Does Zooming Degrade Picture Quality ? - AVS Forum
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post #1 of 14 Old 02-13-2010, 02:24 PM - Thread Starter
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Hi



The Projector has a zoom ratio of 2.00.

In order for me to get the screen size I want ( 133" D ) along with
enough Foot Lamberts I have to zoom in to 1.90 with a 1.4 gain screen.

I can move the projector far back to decrease the zoom but I lose major Foot Lamberts and don't want to go any higher with the Gain.

Oh yeah..I'm not going with a smaller picture so it will be 133" D.


I'm just worried I would lose some of the clearness/quality of the picture by using such a high percentage of Zoom.

Are my worries for nothing or is there some truth to this ?

Thanks For Your Time

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post #2 of 14 Old 02-13-2010, 05:16 PM
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Might depend on the model. It is possible that the lens would be poor near either end of the zoom range, but I don't know if it would with the model you are thinking about and somebody would probably have to test it (although projectorcentral.com or somebody else might have already).

I think the general consensus around here as been that moving the projector further away for the same screen size improves the picture because it uses the sweater spot of the lens, but I think that might not be the case, even not counting the issue of further away for the same image size tending to have less light coming out of the projector, but with higher on/off contrast ratio.

Using more of the exit lens (bigger zoom) may mean some degradation out at the edges of the image, but for the center of the image I think it may improve things. The reason I think this might be the case is that the aberrations in the lens could be smaller compared to the sizes of the pixels as they leave the lens (I'm simplifying somewhat though). I may not be explaining that very well, but one way to look at it is how a piece of paper may seem rough to an ant, but smooth to a human. If you think of little ridges and things in the lens itself (which are inevitable even if small) those are going to be bigger compared to a pixel if you shoot 1920 horizontal pixels over 1 inch of that lens than if you shoot them over 2 inches of that lens. So, shooting so that you use 2 inches of the lens could be better because in comparison to the pixel size the lens is smoother as far as aberrations being relatively smaller.

There is somebody here who has done some measurements of the center of a screen related to this stuff, but I don't want to speak for him and I'm sure he'll provide some more data at some point.

--Darin

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post #3 of 14 Old 02-13-2010, 06:49 PM
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If from a zoom lens of dslr camera point point of view, normally the max zoom will give you the worst picture quality due to ca, lens barrel distortion
,Softer picture, light drop off at the edge. I think the theory should apply.
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post #4 of 14 Old 02-13-2010, 07:48 PM - Thread Starter
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Thanks for the replies ...very helpful.

It would be nice if the makers of projectors would include the amount
of picture quaiity that is lost as the zoom is increased.

It would really help with those of us who want the biggest picture
that their room allows using the zoom without having to sacrifice too much sharpness.

If I had this information beforehand i would know my limitation regarding size
and how much lost of quality I'm willing to lose.

Thanks again guys for taking the time to respond

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post #5 of 14 Old 02-13-2010, 07:59 PM
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What projector? I doubt the vast majority of current projectors (unless super high end) would have the necessary quality to really be able to distinguish differences between long/short end of the zoom. I think you are probably worrying about something you don't really need to...
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post #6 of 14 Old 02-13-2010, 09:20 PM - Thread Starter
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Hi Jason

I posted this in the wrong section. I thought it was in the [b}under $3000[/b]thread when I posted this.

I'm looking at the Panny AE4000u .


Sorry MODS I thought I posted this in the under ..you can move it there if you like.

Blue.


Quote:
Originally Posted by Jason Turk View Post

What projector? I doubt the vast majority of current projectors (unless super high end) would have the necessary quality to really be able to distinguish differences between long/short end of the zoom. I think you are probably worrying about something you don't really need to...


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post #7 of 14 Old 02-14-2010, 04:09 AM
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Projecting the same image size from different distances by using the lens zoom. The zoom effects brightness and to a lesser extent contrast. To calculate the effect on brightness you use the lens f numbers. (f1/f2)squared. For example a projector listed as having a f2.6-2.4 lens works out as (f2.4 largest image / f2.6 smallest image) squared = 85.2. 100 - 85.2 = 14.8% loss of light, largest image 100%,smallest image 85.2%. The effect on contrast is the opposite way round it is higher in the smallest image, and the difference is typically a lot less than the difference in light output, so in this example I would guess about 7% or less difference in contrast. . Since contrast usually has a bigger effect on perceived picture quality than brightness, as long as the image is bright enough, it is usually best to project the smallest image.

Depending on lens quality the smallest image will also usually be sharper than the largest image, but with some projectors the middle of the focus range may give the sharpest image. To determine lens focus and sharpness you want an image of white lines on a black background and face inches from the screen look for the least chromatic aberration (the white does not go sharply into black, but has a colored edge) at the extremes of the screen, or with DLP projectors how sharp the individual pixel structure looks, with DLP you can usually make out the dimple contour in the middle of each pixel.

You also need to take into account the projector offset, the angle its lens aims at the center of the screen. DLP projectors normally have no adjustable up/down lens shift, while some LCD projectors do which greatly improves placement flexibility. Offset is given is a percentage of the screen height that the projector (center of lens) must be mounted from the furthest vertical point of the screen. For example a fixed offset of 136% percent with a screen 62.4" in height, means that this projector would need to be mounted 84.86" above the bottom of the screen for a ceiling mount. 136% * 62.4" = 1.36 * 62.4" = 84.86". DLP projectors sometimes have virtual lens shift to improve placement flexibility but this works by reducing the size and resolution of the image, so is best avoided.

Dlp projectors usually do not have any left/right lens shift, while LCD projectors often do, again greatly increasing placement flexibility. You need to check the lens-screen is absolutely square on, to the screen to prevent key stoning. To check this measure the size of the image top and bottom should be the same length, left and right sides should be the same length. Projectors have digital keystone correction to compensate for not being square on, but this again works by slightly reducing the image size and resolution so for best picture quality it better to not use it.
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post #8 of 14 Old 02-15-2010, 02:42 PM - Thread Starter
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WOW Dovercat thanks for this great long thought out post !


How do I find out the lens F numbers so I can figure out the loss of light % ? I read just about every review and no mention of F numbers.

Yes I read that to get the sharpest image try to stay in middle of the zoom range ..I'm going to push it some so as to get the screen size I want. If I lose too much sharpness/brightness I'll just go smaller.


I'll take the steps you mention below(I made bold) to determine lens focus and sharpness at different screen sizes to see how much I'm losing and if losing
a certain amount is a worthwhile trade off for size.

No problem with projector offset here...plenty of play with the Panny4000.

Thanks so much for your time dovercat



Quote:
Originally Posted by dovercat View Post

Projecting the same image size from different distances by using the lens zoom. The zoom effects brightness and to a lesser extent contrast. To calculate the effect on brightness you use the lens f numbers. (f1/f2)squared. For example a projector listed as having a f2.6-2.4 lens works out as (f2.4 largest image / f2.6 smallest image) squared = 85.2. 100 - 85.2 = 14.8% loss of light, largest image 100%,smallest image 85.2%. The effect on contrast is the opposite way round it is higher in the smallest image, and the difference is typically a lot less than the difference in light output, so in this example I would guess about 7% or less difference in contrast. . Since contrast usually has a bigger effect on perceived picture quality than brightness, as long as the image is bright enough, it is usually best to project the smallest image.

Depending on lens quality the smallest image will also usually be sharper than the largest image, but with some projectors the middle of the focus range may give the sharpest image. To determine lens focus and sharpness you want an image of white lines on a black background and face inches from the screen look for the least chromatic aberration (the white does not go sharply into black, but has a colored edge) at the extremes of the screen, or with DLP projectors how sharp the individual pixel structure looks, with DLP you can usually make out the dimple contour in the middle of each pixel.

You also need to take into account the projector offset, the angle its lens aims at the center of the screen. DLP projectors normally have no adjustable up/down lens shift, while some LCD projectors do which greatly improves placement flexibility. Offset is given is a percentage of the screen height that the projector (center of lens) must be mounted from the furthest vertical point of the screen. For example a fixed offset of 136% percent with a screen 62.4" in height, means that this projector would need to be mounted 84.86" above the bottom of the screen for a ceiling mount. 136% * 62.4" = 1.36 * 62.4" = 84.86". DLP projectors sometimes have virtual lens shift to improve placement flexibility but this works by reducing the size and resolution of the image, so is best avoided.

Dlp projectors usually do not have any left/right lens shift, while LCD projectors often do, again greatly increasing placement flexibility. You need to check the lens-screen is absolutely square on, to the screen to prevent key stoning. To check this measure the size of the image top and bottom should be the same length, left and right sides should be the same length. Projectors have digital keystone correction to compensate for not being square on, but this again works by slightly reducing the image size and resolution so for best picture quality it better to not use it.


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post #9 of 14 Old 02-15-2010, 03:17 PM
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The panasonic projectors have good enough lens systems to not worry at all about quality other than the normal trade offs with max -vs- min distance.


At max zoom (closest you can place projo to get your desired screen size) you get more brightness at the cost of some contrast, at min zoom (farthest distance from the screen for the desired screen size) you get better contrast but lower brightness.



I have to admit, I take the brightness in nearly all instances. So max zoom / closest placement to the screen.
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post #10 of 14 Old 02-15-2010, 03:22 PM
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Panasonic PT-AE40000U user manual p32. Lens F 1.9 - 3.2
You can download the manual pdf via the link at projector central, or at panasonics website.
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post #11 of 14 Old 02-15-2010, 03:35 PM - Thread Starter
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Thanks JD

Yes brightness for the win.

Plus with the high contrast of this panny it will help with the lost of contrast by placing it closer and max zoom.

Quote:
Originally Posted by JOHNnDENVER View Post

The panasonic projectors have good enough lens systems to not worry at all about quality other than the normal trade offs with max -vs- min distance.


At max zoom (closest you can place projo to get your desired screen size) you get more brightness at the cost of some contrast, at min zoom (farthest distance from the screen for the desired screen size) you get better contrast but lower brightness.



I have to admit, I take the brightness in nearly all instances. So max zoom / closest placement to the screen.


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post #12 of 14 Old 02-15-2010, 03:37 PM - Thread Starter
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Quote:
Originally Posted by dovercat View Post

Panasonic PT-AE40000U user manual p32. Lens F 1.9 - 3.2
You can download the manual pdf via the link at projector central, or at panasonics website.

Thanks Dovercat

Another few weeks before I get it and never thought of downoading the manual.

Thanks !

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post #13 of 14 Old 03-05-2010, 12:39 PM
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Our church recently purchased the Panasonic PT-DW6300 (6000 lumens) with a long throw lens. The projector is ceiling mounted with a throw distance of 46 feet to produce a 78"x139" image. On Panasonic's website using the lens calculator for the DW6300 with the ET-DLE350 lens the output is listed as 770lux or 69footlamberts. My sales rep informed me this was incorrect... the difference in f-stops of the lens (standard vs. long throw) would reduce the light output down to 56ftl. So I purchased a light meter and took actual measurements. I get 48ftl (avg.); so our new 6000 lumen projector is really a 3,600 lumen projector.

I contacted Panasonic concerning the lens vs. light output vs. f-stop effects, and why the information they supplied is so errous... ... they are slient.

Again, from actual measurements, the f-stop greatly effects the light output as well as any change from the native aspect ratio of the projector. For this projector the native aspect is 16:10 but using it 16:9 so it really starts with 5400 lumens.

By the way, this light canon produces a very good picture on a flat white wall. Now for a new screen or DIY screen... what to do... PM's welcome
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post #14 of 14 Old 03-05-2010, 12:56 PM
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You are learning fast that rated specs are virtually meaningless, especially in presentation based projectors such as your church purchased. Their specs assume a VERY specific setup to accomplish the rating. Anything besides that...not going to be as good as they say.
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