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I would like to focus an iPhone 5s' torch light into a beam with an angle of around 5 degrees or smaller (hopefully). Is this easily doable with a single lens?

I'm hoping to buy a lens and attach it in front of the iPhone (by 3D-printing a custom case). I just don't know which lens I should get.

This is for an artistic project. It doesn't have to be very precise. Since I will be using around 75 iPhones, I'm hoping the lens to be fairly cheap and easy to get in bulk.

Any input is appreciated. Thank you so much!

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Generating a reasonably collimated beam from an iPhone is only possible if:

  1. You use an enormous lens (several inches in diameter), in which case your beam will be correspondingly enormous.

  2. You are willing to throw away a lot of the light by using a pinhole positioned on the lamp surface in addition to the lens.

The reason these compromises are necessary is that the iPhone lamp is about 4 mm across and thus does not act suitably like a point source unless the lens diameter is much larger. Since a lens has only a single focal point, it will only nicely collimate a diverging beam if the beam originates from a single location. So for a small lens with a short focal length, you’ll need to block most of the iPhone lamp with a pinhole (a “spatial filter”).

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  • $\begingroup$ Thanks a lot for the insights. If I did block most of the light and left a hole 1mm in diameter, would that be suitable? If so, what kind of lens could I be using? (as you perhaps guessed, I am not too concerned by the loss in light intensity) $\endgroup$ – djip.co Jul 9 at 22:49
  • $\begingroup$ @djip.co this is going to depend on what exactly you want it to look like. A 1 mm source will be easier to collimate, but still not good enough if you want a small diameter beam with little divergence. The kind of lens (size, focal length, material, etc.) is all going to depend on practical considerations. $\endgroup$ – Gilbert Jul 10 at 4:08
  • $\begingroup$ I don't really have any preconceived idea of how it should look. I just want a narrower beam than what the iPhone normally provides, hopefully something that looks good when shining on smoke. I may just try a small diode laser plastic lens and see how it performs. I've seen some of them that are 7mm in diameter with a 8mm focus distance. What do you think? $\endgroup$ – djip.co Jul 11 at 0:54
  • $\begingroup$ @djip.co oh, I think that would look cool. Totally worth a try! But you won’t have a nicely collimated beam. It’s still going to diverge quite a bit. Maybe that’s okay? $\endgroup$ – Gilbert Jul 11 at 16:03
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    $\begingroup$ The project I'm working on addresses issues of planned obsolescence and how we deal with "old", unwanted technology. That's why I chose the iPhone as my platform of choice. The project is not about light, it's about reinventing a second life for devices that are deemed garbage... For context, I did a similar project with old plotters: djip.co/art/yoti $\endgroup$ – djip.co Jul 11 at 20:25

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