Trust the Throw Distance Calculators or "The Math"?

[mattvandyk]

I am looking at a BenQ LK936ST. It has a zoom ratio of 1.1x and a throw ratio of 0.81 ~0.89. This is consistent with the marketing materials which say "Fill a 14 foot wide screen from less than 12 feet away" (which would require a throw ratio of 0.857).

I need to fill a 180.4" x 112.7" screen (16:10), but I have a limited area in the ceiling within which to mount the projector. Accounting for the depth of the projector unit itself, the farthest away I can get the lens from the screen is 148".

At a throw ratio of 0.81, this will, conveniently, be almost exactly perfect as the throw distance for a 180.4" wide image at a 0.81 throw ratio is 146-1/8".

HOWEVER, every calculator I've used for this projector -- both the BenQ calculator and the ProjectorCentral calculator -- conclude that the closest I can get to the screen is 162.4", which is well over a foot longer. That throw distance will not work.

Does anyone know why the calculators would seem to be this far off? Are they off? Should I trust the math or trust the calculators? In this case, the difference between the two determines whether this will work or it won't.

Thanks in advance!

[Capz65]

When I calculated throw for mine the Epson calculator was shorter by a foot compared to projectorcentral with exact same values. I ended up placing it by trial and error. I know not much help, hopefully someone can clear the confusion, as I'm considering upgrading to the 936 as well. What could help is if someone with a 935st chimes in with which calculator he used and how close the math is on that model.

[Capz65]

This might help. The throw distance for my Epson is 15-16ft according to the calculators. That's where my projector is now. Then when I do the math the throw distance comes out to only 11ft. So in my case the calculators were right, math didn't work. I don't get why. And now I'm more confused.

[Capz65]

Ignore all that. When I change the calculator aspect to 16:9 the math and calculator values match. Try 16:9 aspect with your values and see if the math works then.

[mattvandyk]

That is EXACTLY it. When I use 16:9, the numbers work out. When I use 16:10, I get the problem described above. I am guessing that's because it's native 16:9 and in order to generate 16:10 it essentially just lops off the sides but the *real* image is wider, and this wider image image (the 16:9 image used to generate the 16:10 height) is what is used to calculate the throw ratio.

THANK YOU for solving the mystery. Now, it's a problem for me b/c I need to fill 112.7" of height, but at least now I know that that means my width (for purposes of calculating distance) is 200.4, not 180.4.

[Capz65]

Glad that helped. It took me awhile to wrap my head around aspect ratio and how to fill the screen. And I'm still making adjustments. Ron at gung ho set me in the right direction for setting a custom resolution on a windows PC. Are you using a PC with Nvidia card by any chance?

[mattvandyk]

Yes. RTX 3090. So, although I'm using the width to determine the throw distance, I'm actually backing into the width by the height. Will set it at the height to fill the screen understanding there will be spillover on the sides, and then eliminate that spill over in the card/windows settings. Is that why you were asking? HA!

[Capz65]

I went through the same back and forth, forward back zoom in zoom out rabbit hole. Ron simplified it. Mount the projector where you fill the width first. Then set a custom resolution in windows/nvidia that fills the height. And swing away.
Next I plan on putting the projector on a linear rail so I can slide it forward and back for movies. And then do it all over again when I can finish the walls and ceiling, and upgrade to the 4k. Neverending. But fun!

[mattvandyk]

Just so Im clear, you mean set the projector so it fills the height (such that the width of the image being projected is wider than the screen, but the height of the image is right on), then do a custom resolution to eliminate the width spillover on the sides, correct? If I set it to fill the width, I cannot "add" height resolution that isn't there.

[GungHoGolf]

mattvandyk see my reply below

[GungHoGolf]

I went through the same back and forth, forward back zoom in zoom out rabbit hole. Ron simplified it. Mount the projector where you fill the width first. Then set a custom resolution in windows/nvidia that fills the height. And swing away.
Next I plan on putting the projector on a linear rail so I can slide it forward and back for movies. And then do it all over again when I can finish the walls and ceiling, and upgrade to the 4k. Neverending. But fun!

That's almost correct. Unless your screen is WIDER than a 16:9 ratio (rare), you want to position the projector to fill the HEIGHT first, then adjust the horizontal number of pixels down on the computer to shrink the width to fit. Otherwise, you're giving up native pixels.

[mattvandyk]

Okay, that makes sense. Thanks!

[Capz65]

Still trying to wrap my head around it. Is "giving up native pixels" the same as saying you're distorting the image? Or stretching the image either vertically or horizontally beyond native? So in my case, I have a 1920 horizontal. By setting the custom height to 1600 or so am I giving up native pixels?

[GungHoGolf]

Still trying to wrap my head around it. Is "giving up native pixels" the same as saying you're distorting the image? Or stretching the image either vertically or horizontally beyond native? So in my case, I have a 1920 horizontal. By setting the custom height to 1600 or so am I giving up native pixels?

"Giving up native pixels" just means you're chopping off some of the pixels on the left and right hand sides of the image, in order to fit a screen that's narrower than 16:9. There is no distortion or stretching, and you're maintaining the full native resolution in the vertical dimension. The part of the image you're left with looks EXACTLY the same as the center of the image if you were projecting at native 16:9 (because it is the same).

[mattvandyk]

Just to be clear. If my screen is narrower than 16:9 (in my case, it's 16:10.925), that's what I want, right? To lop off the "extra" on the side rather than to have it fit the sides and sacrifice the screen real estate at top and bottom?

[GungHoGolf]

That's correct. At your aspect ratio, you'd want a custom resolution (created with Nvidia Control Panel, and then chosen in Windows Display Settings) of 2,160 pixels high and 3,163 wide (2,160 * 16 / 10.925).

[mattvandyk]

Okay, cool. Thanks. I think I get it!

[Capz65]

I was so confused by this until I went back to the thread where Ron helped me out. He clearly said fill the height first then do a custom horizontal. I had a brain fail and reversed it. I get it now.

[Capz65]

So I have a 168 diagonal screen. Ratio of 1.35. Multiply that by 1080 gives me 1458 on the horizontal. Haven't tested at the sim but I think that's the math.