[tomhahn]

With my HT300 I wanted to avoid using digital keystone adjustment. But putting the center of the lens level with the top of the screen meant that tall folks would have to be stoop a little to get under it.


If a raised it a few inches, and tilted it down, I would get some keystoning. But a little, I thought, might be OK. Could I really tell if the screen was an inch wider at the bottom than the top? I tried it, and the answer is no.


So I thought it might be handy to calculate how many inches to raise the center of the lens above the top of the screen to acheive a given tolerable amount of keystoning. I did the math, and it seems to bear out my experiment. So I thought I would pass it along.


You give this formula 4 numbers and it tells you X, how many inches to put the center of the lens above the screen top. Here are the 4 numbers you give it:


W = Width of your screen

H = Height of your screen (distance from top to bottom)

D = Distance for the front of the lens to the screen

K = Keystone width, that is, the desired width of the bottom of the image after the pj is pointed down.


For example, I had


W = 82

H = 46

D = 196

K = 83


That is, I was willing to have the image be 1 inch wider at the bottom than the top.


Here's the formula:


X = D * Tan ( ArcCos((D*W)/(Sqrt((H*H) +(D*D))*K)) - ArcTan(H/D) )


In my case, X was 9.2 so I could raise my PJ so the lens was 9 inches above the screen, slant it down a little to aim at the top of the screen, and only incur one inch of keystoning (1.2%)!


Hope someone else finds this useful.