Projector offset vs optimal image height... how do you achieve it?


Novice Member
Hi all, I could do with some advice on ceiling mounting a projector as I've either got my maths wrong, am aiming for an unusually small screen, or I need to consider having the picture further up the wall than I had hoped. This is my first projector install so I might be missing some vital point.

We're shortly going to be gutting our living room, and while the ceilings are down and we're chasing in power cables we are taking the opportunity to do a home cinema installation. Room dimensions are 4.35m long x 3.2m wide x 2.4m high. As we have other rooms in the house where we can do standard TV watching, the plan is that the living room will do double-duty as a sitting room and a cinema. So it doesn't need to be a stealth install, but at the same time I don't want to be packing the projector away after each viewing, or putting a cabinet in the middle of the room. Door and fireplace location limits sofa placement, which means we are putting the screen on the 3.2m wide end wall, with a window on the opposite wall (black out blind to be ordered!) so we can't do a shelf mount.

We need 4k and 3D to play all of our source material and I'm tending towards the native-4K BenQ projectors. These projectors have a 110% offset, which in BenQ's use of offset percentages means a 100cm-tall image from a projector can be aligned with the top edge of the image up to 10cm below the centreline of the lens. A screen size between 80" and 90" diagonal looks to be optimal when I mark it on the wall and imagine watching a film there without having to dart my head around the place. Viewing distance 3.8m from the best seat in the room to the centre of the picture, around 2.9m from the least good.

In a sofa-seated position, my eyes are 100cm from the floor. Looking at our existing arrangement with a TV, the bottom of the picture is about 70cm above floor level and this feels comfortable. So with a 90" screen giving a picture height of about 1.1m, this means the top of the picture would be 1.8m from the floor. With maximum offset the centreline of the PJ lens is at 1.9m, and therefore the projector mount would need to hang down approx 40cm from the ceiling. If I go for an 80" screen then that increases to approx 50cm (NB these are rough numbers, I've not calculated the exact numbers)

Does that sound about right? 1.9m from the floor seems like a bit of a dangly projector prone to knocks from heads. The BenQ offset seems fairly typical (there is an Epson with a bigger offset but it's "faux-k"), so do I have an unusually small screen in mind or do ceiling mounters just set the picture a bit higher up the wall than my maths anticipates?


Active Member

Benq does not make native 4K projectors. Both Benq and Epson are pixel shift. Benq has 8m vs. 4m pixels on the Epson, but unless you are too close or the screen is very large, you won't see the pixels/screen door effect.

Secondly you can't imagine how big a screen should be. This should be decided after you get the projector and use it on the wall for a few weeks.

In general the ratio used is 10-12" (or more) in diagonal for every foot/30.48cm distance. Doing an average between 3.8 and 2.9 you get 3.35m.
So that would result in a screen between 110 and 130.

While how big the screen should be is subjective, a 80-90 is very small. Getting a small screen is the first major mistake new buyers make. I suggest you aim for the largest screen in that room, which is ~135" diagonalwith the room limitations:

If you get a projector with motorized lenses with memory, and can do a CIH/Constant image height setup. It's basically a 2.35:1 (or other scope format) screen that is used with movies which are usually this format. When it's time to watch a 16:9 format the projector can zoom to fit that screen, and you'll have unused screen on the sides of the 16:9 image.
But a projector with motorized lenses and memory is required. The only one I can recoommend is an Epson TW9400 (6050UB)
It can do ~127" (16:9) in that room and has a lot of lens shift:

For more CIH calculations:

If you insist on a 80-90" screen then get a TV and a cheap used/new 1080p DLP and a drop down screen for 3D.

Thirdly ambient light and wall reflections will affect the contrast/blacks. Needless to say projectors are a dark room device. Assemble the link:


So treating the room is a good ideea.

Regarding offset, Benq's specifications for the W2700 which I assume you're reffering to are not exactly the same in all materials.
I believe offset is 100% (center of lens at the top edge of the screen in a ceiling mount), with 110% lens shift downwards.
This calculator is a bit different (W2700=HT3550):

In any case get a mount with variable distance.

Another option with some lens shift is the Epson TW7100. But not motorized.

To determine how high the screen should be, sit on the couch, close your eyes, sit in the usual position. When you open them the center of the vision should be the center of the screen.

Regarding 3D, DLP is better than 3LCD.
Problem is bright DLP projectors (for a screen ~130") are sports type projectors which sacrifice color/black level for brightness. They also have 60-80ms lag for gaming.
So there's no ideal solution.

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