Can some of you share the techniques you use to find the exact center point where a pj should be mounted so that it lines up perfectly without needing any horizontal lens shift of slight adjustment to the yaw?

I'm going to be removing my Chief RPA and switching it with the RPM and would like to get it more on. I know it is slightly off center because I had to use the Ruby's lens shift feature to center it. Trouble is, the Ruby is long gone and I can't remember whether I shifted it to the right or left and by how much... And my Sharp 10K's lens is not centered with its body so can't really go by that either.

I have a laser light level, which I understand can be handy for this purpose. However I'm not sure what the technique is. For example even if I line the beam so it is going straight up the center of the screen and then across the ceiling, how do I know that its not on an angle on the ceiling? Is it sufficient to just have the beam exactly in the center of the screen at its bottom and its top, and that ensures a perfectly perpendicular line across the ceiling? Or is it not sufficient to line it up this way?

Technically it really isn't necessary for me to worry about this because my next pj (the RS1) has horizontal lens shift, and this seems to be becoming more popular now (i.e. Ruby had this too). And for pjs without this I could always just add the Chief lateral shift bracket that gives me 3" I believe. However while I'm in the process or redoing the mount I figure I might as well try to get it spot on the center.... Thoughts?

My way, which I have never used, is to get a string (Or a tape measure) and mark equal distances from both edges of the screen. A tiny stick pin might work, and you might put a sticky note on the ceiling for the first arc, then mark the spot on that arc that the other side crosses.

Mark one spot a little farther than you want to mount the projector. Mark one spot a little closer than you want the projector. The line between these will be the centerline of your screen.

Maybe those who do this for a living have a better way.

Let me know how it works if you try it. I plan on doing this within the next few months.

Best Regards,
Doug

I thought about that and generally think its a good approach. However in my case I don't have a straight run to the screen (angular walls and so forth) so its more challenging in practice. I'm figuring there has to be a pretty straight way using a laser level - just not sure what the right technique is here.

I would think using a laser measure at a couple of points down the ceiling, ensuring that both screen left and screen right are equidistant at each point would yield the center line?

Mark

Humm. I'm starting to think that if I just measure the half way point at the bottom of the screen, and to the same at the top of the screen, and have the laser level doing a perfectly straight line between the two - then that should result in a perfectly centered perpendicular line running down the ceiling. Anyone care to confirm or reject this theory?

However in my case I don't have a straight run to the screen (angular walls and so forth) so its more challenging in practice.

I'm having a hard time understanding this. If you have a projector pointing at your screen you don't have anything in the way.

Put a table where you think your projector will be hanging. Pin a string (one that wont stretch) to the side of your screen at the same height as the table top. Tape a sheet of paper on the table. Make your archs with a pencil on the paper. When you are done making archs and drawing your center line use a plumb line (something pointed and heavy on the end of the string). Get on a chair or whatever and hold the plumb line to the ceiling and center it over the line you drew on the paper. Now mark your ceiling with a pin or post it.

Assuming you can generate a perpendicular line from the screen that should work. In fact, assuming the screen is mounted level and without warp you only need to do this from the middle of the top of the screen.

I'm having a hard time understanding this. If you have a projector pointing at your screen you don't have anything in the way.

Put a table where you think your projector will be hanging. Pin a string (one that wont stretch) to the side of your screen at the same height as the table top. Tape a sheet of paper on the table. Make your archs with a pencil on the paper. When you are done making archs and drawing your center line use a plumb line (something pointed and heavy on the end of the string). Get on a chair or whatever and hold the plumb line to the ceiling and center it over the line you drew on the paper. Now mark your ceiling with a pin or post it.

What do you mean by "make your archs"? What am I making exactly?

Also if I do a string from the center of the screen running to the back of the room where the table is, how do I know that string is perfectly perpendicular to the screen the whole way? The slightest angle will put my center point in the wrong spot. I cannot measure against the side walls to ensure its perpendicular because the side wall does not run perfectly straight.

use two tape measures. fix one on the top left edge of the screen, the other on the top right edge. walk both back the distance you want the PJ mounted. bring both measures together with equal tension until they indicate the same length. That's the middle.

Make sure the the tape measures are the same "Length". On some cheap ones 10' may not be 10'. Just pull out the two tapes straight the same length first to make sure.

Scott

Bimmerboy said it in an easier to understand fashion.

The idea is that the center line of the screen should be equidistant from each of the top corners of the screen.

Using numbers that make the math easier (a 6, 8, 10 foot triangle)

Lets say that you had a pj that was perfectly centered on the screen and perfectly aligned with the top of the screen. And the screen weas 8 feet away from the pj, and the screen was 12 feet wide, then each top corner of the screen would be 10 feet from teh center of the lens.

Laser levels and everything else will work fine, as long as the level is really flat along teh screen, etc. After 12 or 20 feet, a very small error will be larger.

If you measure from each corner, then small errors will be small.

The idea with the arc, is that you will not know, in the beginning, where the PJ is supposed to be (which is why you're going through this excercise).

So you start by drawing an arc that you feel crosses the center line of the screen. In the example above, you would use a 10 foot arc. Then you measure from the other side and draw an arc at a radius of 10 feet. WHere they cross is on the center line of the screen.

For your PJ, just measure from the corner to about where you want your pj, and then use that length for both arches.

Using a table and the bottm corners of the screen is a really good idea, too. Then you could just use a piece of string with a small weight on it to find the spot on teh ceiling that is directly above that spot.

Almost all string will stretch. A tape measure would probably be about the best thing to use.

With this method you don't have to worry about square or flat walls, or anything like that.

BEst Regards,
Doug

PS, if this seems to confusing, just let me know and I will sketch it out. It really should be very simple. If it seems anything else, A sketch should clear it up.

Bimmerboy said it in an easier to understand fashion.

The idea is that the center line of the screen should be equidistant from each of the top corners of the screen.

Using numbers that make the math easier (a 6, 8, 10 foot triangle)

Lets say that you had a pj that was perfectly centered on the screen and perfectly aligned with the top of the screen. And the screen weas 8 feet away from the pj, and the screen was 12 feet wide, then each top corner of the screen would be 10 feet from teh center of the lens.

Laser levels and everything else will work fine, as long as the level is really flat along teh screen, etc. After 12 or 20 feet, a very small error will be larger.

If you measure from each corner, then small errors will be small.

The idea with the arc, is that you will not know, in the beginning, where the PJ is supposed to be (which is why you're going through this excercise).

So you start by drawing an arc that you feel crosses the center line of the screen. In the example above, you would use a 10 foot arc. Then you measure from the other side and draw an arc at a radius of 10 feet. WHere they cross is on the center line of the screen.

For your PJ, just measure from the corner to about where you want your pj, and then use that length for both arches.

Using a table and the bottm corners of the screen is a really good idea, too. Then you could just use a piece of string with a small weight on it to find the spot on teh ceiling that is directly above that spot.

Almost all string will stretch. A tape measure would probably be about the best thing to use.

With this method you don't have to worry about square or flat walls, or anything like that.

BEst Regards,
Doug

PS, if this seems to confusing, just let me know and I will sketch it out. It really should be very simple. If it seems anything else, A sketch should clear it up.


Sounds like a great technique but not following it too well. Yes a sketch would be GREAT! Thanks!!

I use a "Lateral shift kit" on my RPA - it allows you to move the projector 3" or 5" to the side depending on projector weight. The joist in the ceiling is 2" off dead center in my room. This allowed me to attach the mount to the joist, and have the projector in the exact center of the room (my Pearl has a centered lens). This might be useful for you or not depending on your exact situation, how far off from center your current mount is and if it is in the "right direction" considering your off-center lens and exactly how many inches of play you have.

Sounds like a great technique but not following it too well. Yes a sketch would be GREAT! Thanks!!

Think of the string like a compass. If you put a pencil on the end and use the string to represent the radius of a circle you can use it to draw and arc. Basic geometry will prove that if you draw two arcs from either side of a line these arcs will meet in the center. If you shorten the string after making the first point and make a second set of arcs to make a second point you can now draw a line between the two points. With this line you can now know the exact center and line of travel to the center. With your plumb line you can move up and down the line as needed. Now you can measure from the screen center along the line you drew and determine both your center and distance from the screen for your mount.

The picture is basic and didn't include the arc as it's not so simple to draw them in paint.

If you were looking straight down at the paper on the table this is what your pencil marks might look like...except they would be ...better looking :)

lovingdvd,
Don't worry about slight lens shift. I am going from an RPA and Sony10HT, which was off center by about 2 inches. Being an old View Camera user, the lens shift should have no optical impact unless completely over done (and it's obvious - you get vignetting). As long as the lens was designed to handle the shift documented (IT BETTER HAVE BEEN!!!), you should see no negative impact. I imagine a 2 or 3" shift is probably an 1/8" move on the shift lever. It all depends on the quality of the lens.
I plan on shifting on the fly and watching for distortion. If I see any for a 2" shift, I'll be VERY surprised and then I'll get the Chief attachment (and send the bill to JVC - ;)
Regards,
Dom

Ric. We discussed this in this forum when you were installing your first Sharp. Your laser method is what my guys use and it works fine. You can double check the results by using a string to the corners of the screen frame. Check at several points along the red center line outlined by the laser. As long as the same length of string on both sides, you got it.

That TV character,, Monk, has nothing on you. :)

Lovingdvd,

Did EAS's sketches clear it up? They looked really good. If you want, pm me and I will send you step by step sketches.

Mark's idea of using a laser level and then checking the measurements to the corners is sounds good, too.

Best Regards,
Doug

Humm. I'm starting to think that if I just measure the half way point at the bottom of the screen, and to the same at the top of the screen, and have the laser level doing a perfectly straight line between the two - then that should result in a perfectly centered perpendicular line running down the ceiling. Anyone care to confirm or reject this theory?


Well, that laser light could point left or right of the desired center line and still shine down to cross the center of the screen, like a divining rod people use :D

Let me ask if you had a geometry class in school?
How to draw a perpendicular line to another, in this case your screen, so it is also at the center point of that line/screen with a drafting compass and a ruler.

The compass point at each end of the screen marks an arc some distance away from it. But, you would use a tape measure and a pencil using the same distance from left and right side of that screen. Connect the intersecting arc and the center of the screen projected to the ceiling. That line is the center line 90 degrees to the screen.

ps. you want the lens center on this line as the lens may not be exactly on center line

Thanks guys - yes the sketch was a big help. Makes perfect sense. I like this idea better than the laser level, as oddly enough it seems less prone to error.

My screen is mounted relatively high on the wall, so the top of the frame is only about 4" from the ceiling. With this in mind, I'm going to try and attach the strings to the TOP of the frame and draw the arcs directly on the CEILING.

Since I have an existing mount within an inch or so of the exact spot, I should be able to make the arcs small enough (focusing around the existing "center" point) so that I am not marking up much of the ceiling.

I guess one trick will be adjusting the string length so that the arc curves cross at the exact mounting point I need (distance from the screen, as there is a beam I need to secure to which is narrow so my distance is set).

So my plan is going to be to use a string longer than necessary and attach the pencil by wrapping it around the string a few times, so that I can make it longer or shorter as necessary. I'll do this with one side until it seems to cross at the distance right near my current "center" point. Then I'll lock in that string length and draw that part of the arc. Switch the string to the other side and mark where they intersect, and viola, my true center point.

Actually this is a fairly pointless exercise because the RS1 has horizontal lens shift. However I like to upgrade often and in the off chance that my next pj does not have any lens shift, I probably then won't need the Chief lateral shift bracket (eliminating that piece helps keep the mount as clean looking and flush as possible). Of course this assumes the future pj's lens is centered (some like the Sharp's are not).

At any rate now that I understand the technique it seems like a trivial exercise and I'll learn a little in the process, so might as well get it spot on. I'll keep you guys posted on how it goes. Thanks again for the great suggestions and especially for taking time to make the sketches!

I had the same issue when I went to mount my 11S1, I wanted the the ability to be able to move the projector from left to right and be able to center it, not only for this projector but for any in the future. Draw slides did the trick, hope this gives you some ideas

I mounted my projector first and got the image looking good on the wall, then I centered and mounted the screen to the projector. Worked just fine in my case, no lens shift was necessary. Don't know if doing it this way is a no no or not?

I had the same issue when I went to mount my 11S1, I wanted the the ability to be able to move the projector from left to right and be able to center it, not only for this projector but for any in the future. Draw slides did the trick, hope this gives you some ideas

I wonder if we need to coin a new term...

"Projector Shift" ??? :)

Doug

Thanks guys - yes the sketch was a big help. Makes perfect sense. I like this idea better than the laser level, as oddly enough it seems less prone to error.



Yes, EAS made some nice sketches. :)

As for using the string, do remember that the string will stretch, so you will have to pull with the same force on both arcs.

As for finding the exact distance, you can do that. As you go, you might find it easier to mark a spot an inch in front of where you want the projector, and then a half-inch (or two inches, or wherever it happens to fall) behind where you want the projector, then connect the two points with a short line. Finally, find the point on that short line that is the right distance from the screen.

Pretty much like the second sketch that EAS made.

Good luck,
Doug

I wonder if a patent is in order :)

Finally, find the point on that short line that is the right distance from the screen.

Pretty much like the second sketch that EAS made.

Good luck,
Doug

Yep, that is exactly what I was planning to do. I figure that will be easier than trying ti figure out the perfect string length to get the arcs lined up right at the proper mount distance. Only downside to this approach is that it is a bit more prone to error. I'll let you guys know how it turns out.

At any rate now that I understand the technique it seems like a trivial exercise and I'll learn a little in the process, so might as well get it spot on. I'll keep you guys posted on how it goes. Thanks again for the great suggestions and especially for taking time to make the sketches!


One thing to keep in mind when using a string to do your arcs. Use something that doesn't stretch, like fishing line, so you don't stretch one side of your arc longer than the other, or you will not be perpendicular to the screen.

One thing to keep in mind when using a string to do your arcs. Use something that doesn't stretch, like fishing line, so you don't stretch one side of your arc longer than the other, or you will not be perpendicular to the screen.

Or use mason's line it is made specifically to do things like you're trying to do. You can get it at any hardware store for a few dollars.
http://us.st11.yimg.com/us.st.yimg.com/I/cinemasupplies_1933_79275826

Only downside to this approach is that it is a bit more prone to error.

Well, this sort of thing has been done for hundreds if not thousands of years so I am curious as to what you might think is going to cause an issue.

I use wire instead of string, but really it's not that big of deal. Get your wife or kid to hole the other end against the wall and mark your lines with about the same tension. The more distance between the two sets of arc marks the more accurate your center line will be. Make more thanb two sets of marks if you doubt the accuracy. I'll bet you it's 50% more accurate than using a laser.

BTW: Fishing line stretches more than you think...

No expert but I used a laser level, measure the center of the screen, set the laser level on the mark it projected a line down the center of the ceiling. Next I cut a Triangle out of a sticky dot, placed it in the center of the mounting plate. I then held the plate up on the ceiling when the line was in the center of the triangle I used Drywall screws to lightly secure the plate. Next I measured across to find the width, divided by two, made sure it was in the middle of the mount. When I was sure it was really close I mounted it to the ceiling. Lastly to triple check I measured from Screen edge to the opposite back corner of the mount to see if the distance was the same.

Well, this sort of thing has been done for hundreds if not thousands of years so I am curious as to what you might think is going to cause an issue.


Basically just operator error :). What I was saying there is that if I use this technique to make two points and then connect them with a line, that's a bit more prone to error (both points need to be exact now) than if I I just aim for making one point at the correct distance. No biggie - I'm sure it'll all come together. This is a great technique for sure.

Let's say I use these techniques we've been discussing to find the exact center point and I get my mount all set up accordingly. But then when I put the picture up, let's say it is obviously still a bit off center.

The thing I'm wondering about is how can I easily tell whether it is off center because I made an error in mounting or marking the center spot, versus a case where my yaw is not perfectly parallel to the screen.

I'd imagine that with a small amount of error in the yaw, it could be perceived as having the mount off center - and vice versa.

Does anyone have any tips or tricks for adjusting the yaw and how to tell this difference I'm wondering about? Thanks!

Let's say I use these techniques we've been discussing to find the exact center point and I get my mount all set up accordingly. But then when I put the picture up, let's say it is obviously still a bit off center.

The thing I'm wondering about is how can I easily tell whether it is off center because I made an error in mounting or marking the center spot, versus a case where my yaw is not perfectly parallel to the screen.

I'd imagine that with a small amount of error in the yaw, it could be perceived as having the mount off center - and vice versa.

Does anyone have any tips or tricks for adjusting the yaw and how to tell this difference I'm wondering about? Thanks!
You're over analyzing the situation. The yaw will be correct when the projected image fits within the frames right and left boundaries. :D

Geometry of the rectangle projected on the screen. You obtain perfect geometry and if your rectangle is not centered, your projector is off. Then you use lens shift or projector shift or screen shift.

BTW: Fishing line stretches more than you think...


Well, I guess I am not a fisherman :D
How about a 30ft steel tape measure? :D Then, you can be very accurate, as accurate as how steady you can work overhead :D
Or, one can lay it out on the floor and just use a plumb bob?

Given that the Chief mount has about 4 inches of lateral hift adjustment built in, what's the point of all this?

Given that the Chief mount has about 4 inches of lateral hift adjustment built in, what's the point of all this?

I believe there is a lateral shift bracket that you can add as an accessory, but there is no lateral shift available on the RPA or RPM unit itself without this. Unless I am mistaken, in which case someone please correct me.

Given that horizontal shift capabilities are becoming more commonplace in pjs, and that such a lateral shift bracket accessory is available otherwise, there actually is little point in all this...

However since I am changing the mount and know it is off center, I figure I might as well get it centered while I'm at it given that this is a 10 minute exercise at most (now that I understand the process).

BTW thanks so much for the PM - will touch base tomorrow.

I looked at a RPM mount today at AVAD, and it has built in lateral shift. I didn't see what pecific model it was for. I have one in stock for a Sony and will look at it carefully tomorrow.

Given that the Chief mount has about 4 inches of lateral hift adjustment built in, what's the point of all this?

Believe it or not, my screen has lateral shift built in! Yep, the Screen Innovations Reference series is mounted on a bracket that lets you slide the screen left and right on the bracket in order to get it perfectly centered. Nice!

I looked at a RPM mount today at AVAD, and it has built in lateral shift. I didn't see what pecific model it was for. I have one in stock for a Sony and will look at it carefully tomorrow.

My guess is that what you were looking at was the RPM plus the lateral shift accessory added on. I'm almost certain the RPM on its own cannot do this - it only has adjustments for yaw, pitch and roll. Could be wrong there so let us know after you take a closer look.

LovingDVD...

This thread will soon be longer than the "RS1 questions answered thread" and we can't have that, can we? This may have been suggested, but I'm not reading the entire thread.

If you still have your existing mount up and a pipe protruding thru the ceiling, take a piece of string and run it from one corner of your screen, around the pipe and back to the opposite corner of the screen, pull it snug and cut it off to that length.

Take the string and find the exact middle of the string...mark that with a piece of tape or whatever...just someway you can identify the middle clearly.

Attach (or have two friends hold) the ends of the string at opposite ends of the screen. Hold the center of the string and pull it tight...there is the center point, perpendicular to the center of the screen...mark it. That is the rear of your pipe mounting location...simple, if you have enough hands.

As for determining if you've got yaw and are improperly compensating with lens shift, if you have the projector properly centered, there is only one combination of yaw angle and horizontal lens shift that will produce a square image.

As I recall, there is no center detent in the RS1 lens shift system as a reference. Oh no, there go a few more dropping off the pre-buy list. :-)

This will be easier if you make sure you have the projector accurately leveled as it's easiest to use the upper or lower edge of the screen as a reference point

Adjust yaw and shift in small increments until the height of the L and R side of the image is identical within whatever tolerance you deem necessary...given that you're an AVS'er, I'd recommend .001mm just in case someone else from the forum ever visits your theater.

Cheers,

Ric. I really do know what a LSB100 looks like. I usually package the RPA mounts with a LSB100 and a ceiling plate such as the CMA110. Add a 1 1/2 inch NT threaded nipple from Home Depot and one is done.

I've switched our package now to the RPA and the newest ceiling plate, the CMS115, much nicer looking. I really hadn't looked at it carefully before as my guys, not me, install them. They carry an LSB100 and various nipples on the truck in case they need them. I saw a really nice looking red headed women on the truck a few days ago but she wasn't NTP threaded. The RMA I looked at yesterday did not have an LSB100 attached. I thought it had a lateral shift built in. I'll look at that one again next week.

Tom. Not Rric. He measures in Angstrom units. One Angstrom is equal to 10 to the minus 28 cm. This measurement is useful when measuring atomic distances and to obtain a mounting precision acceptable to Ric.

Ric. I looked at the RPA I have in stock. You are correct. There are only pitch, yaw, and roll adjustments. No built in lateral shift. For that you would need a lsb100 and that's no good for you because it would add further drop from your ceiling. Perhaps the one I looked at at AVAD was different. I clearly remember a horizontal shift slot. I'll check that one later next week.