using any 1:1 pixel map test pattern.
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post #4 of 10 Old 10-04-2012, 08:05 AM
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You never ever ever should use keystone correction. It will seriously degrade picture quality. If you must tilt the projector, one should use a fixed screen and tilt the screen to match the tilt on the projector, thus making the screen once again 90 degrees to the lens.

Lens shift has zero to do with misconvergence. That is a function of chip alignment in the light engine and nothing more.


Chromatic aberration caused by the lens and then because one has shifted away from the center sweet spot and into the curved area of the lens where the lens coatings can not fully correct for the red, green, and blue light bending at different angles when exiting the glass into the air, will cause the grid to become misaligned just as misconvergence will, same effect but a different cause EXCEPT the misalignment will vary in degree because the bend travels over different distances causing non uniform misalignment.

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post #5 of 10 Old 10-04-2012, 11:21 AM - Thread Starter
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Farsider,

With PJ tilted, the center of the lens is 3 inches above the top of a 16x9 image; with PJ parallel to ceiling, the center of the lens would be about 4.5 " above the top of a 16x9 image.

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post #6 of 10 Old 10-04-2012, 11:28 AM
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Quote:
Originally Posted by SOWK View Post

May also be attributed to Chromatic Aberrations
I would also take mis convergence over any level of keystone... Keystone destroys fine resolution detail and you can very very easily tell using any 1:1 pixel map test pattern.

On $1500+ projectors, generally true. However, it does depend on how much mis-convergence vs. how much keystone and it depends on the projector...
I've had some projectors with such bad convergence that I would have much rather had keystone error. I've also seen DLP's with 1:1 pixel mapping where you cannot see the effects of keystone BARELY because the 1:1 pixel mapping ERROR is already there.


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post #7 of 10 Old 10-04-2012, 11:34 AM - Thread Starter
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OK, I changed the name of the thread not to attribute the misaligned colors to convergence. The affect is still not acceptable to me. I will have to check out how much unhappiness the keystone correction causes. I have never used keystone correction on any previous projector as I didn't have or didn't notice the misalignment of the colors so much on other PJs. May have to do a little of each to get the image as good as it can be. I use a electric trapdoor screen so tilting the surface is impractical for me.

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post #8 of 10 Old 10-04-2012, 12:32 PM
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Keystone correction involves a lot of difficult scaling and the image most noticeably will become less sharp. Another solution would be to lower the projector a bit using a ceiling flange and a short pole. Once again another solution would be a better projector, one with a larger diameter and elsewise better lense. Or live with the CA, most likely you will not see it from your seating distance and a little bit of grid non alignment on the screen is a lot less worse than keystone correction.

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post #9 of 10 Old 10-04-2012, 01:19 PM - Thread Starter
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I played a little more and used a test pattern that had a 1 pixel pattern and I could clearly see the affect of the keystone adjustment. So, I played with the PJ tilt and the lens shift to get the least CA with only a small amount of keystone affect which I am able to live with uncorrected. There is still a some CA but not as bad as the original amount. I could only use a 6" extender to lower the PJ as it is hung from an eight foot high soffit and happens to be over a large doorway between two rooms. 6" would help a little but would not permit positioning the image in the center of the lens to make for complete elimination of CA. So, I guess I will just have to live with it until DLP technology with sufficient lumens arrives.

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post #10 of 10 Old 10-04-2012, 03:01 PM
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Sounds like the ticket. a little overscan or zooming out can hide the keystoning. However, you don't have to get the image centered in the lens. Almost all of us have to use some lens shift. As a rule of thumb, keeping the lens center no higher than screen top will not cause noticeable loss of sharpness. Remember that only one edge will be off much from the ideal. You might be quite surprised how much CA disappears if you lower the projector just a few inches. But whatever, this discussion and your willingness to listen and try I think has resulted in a much better PQ set up for you.

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