Projector question

March 19, 2009 9:39 PM

I don't know anything about 35mm slide projectors but I have an idea

for projecting a 35mm slide image onto a wall as an artistic feature, the projection unbroken by passersby.

I believe i need the projector to be above and at a distance of no more than a foot from the wall since this is a high traffic area and i don't want people walking through the picture. Is this easier than it seems?

posted by sarelicar to sports, hobbies, & recreation (17 comments total)

How big an image do you want to project? If you want an image bigger than, maybe 8 inches across you're going to have a big problem, in two ways:

1. distortion (the image will look like it was taken with a fish-eye lense)
2. focus (most of the image is going to be way blurry)

It'd take a pretty amazing projection lense to fix both of those problems. I can't imagine that anything like that can be purchased.
posted by Chocolate Pickle at 9:45 PM on March 19

Actually, it'd be the opposite distortion of a fish-eye lense.
posted by Chocolate Pickle at 9:46 PM on March 19

I guess there's possibly some kind of a magical super-lens or other mechanism that could let you do this, but your standard slide projector? No way. If you want a large image to be projected, you're going to need way more than a foot.

Could you perhaps erect some kind of rudimentary barrier?
posted by turgid dahlia at 9:55 PM on March 19

Drop the image in photoshop.

Deform the image (like keystoning on CRT monitors) so that it looks like what you'd see if you projected it from the image at your required projector position.

Revert image. Deform the image in the opposite direction by the same amount.

Print/Send the image out to be printed as a 35mm slide.

You might have to make the image smaller (ie., letterbox the image but on the sides instead of above and below) and move the projector further away from the wall.
posted by porpoise at 9:56 PM on March 19

Porpoise, that doesn't solve the focusing problem.
posted by Chocolate Pickle at 10:06 PM on March 19

If you visualize the frame of the 35mm slide as a pyramid of light whose point starts at the lens and whose base is on the wall, you can determine how high and far you can go, and how much distortion you'll introduce by doing so.

There are lenses out there that do keystone correction, and they have limits. However, 35mm slide projection being somewhat of a dying art, if you can find the lens that matches your need, it can usually be rented or even bought for a pittance.

Your ideal setup is rear projection with short throw lensing, but since you want to project onto a wall, that's probably not going to happen.
posted by tomierna at 10:12 PM on March 19

Oh, also, if throw distance is an issue, you could rig up a front-surface mirror and project away from your wall, bouncing the image onto the wall.

Drawbacks to doing this include front-surface mirror that big being very expensive, loss of brightness, and additional rigging.
posted by tomierna at 10:20 PM on March 19

You basically want a short throw lens(random site from google), but I have no clue if you could find anything compatible with a 35mm projector. I'm guessing a rear-projection setup is out?
posted by niles at 10:21 PM on March 19

Am I missing something, Chocolate Pickle? I'm old enough that I've used 35mm slide projectors for presentations and I've had them a good 5 meters away from the screen...

I guess the further away you have the projector, the less bright the image is going to be, which might be an issue depending on the levels of ambient lighting.
posted by porpoise at 10:29 PM on March 19

I think what Chocolate Pickle is saying is that if the projector is only a foot or so away from the wall, pointing down at a (severe) angle, the top of the image will require a different focus than the bottom.
posted by rhizome at 10:34 PM on March 19 [1 favorite]

Is it feasible to simply scan the slide to a digital file? Most scanners come with slide attachments. Most all VGA projectors have keystone features built-in.
posted by Burhanistan at 10:57 PM on March 19

The brightness of the image would also vary greatly (it would fall off inversely with the square of the distance from the lens). You could compensate for that too, I suppose but you would then lose overall brightness. This whole idea is really not a good one.
posted by w0mbat at 11:16 PM on March 19

Maybe you could rent/borrow/buy/steal a rear projection screen? You could set it up so that people can't go behind it, though surely it would be clear that going behind the screen would be dumb.
posted by sevenless at 11:27 PM on March 19

I would suggest rear projection, also, because a projector one foot away from the wall and pointing downwards will not show much of a picture without it 1) being small (without some amazingly short throw lens), and 2) showing a serious keystone effect.
posted by Blazecock Pileon at 3:54 AM on March 20

There are ceiling projector mounts for 35 mm projectors specifically for this purpose. The slide projectors have keystone adjustments that should take care of the distortion. This is common for artistic installation. You could test or rent the equipment at an AV rental place, if you are in a city which has such. Googling brings up a lot: Ceiling Mount for Slide Projector Rental.
posted by coevals at 4:44 AM on March 20

If your space/ ceiling height is limited then rear projection is the only real option. You could do it with some mirrors and fancy lenses but you will be looking at serious expenses. The modern and easier equivalent is a Flat Panel TV with built in DVD player. These can be rented.
posted by JJ86 at 6:02 AM on March 20

Another approach would be to get several projectors all shining identical slides onto the same screen, if someone stands in front of one it would just make the image a bit darker but not disappear completely.
posted by Lanark at 12:11 PM on March 20 [1 favorite]