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Questions regarding lens shift

post #1 of 26
1/25/11 at 7:41pm
Thread Starter 
Sorry to ask, but I have been off theater construction for 4+ years and I missed out on a LOT of information.

Of the new projectors in the sub $4500 range, are there any that would allow lens shift for CIH where the projector can be mounted well above the top of the screen? I have been reading up on the JVCs, the Epson 8700 and the Panny 4000.

I really want to mount it as close to the ceiling as possible. The ceiling to the where the top of the screen HAS to be mounted will be about 14".

Thanks.
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post #2 of 26
1/26/11 at 8:47pm
That depends on your definiton of 'well above' the screen. Apparently the new JVCs (X3, X7, X9) have something like 80% shift now where in most cases, 50% was pushing it
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post #3 of 26
1/27/11 at 6:28am
I doubt that you'll have a problem, at least with the JVCs. My HD950 is at the ceiling, with the top of the screen at least 12" down, at a 15' throw distance. But, at least for the JVCs (and I assume the new models are no different than last year's), the manuals specify clearance distances on sides, back and top. Check the user manual(s) to make sure you leave enough room above the pj for ventilation. I had to remove a ceiling tile to allow the pj to "breathe".
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post #4 of 26
1/27/11 at 12:50pm
Lens shift is nice... some projectors do it better than others.

My Sony VPL-VW60 is mounted so it's lens is around 250mm above the top of the screen. The lens shift function adds some keystone trapezoidal to the sides and pincushion distortion to the bottom of the projected image.

It's not much and most people don't see it, but every time I use my projector without my anamorphic lens in place I can see the bottom of the image is wider than the top... as I said, not by much... but I can see it! A slight over zoom of the image fixes the problem, but it would be nice if it wasn't there.

I think I read somewhere that lens shift can add some slight CA due to the light beams path through the optics not being optimal.

Michael.
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post #5 of 26
1/27/11 at 1:37pm
Lens shift isn't all bad: On the JVC HD series (RS10-35) there is a case for using some lens shift as it actually helps improve ANSI contrast due to reflections within the lens assembly. I found that having my projector level with the top of my screen (the 'right' way up on a shelf) allowed enough lens shift to be used to cure some 'ghosting' of images, but not enough to noticably impact on image sharpness (Note that my setup is long throw, so uses minimum zoom, which may bias the result).
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post #6 of 26
1/27/11 at 3:59pm
Thread Starter 
I guess i should qualify some of this with actual measurements! Duhh.

The maximum possible height of the top of my screen (visible portion) puts it real close to 15" down from the ceiling.

The front of the projector to the screen will be about 15'.

The projector will be mounted directly above the middle seat on a 14" riser. This gives a riser to ceiling height of 7' 3" approx. (I had 9' walls to start with and lost a little with the Rsic and double drywall build)

Likely a Carada 1.4 screen, (non AT and no curve)

Since the middle rear seat will have the projector directly above it, I WAS wanting to mount the projector really close to the ceiling. But several post discuss the projector MUST be mounted with the lens inside the screen height itself and NOT well above it. I would think that even the tightest of flush mounts or a hanging shelf will put the top of the lens to top of the screen at roughly 9-11" apart.

I have been trying to catch up on al things theater since postponing the build 4 years ago and was concerned about lens shift + anamorphic lens combo for my 2:35 setup. My projector budget has to stay under $4000 (or close to it) because of a few other things I havent accumulated yet. I like the idea of a LCoS projector. DLP is out because I am not going to risk rainbows for me or anybody else. LCD isn't out, but I would like inky blacks and the LCos seems to be better in this regard.

Oh, this theater will be totally light controled.

Thanks.
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post #7 of 26
1/28/11 at 6:00am
Measurement for vertical shift is generally from middle of lens to top of screen. This would take 1 or 2 inches off your 9-11" measurement. The JVCs are your best choice for maximal lens shift. Given your measurement and an 80% offset, you should be fine with any screen height larger than 35 inches.

Kevin
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post #8 of 26
1/28/11 at 6:46am
I just looked at the diagram of my screen/projector (JVC HD950) arrangement. The dimensions are not too far from yours: 15' throw dist., projector lens at 7'2" (3.5" down from ceiling), screen image area 13.5" down from ceiling. The computed vert lens shift is ~3 degrees. If I have any "keystoning" I don't notice it (and on a 2.35 screen.) The JVC will do a keystone adjustment, but I've read that it should be avoided. Again, allow for clearance above the projector.
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post #9 of 26
1/28/11 at 7:06am
Keystoning should not result from a decent lens shift mechanism, but rather from having to tilt the projector if there is not enough lens shift available. The JVC should work great in the OP setup.
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post #10 of 26
1/28/11 at 4:08pm
Thread Starter 
Awesome to hear, since JVC is on the top of my list. Does the introduction of a lens effect the lens shift at all?
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post #11 of 26
1/29/11 at 6:32am
No it doesn't. The tilt of the lens is adjusted to even out the pincushion at the top and bottom of the screen.

Kevin
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post #12 of 26
1/29/11 at 10:30am
Quote:
Measurement for vertical shift is generally from middle of lens to top of screen.
So if I mount the projector on the ceiling and the middle of the lens is at the same height with the top of the screen, no vertical lens shift will be needed?

I thought that the "default" (with no lens shift) was for the middle of the lens to be aligned with the center of the screen, and that for anything else you needed lens shift.
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post #13 of 26
1/30/11 at 7:41am
You are correct Zoom 123.

My point was only in response to his measurement of vertical shift from the "top" of his lens. I replied that the actual degree of vertical shift was from the "center" of the lens.

Not a big difference, but an inch or two can help!

Kevin
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post #14 of 26
2/1/11 at 12:39pm
Quote:
Originally Posted by Kevin Snyder View Post

You are correct Zoom 123.

My point was only in response to his measurement of vertical shift from the "top" of his lens. I replied that the actual degree of vertical shift was from the "center" of the lens.

Not a big difference, but an inch or two can help!

Kevin

Didn't you say " centre of lens to top of screen" when it appears to be centre of lens to centre of screen"?

I'm not being pedantic because I'd like this confirmed too. I was planning on ceiling mounting but having the screen relatively low. I'll have to post my room sizes up for opinion on the jvc 350 (250 US?). Is there an optimum standard for all pj's or does it really make no difference as long as the machine specs can handle the requirements?
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post #15 of 26
2/1/11 at 1:10pm
Alright already!!

The OP described the height of the TOP of his lens in relation to the top of his screen. I simply replied that the more accurate measurement is to the CENTER of the lens. I was in no way implying that offsets are measured from the top of the screen. I was only responding to his particular measurement description.

Each projector has a different amount of vertical lens shift, measured from the center of the screen. In the case of the JVCs, this is 80%. This means 80% of the screen height above the center of the screen. This would mean for a 50" high screen, the center of the lens could be positioned up to 15" above the top of the screen.

My understanding is that if possible, it is best not to use the entire lens shift, but to try to limit the vertical shift to the top of the screen. I currently use a JVC and need to use much of the shift. I personally have not noticed a significant difference.

Kevin
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post #16 of 26
2/1/11 at 2:02pm
Thanks for that! I never really understood in practical terms what 80%shift meant. So are you saying that as a rough guide (in my case) I should only shift the picture down so that the centre of the lens is level with the top of the screen?
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post #17 of 26
2/1/11 at 3:50pm
Quote:
Originally Posted by BroncoSport View Post

Awesome to hear, since JVC is on the top of my list. Does the introduction of a lens effect the lens shift at all?

The projector will be fine mounted higher. Your A-lens will need to be tilted down to even out the pincushion though. The higher up, the more tilt required.
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post #18 of 26
2/1/11 at 5:03pm
Quote:
Originally Posted by bigjas View Post

Thanks for that! I never really understood in practical terms what 80%shift meant. So are you saying that as a rough guide (in my case) I should only shift the picture down so that the centre of the lens is level with the top of the screen?

Optimally, yes. As I said, though, my projector lens is above the top of the screen, and I haven't noticed deleterious effects.

Kevin
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post #19 of 26
2/2/11 at 5:32pm
Quote:
Originally Posted by Kevin Snyder View Post

Keystoning should not result from a decent lens shift mechanism, but rather from having to tilt the projector if there is not enough lens shift available. The JVC should work great in the OP setup.

Great info guys. Noob that I am I though I had to install my projector on an angle when I ceiling mount. Glad to hear my RS40 on order has great lens shift which will not degrade image quality.
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post #20 of 26
2/2/11 at 9:08pm
I was looking at the tech specs sent to me by JVC yesterday their diagram shows the centre of the screen as 0% and 80% means 80% of the screen height above centre line! That is allot of range.
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post #21 of 26
2/2/11 at 11:44pm
Here is the diagram for the JVC

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post #22 of 26
2/3/11 at 5:50am
Wow allot of range indeed. I have a hard time visualizing whats going on inside with the lens to be able to shift like that. Does the lens go on an angle?
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post #23 of 26
2/3/11 at 4:02pm
My turn for a noob comment as I can't fathom out that image. When the pj is centred to the screen (as at 0%) surely 50% of the image is below this line and 50% is above? If that is true then the increase from 50-80% isn't that much?
Or, does for example, a 100% lens shift mean it is possible to move the image up or down by a full screen height i.e. The top of the screen can be moved to where the bottom currently is (or vice versa). Obviously we are talking 80% here so not quite the full distance I've attempted to describe. Or have I got it all wrong?
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post #24 of 26
2/3/11 at 4:04pm
Please see the discussion from a few posts earlier. 50% lens shift means center of the projector lens can, at maximum, be positioned even with the top or bottom of the screen. An 80% shift works as follows:


Quote:
Originally Posted by Kevin Snyder View Post

Alright already!!



Each projector has a different amount of vertical lens shift, measured from the center of the screen. In the case of the JVCs, this is 80%. This means 80% of the screen height above the center of the screen. This would mean for a 50" high screen, the center of the lens could be positioned up to 15" above the top of the screen.


Kevin
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post #25 of 26
2/3/11 at 4:11pm
Thank you again. I get it now thanks to your reminder and the image now also makes sense. So I was right in the first part of my post. It doesn't seem that great a difference to me between 50-80% but obviously I'm a complete novice and the jvc may be much better at shift than a lot of other makes out there!
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post #26 of 26
2/9/11 at 2:54pm
How much does the image degrade on the JVC when shifting it by the maximum amount? I would want to place it at 50-60% up and as far right as it will go.
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