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Home made slide projector project


Patrick Cooper

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Working on a project at the moment where I'm making a very basic, crude DIY slide projector with a few parts found around the home. Nothing fancy or elaborate. Sort of like a MacGyver slide projector (if he ever needed to make one.) A cardboard box with a circular hole cut out of it will be used to house the Canon FD 50mm f1.4 lens. I'll also make a cardboard stand for the slide to stand up vertically.

Though I'm curious about the light source. Obviously, I want a nice, bright image projected. But I don't want a light that is so bright that it would cause harm to the slide. Most people know what happens when the film gets stuck in the gate of a movie projector. The stationary frame will burn. That actually happened to me once with a super 8 movie projector. I was running a film of mine where I had shot a time lapse sunset and I watched one of the frames freeze on screen and then burn. I stopped the projector straight away.

Although I have used slide projectors in the past, Ive never taken a look inside them. Ive checked online and I notice some of them use 150w and 300w globes. Would there be any kind of diffusion panel to help project the slide? There's a youtube video on how to make a basic slide projector and they say one of the parts you need is a ground glass. But they don't actually mention how the ground glass is used. I'm assuming that the ground glass would be placed between the light source and the slide. I wonder if baking paper could be used instead as an alternative.

The slide has to be very close to the light in order to get a focused image with the lens so it would be a pretty tight space for a diffusion panel.

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Ah yes, Ive just had a look online about slide projector components and indeed, a condenser lens is needed.  Yea I could look online for a spare one or one from a broken projector. I guess I could purchase a convex lens from an online store (magnifying glass etc) though I guess it would need to be the right focal length. And I see that a diffusion panel is not required.

Though I do plan on having a rather short, casual slide show with some family members fairly soon so my crude setup will have to do in the meantime. It does project a slide on a wall so that's the main thing. Obviously, the output won't be as good as a commercially made slide projector. I'll probably use a regular household bulb in a desktop lamp. I am a little worried that the slide will be positioned very close to the light. I guess having a short projection duration with each individual slide would be wise. I notice a lot of commercially made slide projectors have built in fans for the bulb. Would that simply be to extend the life span of the bulb or reduce the heat on the slide (Or both)?

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condenser lenses are aspherical - that's why in projectors they're mostly molded glass to reduce cost, which is good enough for the light beam to be collimated. it's the projection lens that determines the image quality. the condenser just makes sure you have an evenly lit slide from center to corners.

the fan probably serves both purposes.

Edited by David Sekanina
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It used to be said that camera lenses are not intended for projection- they have cemented elements and the cement is susceptible to heat and may discolour. Projector lenses are uncemented. But if you're using a LED source the heat is much less. Still, as you intend to do, minimise the time the lamp is on.

Nowadays you could use a high-wattage COB LED as they are more of a point-source.

Edited by Mark Dunn
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Strongly suggest you do more research on the proper spacing of the condenser lenses, lamp, slide and objective lens.  You should get acquainted with the basics of optics and ray tracing, which isn't too difficult, but will help you get better results.   In all projectors of this type, it is common to arrange the condenser so that an image of the lamp filament forms at or near the aperture stop of the objective lens.   And yes, heat can potentially damage the lens.  If you're using low wattage, it's less of a problem, but then you won't be able to project a very large image, either.  All in all, it's a good way to learn about optics. 

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Previously, I made a camera obscura out of cardboard with a double convex lens. So I used that convex lens as a condenser lens last night for the DIY slide projector. I experimented with placing the slide and 50mm lens at different distances from the convex lens and got decent results. I couldn't see any difference when I varied the distance in small increments. Though when I removed the convex lens from the set up, I got a larger projected image (with just the 50mm lens and slide - moved closer to the light source.) 

The heat potentially damaging my Canon FD lens does have me concerned. I think I'll stop using it for this project. Somewhere I do have a Takumar 55mm lens that Ive never used. I could employ the Takumar for this project (if I can find it.) I think it might be the f1.8 version if I recall correctly. Though I haven't seen it in ages. 

 

Edited by Patrick Cooper
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The condenser lens must be of larger diameter than the diagonal of the slide or your light cone will not cover it.  Studying the principles a bit, as I suggested, will help you understand how it all works and apply actual measurements to your set-up.  Admittedly though, the condenser system does complicate things.
You could get away with a simple diffuse light source behind the slide, i.e., just the lamp behind a piece of ground glass. (Your baking paper idea might catch fire).  Put a mirror or even just a piece of white cardboard on the other side of the lamp to bounce all that lost light back into the slide.  Definitely works and no math needed.

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Dan, the double convex lens I have certainly does have a larger diameter than the slide I'm projecting. Though yea the more simple approach is more attractive to me. And yes, I did consider the fire risk with the baking paper! I was once on set as an extra on the Australian movie Hey Hey It's Esther Blueberger and I could see smoke coming from a nearby light with a coloured gel attached. I could try and locate some ground glass. Do you reckon the gg could be effective in reducing heat from the light source? And yes, a good idea about having a reflector behind the light - just like with a conventional slide projector. 

I did manage to locate that Takumar 55mm lens but it turns out that it's f2 rather than f1.8 (my memory must be placing tricks on me.) 

 

 

 

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Conventional ground glass is not going to block much heat.  There is such a thing as heat-absorbing glass, which was used in slide projectors for this very purpose.   Not certain where to buy some, but I used to get all kinds of experimental optic supplies from places like Edmund Scientific and Surplus Shed.  Both are still in business, so you might do a search through their online catalogs.

The speed difference between your lenses is negligible.   Not a problem. 

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Actually, ground glass and / or heat absorbing glass is going to raise the budget quite significantly for what was originally going to be a dirt cheap DIY project. Though I do know that some people convert regular glass to ground glass like this example here:

Though he doesn't really mention if he does the other side of the glass or not.

And actually, I was just thinking - a reflector behind the light may not be that effective in my set up. I would likely be using a desktop lamp which would be very directional. A lot of the light would be aimed in one direction - towards the slide.

 

 

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Ground glass used to be pretty cheap from places like those mentioned.  I have also made my own, like the example you've shown.  The grit wasn't too expensive.  In my case, I made a focusing screen for a view camera, so for that you only grind one side.  For a diffuser, probably doesn't matter if you grind both sides.

Yes, naturally, any light fixtures with built-in reflectors will serve your purpose.

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I see the prices for ground glass are quite reasonable at Edmund Scientific. Looks like it's not quite straight forward to find small sheets of regular glass online. I wonder how perspex would fare if I rubbedd some silicone carbide powder on it. 

Over an hour ago, I did some testing with the Takumar 55mm f2 lens - projecting slides on to a white painted door with a small LED light. I'm quite impressed with this old M42 mount lens. Ive never mounted it on a camera but I like what I see with it's performance as a projection lens. Nice and sharp with good detail. But I can only see so much with the amount of ambient light around (not really a dark room.)

 

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Good point about the perspex. Speaking of heat tolerance, do you think it would be risky to use the type of opaque plastic that some orange juice bottles are made of? By the way, this DIY slide projector is only a short term project. I doubt I'll use it for much more than a few slide shows. Maybe less. And the slide shows themselves would be relatively short with a small number of transparencies projected. I guess I could always replace the opaque plastic with another piece if it's affected by the heat. 

I think I might use alfoil for the reflector. 

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Oh yea I'm sure that the polyethylene would have a limited life span being subjected to the heat. Though as I mentioned before, it wouldn't take much to replace it. Additionally, I'm planning to add a fan. I'll cut a small opening in the side of the rear box right next to the light and blow cold air directly in there. My only concern is ambient light getting into that opening (although obviously, I'll do the projecting in a darkened room.) I'll try and place the slide as far away from the light as possible while still getting a focused image. 

Ive just discovered that you can buy little compact battery operated fans. One or two of these could be placed inside the box. Though I'm not sure how powerful they are. They may only be very mild with their output but that's just a guess. 

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I might go back to the original idea of using ground glass between the light source and the slide. It might be safer for the slide compared to opaque plastic.

Though I do have another query. Would there be much of an issue if there is some empty space on the left and right sides of the ground glass and above it as well? It would basically mean that there is some undiffused light passing around the ground glass into the lens. If this would likely cause issues, I guess I could place fill up that empty space with black foam. 

When sourcing glass, I could grab a cheap picture frame. Then convert it to ground glass. 

 

 

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