How do you Calculate Optimum Throw Distance?

Mount it in the middle of the throw range for a little leeway.

Actually, the best place would be at the furthest throw distance for a given screen size. The "purest" part of any lens is in the center, and that's the portion you'd be using for a longer throw distance. You may not notice any difference though. Get the projector, and test out the images within the throw range you're working with. If you notice no difference, place it where it makes most sense for you.

Of course it becomes a somewhat delicate balance of fl's -vs- zoom -vs- throw distance -vs- screen gain.


I don't think there is any one answer on this. You have to balance the above for any room / projector / screen combo as to what will really be best.

I was putting mine closer to the minimum rather than the max distance, this was partly due to room restrictions, but mostly due to FL, I wanted a bright picture for ambient light situations (superbowl party).

Great point avZen. Unless the projector is an absolute light canon, I tend to be more on the side of max zoom to keep the fl's high.

I really don't know the answer for best picture quality. I have heard some say mid zoom; Kirk above says max throw. I suppose it could vary based on the lens design. Brightness, OTOH, is best at shortest throw. Zoom lenses are "faster" (allow more light) set at wide-angle than at telephoto. Projector calculators show this effect. Just watch the foot-Lamberts number plummet as you increase the throw while holding screen size constant. Optical experts, please chime in.

I'm not an expert, but I will chime in. You may be able to calculate a small difference in brightness based on a change in throw distance, but you'd need a light meter to do so. A foot lambert measurement doesn't take throw distance into account. It only measures the amount of light hitting a specific area, along with any screen gain. I'm sure others here can give you a more thorough or specific calculation, but in a nutshell, there it is. I've seen many projectors in action, and have moved them around quite a bit. I don't see much, if any, difference for a given screen size. YMMV.

The projector calculator at projector central does.

aye, the projector central calculator does in fact estimate foot lambert based on throw distance, just remember its a calculated amount and based on maximum lumen rating (which will never be hit in any HT)

Exactly, so if you are seeing a low fL value there, don't count on your setup to do any better.

Not flaming anyone here, but please show me how the FL rating changes based on throw distance. If you change the throw distance on the calculator, the image size also changes. Of course that will affect the FL rating. However, I've yet to see on this calculator where you can see the FL rating between two throw distances, with the same size screen. Maybe I'm missing something, and if I am, I'll be the first to admit it.

ETA: This quote from Da-Lite's site, on how to calculate foot-lamberts:

Foot Lamberts, conversely, are a unit not of illuminance but of luminance. As such, they measure the intensity of light per unit area leaving the screen. Foot candles are dependent only on the total amount of screen area and projector output. Calculating foot Lamberts requires both of those variables and screen gain.

http://www.da-lite.com/education/ang...ils&issueid=15

Also, this quote from Runco:

Projection light output basics:

The projector outputs a fixed amount of light
As screen size increases, foot-Lamberts decrease
Gain on the screen will increase the foot-Lambert reading
Negative-gain screens used for better black levels will decrease foot-Lambert value
Doubling the screen size diagonally results in 1/4 the brightness of the smaller screen
A formula can be assigned to every projector to calculate its true foot-Lambert measurement capability for any screen

http://www.runco.com/csms.html

You are missing something:

Go to the Pro calculator page.
Set for manufacturer and model.
Set your screen size on the right.
Go to ZOOM BOX on the left and drag the THROW RANGE up and down.
Observe the fL value change 2 to 10 fL depending on zoom of PJ.

As was mentioned, this is optimum based on stated lumens of PJ, YMMV, however, expect a lower fL value and plan accordingly.

Thanks for the clarification. Not something that I had seen before. Just for test purposes, I used the Epson Home Cinema 1080, and a 100" diagonal image. The FL ratings changed from 9FL at 20'11" to 15FL at 9'9". How much of a difference anyone will see will be based on that individual, but yes, there is a difference. I stand corrected.

ETA: This is interesting, as I've heard from many engineers stating that throw distance for a given image size doesn't have a significant impact on image brightness. Physics laws and all that, etc. I haven't seen any differences, and we do shootouts quite regularly. Next time we have one I'll see what gives.

On the reflective side from the screen that should be correct, no impact as long as the distance from the screen remains the same.

Now on the refractive side from the lens itself, I am pretty sure this makes a difference. Attempting to think back to college physics here, but I don't want to think to hard on it or I may hurt myself.

From what i have gathered on different threads the best FL to try to obtain is between 20-25 on the Pro Calculator for optimal performance with some ambient light, this may sound high, but its only cause its calculating based on max lumen output. This isn't a physical measurement that I have made...just a loose guideline (and i stress loose) that I have based my build on from discussions on several threads here on these forums.

The calculator is meant to be a general planning tool, it won't give you super accurate data.

it took me a few times to figure out how to move the projector throw without changing the screen size too. I also have been wondering why the FL changes depending on distance, I hope someone that knows the answer might post the reasons.

Guys, here's an excellent reference that helped me understand the relationship between zoom range and brightness for a fixed screen size. (Scroll about halfway down the reference.)

As the DaLite article points out fL are not the same as brightness.

"Even though we now know how to determine luminance, we have still to be careful not to confuse it with brightness. In screen terms, luminance is the measurement of light from a surface while brightness is the subjective appearance of that surface. To drive home this point just one more time, 60 foot Lamberts of luminance will be twice as great as 30, but it won't be twice as bright. "

A good idea of what fL means in terms of viewing is found in their discussion of a cinema screen versus a tv screen. Optimal cinema fL with near total light control is 16fL (DaLite's number). For a tv in normal room ambient light conditions, the fL need to be 50.

My screen is in my TV room and I have some light control. My screen gain is 2.2 so if I crank up the lumens on my HD70, I can get >30fL from my screen which helps out during the day for sports. With shades drawn and the doorway shaded or in the evening when light can be controlled or does not exist, I crank the lumens way down (brightness is -16 and brilliant color is 1 or 2) to get a great picture more suited to cinema. I have these two conditions covered by tweaking two separate modes so all I need to do is change the mode on the remote.

found this link that explains why FL increases/decreases due to throw distance:

http://proav.pubdyn.com/Hands_On/May...rallaxView.htm