Turn a Regular Video Projector in Short-throw Model for ~40$

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As a video artist, I like to perform video projections directly from the stage. I appreciate this approach because it's easier and faster to install than to hang video projectors on grill-top or less complicated than other installations. Done well, this technique has good results especially with retro-projection, but the efficiency is often limited by the depth of the stage.

To fix that, the easiest solution consists in using short-throw video-projectors, but it may be expensive especially when you're looking for high light-power (e.g. almost 3000$ for 5000 lumens). So I've looked for a cheaper solution to turn my 6000-lumens-video-projectors into power full short-throw projectors.

I've done a lot of research and tests with mirrors; interesting but less practical to use, to adjust and too fragile to transport. And finally, one day when I was taking pictures, I just thought about how the light goes through a wide angle objectif. That's it ! I was looking at the same principle..... but from another perspective. ^^

That's for the story. The solution was easier: just find a way to fix and adjust a kind of wide angle lens in the same way as my video projector's light field.

Step 1: What Do You Need ?

To do so you'll need:

  1. a wide angle lens converter like these below: ~25-30$

    https://alturaphoto.com/altura-photor-52mm-hd-wid...
    https://www.amazon.com/Altura-Photo-Professional-...

    Even if I chose the lens randomly, this brand is a good one. There's a number of different models and quality available on the web....

  2. a short squared aluminium pipe (e.g. like 15x15x~300mm) ~2$
  3. some screws and bolts (M6 metric screws, autostop nuts, rings, handle,...) ~10$
  4. access to a 3D printer (ask your grand-ma, she has one !), free

Step 2: Prepare Your Video Projector to Receive the Lens Support

Because there's too many types of video-projectors, it is impossible to propose a common support that matches all projector. I'm showing you, in my situation, the technique I used to fix the aluminium pipe and I'm sure that your creativity and ability will help you to find a way to have the same (or a better) result.

I used a wooden board as support below the projector. I use it to hang the projector and I took advantage of this support to fix on it a counter-pipe that will receive the aluminium pipe. I think it should be easy to create an equivalent with wood or hardware components. The important thing is to take care : the pipe must be inline and as aligned as possible with the objective of the projector.

Step 3: Print Your 3D Parts

To have a fully adjustable installation, I used three different parts. As you may see, the parts look quite massive. at the beginning, I put too much torque on the screws and the clamping sections broke ; so I increased the size. It's also explained by the fact that, in order to use it, the system must be robust enough to resist the fast manipulations, and to be rigid enough (without any movements) to allow a long installation (in my case, up to 8 hours).

Base support: makes the liaison between the aluminum pipe and the support. It allows the lens to be moved away and adjust the vertical position regarding the projector lens.

Spacer: allows the adjustment of the lens' height on a vertical axis

Lens support: clamps the lens and allows to adjust the lens' inclination

Base support.step

Lens support - ø58.4mm.step

Lens support - ø61.6mm.step

Spacer.step

Step 4: Assemble the System

All screws and bolts are metric M6. Feel free to modify in order to have a more compact system.

To allow fast adjustment I chose a kind of plastic handles (but it's not needed). As you may see on the picture, commercial screws may not have the right length to fit the system perfectly (or like me, you don't want to buy new one ^^). So, you can adjust with some washers, saw the screws, adjust the size of supports to fit,...

If you want to use multiple lenses, you can pre-mount the lenses on the support to have a faster switch. Using different plastic colors allows to easily identify the model of the lens support.

Step 5: Calibrate...

Once the system is mounted, use the different adjustments to place the lens in the center of the projector's ray of light. Display an image that uses the whole illuminated surface of your projector (like a test pattern), have a look from the side and adjust the lens to display, through this, all the rectangular images (when there is a bit of dust on the lens, it's easier ^^). Do not try to fit exactly the rectangle to the border of the lens, it's better to have a small gap to use the center of the lens more (see picture).

Step 6: Enjoy !

From an optical (or professional) perspective this system could not replace a real short-throw video projector; but it has saved my "lives" a number of time... :)

Result: (with a 0.43x lens)
The result is acceptable especially when considering the price and the work time (~10 hours). This lens system allows to gain almost 125% of added surface. Without lens (projector at left), it displays a 51x38.5cm image and with the system, it jumps to 76.5x58 cm. It increases the dimensions 1.5x, and the whole displayed image around 2.2x.

Experience feedback after a couple years:
The weakness of this system is the distortion and chromatique aberration generated by the lens (see pictures). The wider the lens angle (or ratio), the greater these problems will be. A good compromise for me is situated around x0.5 (I use the x0.47 lens 90% of the time). In my opinion chromatic aberration is often negligible because it is especially noticeable when pure white is projected.

You can fix the distortion by different means: with the help of a mapping software, by deforming the projected surface, modifying the image or video source,... It also should be interesting to try with different lens quality or suppliers.

Anyway, in my opinion, this solution is reliable, has an unbeatable quality/price ratio and opens up interesting possibilities in terms of video projection (through prisme, light rays transpositions, specific lens effects,...)

Thank you for your interest. Enjoy and I look forward to reading your feedback !!!

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