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Discussion Starter · #1 ·
For a single lens LCD/DLP projector with a zoom lens, you can project same screen from range of distance using zoom.


Is it better to


1. place pj far away from the screen and not use zoom

2. place pj close to the screen and use zoom


Anyone has experimented with this? any theory?


I think #1 was better... but then again may be not.
 

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Hi,


I donno know if the "zoom" on a digital projector is a MYTH or not! I do know most new users have some VERY WRONG idea about the "zoom".


ALL the "zoom" on ANY digital projector will ONLY SHRINK the picture at a given throw distance. It means if you need to place the projector far away (at the end of a room etc), you can still use the "zoom" to make the picture SMALLER to fit your screen.


So JSW, in your case, both condition you listed are wrong!


1. if you place the projector far away and NOT use zoom, the picture will be HUGE.

2. if you place the pj close to the screen AND use zoom, the picture will be very SMALL.


You should know what to do now! http://www.avsforum.com/ubb/smile.gif


regards,


Li On
 

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Discussion Starter · #3 ·
Alrighty then...


Let me restate my question. Project a same size screen by:


1. placing the projector far away and use zoom.

2. placing the pj close to the screen and not use zoom.


Which is better?


 

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Globally, no difference. The zoom is there only to help adjusting the image to fit exactly your screen once the pj is fixed. Zoom ranges are usually quite short, from 1:1.2 to 1:1.5 at most, not enough to show weaknesses. On my projector, zooming in/out didn't revealed any visible weakness of the optics in either position.

In a tri-LCD projector, the prism, panels' convergence and uniformity are much more challenging. And for any digital projector, the uniformity of the light path is also critical.


But your question may still be interesting optically speaking. I don't know very much about optics, but here are my feelings.

For the same image size, when the projector is at its nearest position from the screen, a bigger surface of the lens is used. If the lens is bad you may see more vignetage in that position (bright uniformity, corners darker than center); there may be also some distorsion, the image rectangle not being rectangle anymore (pincusion) or not being in its correct aspect ratio (4/3 or 16/9); you may also see some color convergence issues showing a "blurrier" image.

Now, if you see any of these symptoms differently from all zoom positions of your projector then its lens is very, very, very bad. I doubt any is that bad. The definition of any motion pictures (film or video) is quite poor, at least compared to the definition of a simple 24mmx36mm photograph (from 10 to 20 millions "dots"). So if the optics of a projector is as well done as the optics of a photograph camera, you shouldn't worry about the zoom quality of any digital projector.


Carlos
 

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Wouldn't placing the projector as close as possible provide an improvement in brightness?


Scott
 

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If the "rules" of a camera zoom lens can be applied to a projector lens, then probably. But again, the projectors' zoom range of is short, not enough to see a difference.
 

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isn't there a difference in the amount of light passed (speed of the lens) depending if it is zoomed in or out? or does it not matter within these small ranges?
 

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JSW,


One quick note. At maximum throw distance, the size of the image at the exit of the lens is the smallest. If you think you might want to use an anamorphic lens at some time in the future, small beam size is desirable to prevent viginetting (if that is a word).


Jeff
 

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The central portion of any lens provides the sharpest image; look at the improvement in sharpness and depth of field provided by taking a photo at a high f-stop.

So, in theory, projecting zoomed out (smallest image) should be best.

However, it is also true that the video images we are watching are not very high resolution, even HD, and any decent lens should be up to the task at any zoom ratio. In short, it probably doesn't make a difference, unless using an outboard anamorphic lens, in which case the smaller image is better.

Other factors, such as what projector position works best in your room, may take priority.
 

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Without getting into any of the optical reasons or arguments there is the issue of utilizing a 16:9 screen with a 4:3 projector...


Even of you decide to go with a 4:3 screen now you may change your mind later and if your careful you don't need to do a projector remount...


By having your projector positioned carefully you can usually get your full panel to pillar box in the center of a 16:9 screen and then zoom to give a 16:9 panel still on the screen (but you must top align your image)...


Of course this becomes moot if using anamorphic lenses or a custom 16:9 resolution but if you are a little low on the light output side (as I am) and want max brightness / resolution for 4:3 and 16:9 this is the best way I can suggest...


I cant see any visible difference with a Davis clone (but I read these had good optics) so I just mounted for maximum flexibility...


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It all depends, I think, on the engineering of the lens. That person tuned a set of parametric equations to reach a goal. I suspect the end points of the focal range are most difficult and are determined by limitations on element movement, element quality and aberration thresholds. But you would really have to test each lens type, and each serial number may vary from the specification. Were I you, I would let convenience dictate.
 
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