Lens shifting may not distort, but it decreases light transmission. Personally, in our design I'd much rather lose image clarity over lumen output any day.

Well it seems to me that its only because the lenses aren't big enough, a lot of light misses the lenses when they are offset. However if you had a slightly larger lens, that shouldn't happen. If it still looks darker, I'd say thats ok so long as its not much, we can afford to use some light if we use a reflecor.

Anyway, my recent experiments with keystone correcting in the triplet have been more succesful then the previous lens shifting tests. Didn't notice any light loss.

I think I've done about all I can do untill I get my lumenlab projector finished, and buy an extra triplet for me to experiment with/ruin.

For a while I kept trying to tilt the front lens and tilt the screen to be paralell with it, but it still didn't correct the focus, however last night I found that if you tilt the front lens, the screen has to be tilted in the opposite directon to stay in focus.

I managed to get an angled projection into perfect focus last night using this method, however once I did I noticed that the picture was a slight trapazoid, though it was in perfect focus.

Then what I tried to correct it was tilting the tv and slightly readjusting the lens, this corrected the trapazoid and kept it in focus. It seems to work though I didn't have time to fine tune them to get them into clear focus so I could be absolutely sure. It was late and I had to go to bed, I'm going to do more experimenting tonight.

Btw, I think I may have tilted the screen in the opposite direction I drew, I don't remember, I'll post more after I've done more experimenting.

Interesting. It appears to me that your perfect version actually has the front lens and the green line (representing the front fresnel?) more or less parallel. And if you drew the screen the opposite direction of where you actually tilted it, then the screen would be more or less parallel to the front lens and the green line. So, this begs the question- why not just tilt the rear and middle triplet lenses leaving the green line and front lens (and screen if drawn oppositely) untilted? This should be more or less the same as the "perfect results" drawing. The effect should be the same because the relative angles would be the same. Unless of course the amount of tilting for each component was different from each other component but just not drawn that way. My explanation isn't very clear so maybe just tilt your drawing counter clockwise until the green line is vertical and I think you'll see what I mean....

This may be where the larger lens is necessary since you wouldn't be able to tilt the rear and middle triplet lenses much in the current LL design without losing part of the image seeing as the image should pretty much fill the triplet. Tilting the triplet lenses would then reduce lens area relative to the image and cut the image off. Yeah, definitely need oversized lense relative to image size. Maybe the pro lens would work with a 15" lcd to enable lens shift....and before anyone says it, yes I know using the pro lens will increase throw distance and decrease the keystone angle- I don't necessarily have the room to place the projector ideally so I'm just considering the possible options.

No that green line would represent the lcd. I'm not sure how you would do this with a collector fresnel, you need it to point all the light towards the lens. But some people have said tilting the collector and alling the triplet so its parallel to the collector, instead of the lcd works for keystone correction, maybe tilting the front lens as well will get rid of any distortion caused.

You can't just tilt the lens, if you want to move the lens instead of the lcd, you would have to move the lens sideways but keep it pointed at the center of the lcd.

I don't remember which way you are supposed to tilt the screen, I think the tv, the front lens, and the screen were all paralell.

Mikau,

I meant that since your were tilting the lcd, the front triplet lens and the screen in parallel while keeping the other two triplet lenses vertical, that you should be able to achieve the same thing by just tilting the other two triplet lenses and leaving the lcd, front triplet lens and the screen untouched. See the image below. This should create the same lcd, lens, screen relationships as when you apparently got a "perfect" result (as labeled on your pic). Or maybe I didn't understand your diagram? I know a few other LL PJ builders keystone correct by tilting their lcd, fresnel, the screen and the entire triplet, or some combination thereof. But I imagine that if you could get away with just tilting 2 of the 3 lenses in the triplet that would be preferable (assuming taking the triplet apart isn't an issue and the triplet is large enough to accomodate tilting without losing image- big assumptions I know).

I'm still trying to find a good explanation or drawing of how lens shift in comercial projectors is achieved. I haven't found anything good but I have found quite a bit of info on the use of "tilt-shift" or perspective control lenses in photography. They appear to work on the same principle and are even stated as useful for "keystone" correction (that's the perspective control part apparently). But still no good diagrams yet...

Chrisballas you are making the same assumption I made, you would think that if you can tilt the lcd, then you should be able to just tilt the lenses instead and achieve the same results, however, tilting the lcd, and tilting the lenses is two entirely differant things.

Heres a diagram. In A, the screen is tilted, the lens is pointed towards the center of the lcd. In B, the lenses are tilted, you can see why this wouldn't work because the lens isn't facing the center of the screen and it won't be able to get the whole image in focus. C is how you would move the lens instead of moving the lcd, you have to move the lens SIDEWAYS and keep it pointed at the center of the screen.

And btw, I really appreciate the research your doing trying to figure out how the lens shift mechanism works.

You rock!

But this particular method we're discussing right now is more keystone correction then lens shift. Well actually its more like keystone correction USING lens shift.

(sigh)

I've gotten some pretty clear pictures. Again, it appears to work, its just a question of if it will work wih a lumenlab projector and how much tilt will it allow.

In the picture below, the top diagram shows the method that appears to work.

The bottom two are theories for doing it with the lumenlab projector.

In the middle I have the collector behind the lcd, and the lcd is just tilted. This might not be a good idea as I think lcds are supposed to have the light going straight through them.

The bottom shows tilting the collector like you would normally do for keystone correction.

What programs do you guys use to test this and make the pictures ? or do you just guess and draw them in mspain or something ?

They are just simple paint drawings. I do have autocad but theres no reason to draw it to exact dimensions since I don't have any they are just theories. Sometimes its easier to just throw a quick rough drawing together on paint.

As far as testing, I'm just using a cardboard box tv projector with a good quality projection triplet. I've had success with keystone correction to some degree using this method, but theres only so much you can tell from the size and brightness of a picture tube tv projector. Anyway, I pm'd joechevvy about it and he seemed interested, he said he's going to try experimenting with the idea as well.

I couldn't be happier

Btw chrisbballas, I think this is how you would allow yourself some shifting/tilting room. By putting the collect a little further back to shrink the light beam, that will give you a little room, it will probably be a little dimmer though.

Mikau,

I thought about moving the collector fres closer to the lcd to reduce the image size as it hits the triplet. I would hope that would give enough room for at least a modest amount of lens shift. Not sure how it would impact image brightness. I had thought some time ago that the standard 15" collector fres could be used with the pro triplet by moving it forward but was told there is too much light lost as the collector fres is less efficient the farther away it is. Perhaps being closer to the screen the fres might collect more efficiently and compensate for light lost by not filling the triplet completely? Anyway I'm thinking of ways to make my projector such that the triplet mount is flexible enough to accomodate lens shifting. That might be a different problem all together...

Chris

Why is this kind of correction called "keystone" ? Is it for the same reason the fresnel in some OHPs is tilted ? saw that today when servicing a bunch of OHPs, the one with tilted fresenl had a almost square image and the others that where normal had a image that got smaller in the lower area and bigger in the upper area where it also looses focus. Is this also keystone correcton ?

Don't know if this is too simple of a fix for most but what about simply use 720p via upscaler in 16:9 mode. This will produce black bars on top and bottom. Just move the black bar to the top of ceiling edge. This will raise the height of the center and lessen your angle degree. Just a thought.

Well keystone correction pretty much changes the focus distance so the screen can be tilted, but remains in focus.

Chrisballas, yeah I believe its true that when the collector is placed farther away from the triplet, light is lost, but the reason keystone correction works is because the collector is titled, so the top of the image hits the lens sooner then the bottom, so one side is bigger and the other is smaller.

So this causes one side of the projection to use more area of the lens, and the other side of the projection to use less area of the lens. that produces the angled projection.

So, why not instead of using one side normal, and one side less, why not use both sides a little less, any dimming this causes, will at least be evenly distributed and if done right there will be no focus distortion.

Well gc arter, that would work but you are sacrificing a lot of resolution if your not using 720p or a widescreen format.

Yes, I know. I have my projector ceiling mounted and made it where I can swivel it up or down. As I understand it, anything in 720p will be sent out in widescreen format. IN actuality on a 15inch monitor it will be 1080x576p when set on 720p with the upscaler. Still it will provide decent resolution while at the same time helping to alieviate some of the key stoning problem.

Did a little homework on this cquarter.

It seems when watching something in a full screen format (or in any format for that matter) you only have to have the screen big enough to fit the number of dots that the movie or game your playing displays. If your playing a 480p game, you shrink the image size and pan it over without distorting the resolution so long as theres at least 640x480 pixels still inside, because the image will only display resolution as high as the signal the dvd or game is giving it, regardless of if the tv is capable of producing a higher resolution, or not. A 480p movie will look just as good on a 480p tv as it will on a 1080i tv. Of course the opposite is not true.

So that can be done. and the higher definition formats use widescreen, so both work well for getting the projector out of the way by simply panning the image down.

The obnoxious part is you would have to manually stretch and shrink the image size whenever you switch from widescreen to full screen. If theres some sort of upscaler that can toggle between full screen and widescreen, then keystone correction will be virtually unneccesary. Though I'd still like to work on idea's for improving it.

Well here is my thought on the matter but I'm going to state up front my ignorance on the whole matter since I haven't researched indepth. But if one sets their upscaler on 720p I would think it is would be automatically place it in 16x9 mode since 720p's ideal resolution is 1280x720 and represents a 1.78 ratio. Seems like the upscaler would have to stretch the source signal and then squash it to fit the 1080x768. This leaves the black bars on top and bottom. I would think all signals could be done this way however since DVD's and most source media is 480i upscaling all the way to 720p may create a great deal of junk and artifacts since it must extrapolate almost 1/2 of the video being shown. Also I have no idea on how great a 720/16:9 looks from the upscaling and squashing. The images may be subpar to use this as a way around the keystoning issue.

Well like I said, the lcd only needs to have a big enough picture to fit the number of pixels the signal is giving it. The 4:3 definition is at maximum, 480p or 480i, I'm not sure what the differance between the two are, maybe 480i is widescreen. But anyway, the full screen (4:3) formats seem to have lower resolutions, so you can shrink the image size down a great deal and still maintain enough pixel room to display its maximum resolution. Since the higher resolutions are set in widescreen mode, then you can just pan the image to the bottom of the screen since you'll be forced to use letterbox anyways. Toggling between the two is were the trouble would be.

What we need to is set the screen to display widescreen all the time, and when you watch/play something in fullscreen, black bars appear on the sides, instead of top and bottom. A 1080x576 image can fit a 640x480 image inside it, so that would work. It would be smaller, but it would still work.

Ok. I did a little more research on this.

It is true that the full screen formats use lower resolutions (480p) and the high def formats are widescreen, this works well, you can scale down all your images to widescreen and pan it to the buttom of the screen. Use your scaller to play the full screen formats inside the widescreen box, with black bars on the sides. This will make full screen formats smaller but you will not sacrifice any resolution if the box is big enough to fit 480p (640x480 pixels).

The only thing that throws a wrench in this mechanism is fullscreen dvds, and dvds that use a 1:1.85 ratio. These are dvds using the same high def resolutions made for widescreen tvs only the picture is zoomed in. This makes a bigger picture and better resolution (assuming your watching a dvd that displays higher resolution then your lcd such as a 720p or 1080i dvd). But some of the picture is choped off. While full screen format dvds can generally be avoided, about 50% of dvds use the slightly zoomed in 1:1.85 format. While you could keep it the same size, you might want to make it slightly larger so you can appreciate the closer look and better resolution since your sacrificing a little of the sides of the image in that format.

I need to experiment with scallers to find out how I can get these three formats to work together without stretching or shrinking the lcds image manually everytime you change formats. With the widescreen format you should be able to place the projector almost centered on the top of the projection screen, same on the full screen image, and on the 1:1.85 image the projector would be about halfway between the center and the top. This might be enough so this could work.

Since this is the keystone thread anyway, what would happen if the front lens were tilted instead of the front fresnel? Would there be any ill effects?

Well tilting the entire triplet doesn't work. However if you take apart the triplet and tilt the front lens, it produces a trapazoid in perfect focus, tilting the screen seems to correct this though. It does work to some extent, but I'm waiting to finish my lumenlab projector to put this idea to the test on a lumenlab projector. So I'll keep you posted.

The absolute best thing would be if we just had a slightly differant triplet and collector, a triplet with more magnification so we can place it farther away, and a collector that points light towards the bottom, like the picture below. The triplet has the same viewing angle, but higher magnification so it can be placed farther back and offset. The fresnel is either a special made one, or a large one cut in half.

In my oppinion. This should be the standard lens setup. Because who doesn't want to be able to place their projector on the ceiling or floor, completely out of the way?

I agree, I'd like to build the projector to go on the ceiling or floor, completely out of the way. I like the idea of a collector that points light towards the bottom. I'm trying to design a widescreen PJ so cutting a large fresnel in half (or at least somewhat off-axis) seems to make sense.

Here are some raytraces (animated gif) of keystone correction. The first shows an on-axis collector fresnel that is tilted to correct the keystone. These are 2-d diagrams, please refer to the relative spacing between the rays on the top and bottom ("squashed" vs. "stretched"), I think that demonstrates the keystoning.

Unlike the last post this assumes pj on the floor not ceiling.

Ignore the projection lens in all these diagrams.. I don't think I understand much about that lens yet. I wish I'd done the raytraces (esp. #3) without the lens. Please let me know if there's anything else I did wrong, I'd be happy to post corrected raytraces.
Charlie

Trace #2 This tries to trace an offaxis collector fresnel. I kept the projection lens at the focus of the fresnel, for what that's worth.

It seems very effective at raising the image, but there is the same keystone problem. In trace #3 I tried to correct it with tilting.

Here's trace #3. To me it looks like a combination of tilting the collector fresnel and designing it to be off-axis might be effective.

For this trace I started to play with the position of the projection lens. I actually have no idea what I'm doing here, and this raytrace is basically incorrect in many ways. I'm currently working on how to raytrace a triplet. Only then could I start to model the diagrams Mikau has done in MS paint.

Meanwhile, I read in another post http://www.diyaudio.com/forums/showthread....10&pagenumber=6

Maybe the idealized thin-lens that RAYTRACE here uses, can't really simulate off-axis fresnels? I don't have any fresnels yet so can't experiment...

Another comment: I used an Infocus X1 for awhile and it projects "up" with keystone correction by default. It is fairly short-throw but it did a perfect job of optical keystone correction, somehow. How---?

edited down, dam these raytraces turned out big

That is the sexiest thing I've ever seen!

Very very interesting. But notice the extreme angle its not hitting the lens. That may be too much.

Ok. Heres some analysis. Moving the lens farther away from the lcd results in a smaller picture, closer up. Moving the lens closer to the lcd results in a bigger picture, farther away. That said, you would think that you could achieve keystone correction by tilting the lcd, one side is closer so the projected image will be smaller and closer, the other will be farther and bigger. The problem with this theory is that the ratio of the distance of the lens to the ditance of the screen changes depending on how close or far it is.

So if you tilt the screen, the proper focus points for where the lcd should be would be curved. As shown in diagram A below.

Now I suspect tilting the fresnel helps to compensate for this BUT the fresnel is still flat. If you could mathematicly calculate exactly where the fresnel needs to be in each location you could create a curved frame and BEND the fresnel on these points exactly, that would in theory produce a perfectly focused image.

Moving the fresnel is differant then just tilting the lcd, and the fresnel distance and angle obviously has an effect on the lcds focus distance (since it works for keystone correction) But obviously there are some aberations produced. The fresnel may have the same issue with irregular magnification so it may need to be first curved to compensate its own focus, and then curved from there to compensate for the triplets focus.

My only worry is I have a sneaking suspicion that this will produce a trapazoid, in perfect focus. However, maybe this can be compensated for by tilting the lcd as well as the fresnel. The other is we can't bend the fresnel too much without making it too small to catch all of the lcd.

You would be lost simply trying to bend your fresnel. There are an infanite amount of combinations and the curvature varies based on projection size and screen tilt. Also if the starting point of the fresnel would have to be tilted, and you had to adjust the lcd as well to compensate for the trapazoid, there are an awful lot of variables.

In otherwords, I think the exact curvature would have to be calculated mathimaticly, you couldn' t just manually adjust to find it.

How precise would the curveture have to be? I'm not sure exactly, but I'm sure it would work better then a flat fresnel. The accuracy would depend on howmany points you plotted and how precisly you bent the fresnel around that point. You could use push pins, a curved grove or maybe attached to square fram with a curved front.

How would you calculate the differant focal distances of the lcd? You would have to calculate that based on the distance of the screen in various points. The more points you plot, the more accurate it will be.

To figure this out mathematicly, we'd need to know exactly how the fresnel effects the image based on distance and tilt. This may also vary in differant area's of the fresnel. This would require a lot of experiments to calculate a formula to figure out exactly how the image is effected. This formula would have to be used with the focal distance formula for the triplet.

In otherwords, it would be a highly scientific mathimaticle procedure to figure this out, and create a formula to plot the points of the curveture. But once you had it, you could just punch in the distance and angle of the screen your projecting on, and it produces the points to bend your fresnel around and how far to place it from the lcd AND what angle to tilt the lcd (if thats required).

We need that guy from N3mbers!

By the way, bending a fresnel one way is very easy. Two ways is whats hard. This should only require one way.

Keep in mind that the fresnel may not (and most likely will not) simply have to be curved around the points where the projection focus is.

There are an awful lot of things to be considered here, so we're going to need a math genius.

But I have confidence that it will work, if you can find the right curveture. But I doubt that can be done manually, So we need it to be done mathematicly.

You could try bending your fresnel by hand when keystoning to see if you can get any improvement I suppose, you would have to tilt it as usual and pull the middle in slightly, but again, it would be very difficult and you might ruin your fresnel.

Just want to clarify a few things and perhaps add a note of caution.

Not trying to stifle innovation here but Mik where have you read about bending fresnels? (just asking because you sound fairly convinced) Maybe i've missed a good plog or two but i haven't noticed the method employed anywhere. I will admit the idea sounds intriguing and maybe possible with the "perfect bend and placement", otherwise...hmm.

Just a note of caution for the noobs before they begin trying to bend their fresnels with a karate chop or two

How do i know bending works? Well to be honest I don't. But why not? The fresnel should bend most around the thinest area, which is the gaps between the triangles in the fresnel, so the triangles will not be bent, they will just be tilting.

Here's a pic I found which seemed very encouraging. This looks just like the adjustment on some commercial projectors. I highlighted the red because it sounded to me alot like some of the stuff in this thread, ie. on the right track?

Nice! Where did you find that?

I'm gonna pm joechevy about this. We're doing some experiments with lens shift.

Any more info on that lens?

HOLY SMOKES! I just found the page. Tell me that thing is an attachment you can put in front of any projector!!!!

WHERE DO YOU GET THAT THING???!!!!!!

Hmmm...well the only place I found it available is a place where you can "rent it" for 25 bucks.

So that thing must be an expensive product only for commercial use. :angry:

Yeah, just checked the pricing. Everything seems to be 1000 dollars and up. But at least we finally have some information on the lens shifting system.

Back to the fresnel bending idea. Where were we?

i found it in the middle of a google fest, it showed up under "projector keystone optical correction". There might be some other useful stuff in there. 47000 results :-]

I'm pretty sure the Epson 500 has a similar pair of knobs to twist. They call it "manual lens shift". They list "keystone adjustment for fine tuning" as a separate feature.

I wonder if this type of lens only works well if the input is already a collimated / parallel beam? Or does the first lens of the triplet do that for us. I'm still trying to understand the basics of that triplet! Optics 101 (or maybe 201?)

Not sure. I emailed the company of that gnk xypc keystone correction lens for info on pricing and dealers. And they said its been discontinued but might be available on ebay.

For now I'm going to think more about bending the fresnel.

Charlie if you could do me a favor, could you post a still picture of the drawing of the keystone correction after its been corrected? I'm trying to compare the two and see if ALL of those rays have been corrected.

If not then we can try readjusting the collector, then see if other rays are corrected. That would prove that a curve is required.

Bending the collector might mess it up a bit, but it might mess it up precisly the way we want it.

The tricky part is when bending the collector is a lot differant then just tilting. Its like tilting each piece of the fresnel in differant spots. Only differance is they are connected, so tilting it effects the height of the area's of the fresnel. But again, these aberations may be what we need.

Here is the pdf for the GNK.
http://www.navitar.com/download/gnk_keysto...nstructions.pdf

All the pricing I've found for it is call us.

still workin on it...

Here is the one I think you wanted -
The RAYTRACE demo doesn't allow saving, so I lost yesterday's model -
I still had the gif though, here is the 3rd frame and zoomed up the screen. Do they look even?
I'm working on another model using RAYTRACE's "measuring stick" tool.