If you have old camera lenses, a macro bellows and a high power LED flashlight, you can use those to build a battery powered projector.
As high power LED lamps are much more efficient than classic slide projectors, and camera lenses are often brighter than their often cheap projector counterpieces this can be a very powerful combination.
I've done projections over 200m distance, using a Cree XM-L T6 light and a 400mm f/5.6 lens.
Step 1: Select the Right Flashlight
The outcome will depend largely on the flashlight you chose.
The ideal flashlight would evenly light a tight spot of 45mm diameter (the image area of a 35mm slide) at very close distance and as bright as possible.
Indicators for good flashlights are:
- ultrabright (around 1000 Lumen, comparable to a car headlight)
- with Cree XM-L or XM-L2 LED's (those are most efficient)
- long range over 100m, a deep parabolic reflector with a diameter of ~45mm
- enough juice: at least 4xAA or 2x18650
An example for an affordable matching flashlight is the chinese SomeFire X8 model (for example UltraFire X8).
Step 2: Pick the Right Lens
Projecting works best with very bright or very long lenses (but not with zooms!).
Here's a list of lenses that have proven useable, together with their estimated 2nd/3rd hand market price:
- 50mm f/2.X ($5..$15)
- 50mm f/1.X ($15..50)
- 135mm f/2.8 ($15..25)
- 200mm f/3.3 or f/4 ($20..40)
- 400mm f/5.6 ($30...)
- 500mm f/8 ($30...)
If you don't know what to chose, i recommend to select one of the following lenses
- 135mm f/2.8 -will project a spot at a distance (5 Meters up to 50m)
- 50mm, which projects a larger image at shorter distances (2 Meters up to 30m)
f/1.4 is ideal, but f/1.7 and f/2.8 will work too (though darker)
Step 3: Prepare the Bellows
You will be cutting slits into the bellows in order to position the 35mm film just where it would be in a camera or slide projector.
In order to do that, just extend the bellows halfway and measure a distance of 44mm from the lens flange. This should be within +/-3mm to the exact flange focal distance of your lens (you can adjust it with the bellows)
Now make two incisions to the left and right side of the bellows, and insert the 35 mm film.
Just be careful to make them horizontally centered, symetrical, and not too wide.
Insert the 35mm film with the image bottom-up.
Step 4: Attach the Flashlight
You will have to find some way to attach your flashlight.
I used M42 extension tubes and gaffer tape, you might find another solution. Whatever works :)
Step 5: Start Projecting
Congratulations! You should have a working projector.
If you'd like to have this in a more solid form please have a look at the GuerillaBeam 3d printed projector. :)