# Projection Design Focus Tolerance

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This is a tough one that I've always been unsure about. How do you determine a focus tolerance when a projector is at an angle or projecting on to a curve? Usually I just focus to the middle point and that does the trick (if you're far enough away), but how does one simulate or calculate that?

I've seen this called projector "depth of focus" and "depth of field" - though depth of focus makes more sense to me.

Here's a good example –

I'm projecting on a curve. The projector coverage is 24' wide x 15' tall, but is covering a curve that is 24'10". That means the focus variance is 2'10', where the difference between the closest surface and furthest surface is 2'10". Normally I'd focus to the middle (approx 1'5"). Is there a way to simulate this to see if it's acceptable? Do I need to determine this based on viewing distance?

If I'm blending multiple projectors, would I need to focus to the blend area instead of the average?

I've found this thread on Reddit, but I don't understand any of it! Optics and micron calculations are a little above my pay grade 😞

Edited by trashcan
• 5 months later...

Two ultra short throw PJ's blended? That doesn't answer the question but it cuts the variance in half. Guessing that doesn't solve your real-world challenge though...

I think projection has too many variables for this to be a simple calculation. The quality of the lens and the size of the image source can both affect the calculation. Some of the illustrations here are helpful. https://en.wikipedia.org/wiki/Circle_of_confusion Certainly, the larger the image source, the grater range of focus you will have (there's a reason that Pani projectors use 15cm slides, and Kodak used 35mm…), and a longer throw with a narrower lens is going to have better depth of field.

I would actually imagine that projecting onto a curved screen might have better depth of field than a flat screen, at least with a wider lens. The edges of the screen are actually at a more consistent throw then the center of the screen when the screen is curved. Projection lenses actually act to counteract the natural curvature of the image.

Edited by JBenghiat

@JBenghiat that's really good intel. I guess it is a too-taste / experience based judgement call most of the time. Or you just do some tests to see what is acceptable.

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