CurtPalme.com Home Theater Forum

lyd · Joined: 15 Sep 2007 Posts: 390 Location: Lake Mills, Wi

Howdy again. I have reviewed the primer and done some searching, but not quite gotten a handle on this yet.

I am trying to determine what height relative to the screen I should be shooting for. My initial impression had been that I want to get the horizontal center of the lenses as close to the center of the screen as possible, then adjust the angle of the projector on the mount as required. A number of things I have come across later seem to be suggesting that I am better off going with a specified vertical offset relative to the screen, which would keep the built-in 10 degree projection angle as-is and the projector level on the mount.

Can anyone help me determine what I want to be shooting for here? Feel free to just point me toward the relevant information, if that is appropriate.

To sum up, this is a Marquee 8500 with HD144 lenses and a 4.5' high 16:9 screen (96" x 54").

My continued thanks for all the help.

lyd

Heywood Jablome · Joined: 12 Mar 2006 Posts: 1548

Dead perpendicular (centered in both axes and angled so the green tube is perpendicular to the screen) will give you zero keystone, which maximizes light output and the ability of the projector to resolve higher resolutions.

That said, typical front projection setups can't suffer this location... the projector will be in the middle of the room, in the middle of the air. Some have really high ceilings and really steep stadium seating, where the PJ is mounted in the risers, but besides that... probably not real practical.

The manuals for any given projector will list mounting options, but typically the PJ is parallel to the ceiling and the lens EDGE lines up with the screen edge.

Now, a rear projection setup, with or without mirrors, can be made zero keystone IF you've got the real estate behind the screen.

garyfritz · Joined: 08 Apr 2006 Posts: 12088 Location: Fort Collins, CO

Many projectors have a built-in "tilt" angle, so that when you hang them level they shoot "down" to the screen center. My XG and Dwin were like that, and I think the G70 was too.

I mounted my Marquee (with HD145's) level, thinking it would work the same way. This resulted in a centered raster shooting way high. I had to raise up the rear end of the projector (rear mounting feet about 1" higher than the front feet) to get a centered raster to project on the center of the screen.

My screen is about 85" wide and I mounted the projector at the appropriate distance for that. Actually I'm a few inches farther away than I should be for the HD145's but the tilt angle should be about the same.

Your angle will vary of course, depending on how far your screen center is below your projector, but that's what worked for my room.

lyd · Joined: 15 Sep 2007 Posts: 390 Location: Lake Mills, Wi

I think I am still confused.

Let me try asking this a different way....

The "built-in" angle of the 8500, the angle of the lenses with the base level, is 10 degrees, so I will treat that as my maximum. The manual says the allowable angle extends to 15 deg., but there is no reason for me to go past 10.

For my 96" screen, I calculate 120.5" (rounded) as the theoretical optimum distance for the lens face, using 1.144*(96+9.3). (HD144 lenses)

The vertical center of my screen is 5' 4.5".

So I extend a line perpendicular to the screen plane at that height, for that length, and that's my Adjacent.

Opposite/Adjacent=Tangent and Tangent*Adjacent= Opposite, so I calculate the following.

Maintaining the angle of 10 degrees, the bottom of my projector will be about 7 feet off the floor.

With the bottom of the projector at 6' 2" off the floor, about the lowest I think I would want to go, I can achieve an angle of about 7 degrees.

I understand that these calculations make a couple of fudges, but is this a roughly valid way to determine what my projection angle will be at a given mounting height?

If it is, just how much difference will that 3 degrees make in practice?

I'd rather have the thing up at 7', of course, but if it is a tangible payoff I can definitely make the rationalization for going lower.

lyd

**AnalogRocks** · Posted: Wed Oct 24, 2007 5:28 am Post subject:

I think the easiest way to do this is just remember to have the projector within the range of the top edge of the screen to the bottom edge of the screen. You will have enough adjustment in the menu's to get it set up right.

EDIT: spelling

Heywood Jablome · Joined: 12 Mar 2006 Posts: 1548

lyd · Joined: 15 Sep 2007 Posts: 390 Location: Lake Mills, Wi

Heywood Jablome · Joined: 12 Mar 2006 Posts: 1548

I'm certain the brightness will not scale linearly, but am too far removed from my physics classes to estimate the real impact.

I'm no Marquee expert, but you should get there with acceptable sharpness if you pay particular attention to static and dynamic astig.