How does lens shift affect sharpness? - AVS Forum
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post #1 of 18 Old 10-02-2012, 05:16 PM - Thread Starter
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I am about to buy my first projector and I am looking at the new Panasonic, the new Sony (50ES) and the JVC X55/X75. I would like for some of the experts to educate me on how lens shift affects sharpness. My assumption is that it will degrade it but is it only noticeable if you are 5 feet from the image.

My eyes will be about 13 feet from a 120 inch screen.

The Panasonic AE8000 is attractive since it has the most lens shift of the projectors I am considering but will I see a degredation of sharpness? Any examples you can post would be cool as I am a real sharpness nut.
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post #2 of 18 Old 10-03-2012, 04:40 AM
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It varies depending on the convergence itself and other factors, in rare cases it could actually improve it slightly in the case that a slight CA hides a slight convergence error, but generally it only degrades it slightly. It can in some cases degrade it more, but it just depends on luck and every projector will be slightly different.

It usually affects it less than the variance of convergence luck, so it is not a huge concern. More of a concern is getting a projector with good convergence in the first place.


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post #3 of 18 Old 10-03-2012, 06:01 AM
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I've never seen it confirmed with the newer models, but back in the RS20 days, there was a measurement that showed how using some lens shift actually improved ANSI contrast and I experienced this with my old HD350(RS10) in the form of slight ghosting of high contrast such as end credits, when I first installed it dead screen centre and zero shift: I later moved it higher up which meant it needed some shift applying and the ghosting went (note this is not 3D ghosting) so I think there is some credence to it. Even if the shift slightly softened the image it wasn't noticeable from my seated position, but the ghosting was, so I chose the compromise that made the most effect from my seated position rather than 5 feet from the screen. I'd rather have improved ANSI contrast and slight softening than the other way round: I'm getting an RS45 soon, so I'll find out if that works the same as my old RS10...

Zooming: Been there, done that, bought the lens...
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post #4 of 18 Old 10-03-2012, 08:00 AM - Thread Starter
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Thanks guys. It sounds like I need to check convergence as soon as possible on any projector and I will try to not go to the limit of the vertical lens shift. After getting my projector I will post some photos of the convergence pattern and see what you guys think.
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post #5 of 18 Old 10-03-2012, 08:02 AM
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How does lens shift affect sharpness?

Basically lenses for the most part perform best when the image exiting them exits from lens center and the smaller the exiting image the better. This spot is called the G, whoops, the sweet spot. As one uses lens shift, the image which stays fixed entering the lens, hits a different entering point on the back of the lens, because one is shifting the lens (moving it) causing the exit point on the front of the lens to differ. One other goodie to remember is that white light is comprised of many frequencies and each of these frequencies diffract differently as they exit the lens into the air surrounding the lens.


Lens generally are flattest in the center and tend to curve as one moves away from center. Lens manufacturers coat their lenses to try and compensate for the different bending of each light frequency, but this compensation is fiqured for light at lens center and not on the curved part of the lens glass. The more from center, the more the curve and the greater the different degree of bending. Because of the bending, red, green, and blue light exiting the lens will not hit exactly where you and the lens designer would like them to go. This error is called Chromatic Aberration or CA. It looks if you put up a grid of white lines this error looks exactly like misconvergence except that the error will vary across the grid.


Besides CA, lens just don't focus as well in the curved (non G spot, whoops again, non sweet spot) area. The more one is into the curved area and the longer the throw, the more unsharp will be the image. Of course, sit far enough away and one can't see anything on the screen except a blob of light. Then again, one can sit real close and still have impaired vision. So there is no real answer without knowing a lot of detail about ones set up and eyes.

But a rule of thumb is, the less lens shift one uses, the better. Lens shift is a convenience feature. The better and wider the lens the more it can be used to shift away from center without hurting the image.

Now there is something to be said from moving off of dead center a click or two vertically. Never horizontally because ye old 16/9 or greater is wider horizontally than vertically and is much closer or on the curved area already. Some lenses are less than ideal and light bounces off the inside barrel and result in some ghosting if exactly centered. Use lens shift if you need to, but don't use it to the extent possible just because its posssible.

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post #6 of 18 Old 10-03-2012, 08:43 AM
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Well said.

Maybe your wife needs to read that as well, lol.

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post #7 of 18 Old 10-03-2012, 09:41 AM
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Not all women have a g spot but all lenses have a sweet spot.

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post #8 of 18 Old 10-03-2012, 10:01 AM - Thread Starter
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Mark... thanks for the excellent information. Any specific thoughts on the Panasonic vs the Sonys vs the JVC if I am looking for the best sharpness but also want to be able to mount my projector about 10 inches above my 120" screen? Due to the set up of my theater room I don't want the projector hanging down too low at the upper riser area.
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post #9 of 18 Old 10-03-2012, 10:15 AM
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First of all you need to determine whether placing lens center at 10 inches above screen top for a 58 Inch tall screen is possible with each projector. One would need 10/58 x 100% which equals about 17.25 percent. Lens shifts are usually specified from center so you will need a shift of 67.25% or some measure it from bottom or top going the other way, 117.25. As to the specific projectors, my guess is the JVC has the best lens here but I would neeed to look up the available vertical shifts on all three. Lens quality wise, I think the JVC trumps the Sony 30/50 and Panny. I really hate mounting anyprojector higher lens center wise then at screen top and I would consider raising my screen higher to lower the lens distance to screen top. Many will tell you I do it and it is plenty sharp for me. But careful and critical viewing will disclose the truth into what I speak here. If you are not a sharpness freak, I wouldn't be so critical as to your set up.

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post #10 of 18 Old 10-03-2012, 02:16 PM
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Lens shift can cause slight bowing due the size of the beam and curveature of the lens. I use minimal shift, mostly in the vertical range, but I do get a slight curve on the bottom part of the image where the center of the image is maybe 1/4" higher than on the corners. It is only noticeable when the lights are are slightly and you can see the white portion of the screen.

As Mark said lens shift is convenience feature. The best setups require no shift, but many of todays projectors with shift require the projector be mounted so the lens centerpoint is dead center on the screen. The fact is that many rooms can't accomodate that. I would bet that the vast majority of members that have a projector here have them ceiling mounted with the lens close to even to the top edge of the screen.

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post #11 of 18 Old 10-03-2012, 04:22 PM
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Lens shift shouldn't cause any bowing. What will cause bowing is the center of the screen along the edges being closer to the projector than the edges at the corners if the bow is an inward one. You need to shim the frame against the wall.

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post #12 of 18 Old 10-03-2012, 05:01 PM
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It is very slight. It is not a fixed frame screen. In fact it is a tab tensioned screen. It appears to be flat. I had a very similar issue with my 1080UB.

I can't remember the source but I did read an article on on of the enthusiast sites that claimed bowing is possible with lens shift. I would imagine the smaller the beam through the lens the less bowing affect. In my setup I have my HW30 near the min throw range.

Again we are talking 1/4" or less and it is not noticeable unless you have ambient light.

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post #13 of 18 Old 10-03-2012, 05:07 PM
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The fact that you had it before would indicate it is a screen problem. I suspect you batten bar is slightly bent or warped. How large is the screen? A new batten bar is cheap and easily installed. Shoot a laser pointer accross the screen bottom. Just a slight difference in distance will cause a curve. If you zoom in and get away from the bottom screen edge I bet the curve becomes a straight line.

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post #14 of 18 Old 10-03-2012, 07:57 PM
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[QUOTE=http://www.projectorreviews.com/advice/general/opticalkeystone.php]
Even optical lens shift does technically degrade the image, but not in the way of digital keystone correction. The result is a slight bowing of the image. Let's stick with a table top projector for now. If you are using lens shift to keep a rectangular image when the bottom of the image is above the lens, you will probably find that if you look at the top of the projected image, that the corners are a little higher up than the top center. If your screen is a 100" diagonal (about 87" wide), the actual amount of bowing will depend on a number of factors, but at worse the center top might be an inch or so lower than the top corners. Since your screen no doubt has a border, you aren't likely to notice unless you are looking for it.[/QUOTE]

It could be a slight bow on the bar but again it is so slight I don't notice it.

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post #15 of 18 Old 10-04-2012, 07:23 AM - Thread Starter
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Mark,

I can't raise my screen any higher due to a custom valence that I had the carpenters build to hide the electric screen. So my takeaway from this discussion is that I need try to mount the lens center as close to the top of my screen as possible.

My other option is to mount at the back of my room but that is 23.5 feet from the screen and that just happens to barely fit within the throw capability of this projector.

So this leads to another question. My theater room is painted dark and completely light controlled so I am not too worried about brightness but I do worry that sharpness will suffer if I am at the extreme end of the zoom lens. Any experience with using projectors at their zoom limit?
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post #16 of 18 Old 10-04-2012, 09:11 AM
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I assume you have a fixed frame screen.If so, you can tilt the projector down and thus use less lens shift. Tilting the projector down will cause keystoning but there is a very very easy fix. lenses need to be perpendicular to the plane of the screen. Just tilt the screen bottom slightly up. we are talking only a degree or two here. Shim using a couple of washers or whatever. When you tilt enough up to restore the perpendicular geometry, you will not see any keystoning, you will have a perfect rectangle assuming the lens is not shkewed left right, that is you need to make the left right endes of gthe lens also parallel to the screen.After you obtain getting the lens surfave parallel to the screen, you may need to use a click more or less of lens shift to reobtain vertical center on the screen. Give me a call if you want to discuss this. You will never notice a screen tilt of a degree or two.

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post #17 of 18 Old 10-04-2012, 09:44 AM
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Quote:
Originally Posted by mark haflich View Post

The fact that you had it before would indicate it is a screen problem. I suspect you batten bar is slightly bent or warped. How large is the screen? A new batten bar is cheap and easily installed. Shoot a laser pointer accross the screen bottom. Just a slight difference in distance will cause a curve. If you zoom in and get away from the bottom screen edge I bet the curve becomes a straight line.

It is a 100" Elite Cinetension2 screen. Thanks for the suggestions, but it is so slight that I don't think its worth the effort. Compared to the wavy mess that was my non tensioned cheapo Optoma GrayWolf rev 1, this screen is a huge step up. Now if it was a $1k+ Stewart screen I might demand perfection. But I also think that explanation from ProjectorReviews.com does have some validity. I had both projectors set up at their near min throw and both were using a significant amount of vertical adjustment.

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post #18 of 18 Old 10-04-2012, 10:30 AM
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I realize that this is now a purely acaedemc discussion. But in the world of projector lenses, the only way to get a curved line where a line should be straight is to have a bow in or bow out in the screen surface. Moving the lens up or down or left right (lens shift) in relation to the fixed image hitting the lens. this frequently happens with large fixed screens (say in the 12 ft wide class) where the frame flexis. A non straight wall can cause it to depending on the mounting configuration. With a roll type screen, the most likely cause is a bent batten bar. I don't blame you in ignoring it if it is very slight. The fix is easy. Just open one end of the batten bar pocket, remove the old bar and insert a new one.You probably don't even need to close the open end but a little piece of velcro on the inside of the pocket at the end will do the trick. batten bars are cheap. Shipping will cost much more than the bar which should be under $20, perhaps even under $10..

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