I am currently stuck in the wilderness, and I have been out here for a while. I want to focus light. There are various reasons for wanting to do this, with the two biggest reasons being for vision and for fire starting. It does not need to be anything fancy, not even to help with the vision: my eye sight is just barely too bad to make out the north star easily, so a slight magnification would help.

Are there any resources that I can forage or anything I can create while in the wilderness for focusing light similar to a glasses lens or a magnifying glass?

Available resources

My glasses for my poor eyesight are either lost or smashed to pieces. All of my clear bags have long since been punctured, and I have no other clear containers for holding water in, otherwise I might try to use water to focus the light.

I am assuming creating glass from sand is not an option since I have seen backyard tinkerers with the proper academic knowledge and decent tools fail to create good glass. If you think this is an invalid assumption, then feel free to challenge me on that.

  • I have left this somewhat broad, as I assume that is necessary to get an answer at all. If I am mistaken and this is not as difficult a task as I am thinking, please comment and I will narrow the question and/or split it into multiple questions.
    – Loduwijk
    Commented Sep 24, 2018 at 22:38
  • Not helping with the question, but I know of four methods of finding North using only stars with apparent magnitude above 2, well spaced across the right ascension. One of them an eight-year-old girl taught me!
    – dotancohen
    Commented Sep 25, 2018 at 10:06
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    Keep in mind that only glasses for far-sighted people magnify/focus. Short-sighted people need diverging lenses. Commented Sep 25, 2018 at 10:24
  • There are much easier ways to start fires, such as by friction or by striking sparks from metal. Commented Sep 25, 2018 at 17:52
  • 1
    Not an answer because I don't know if it would work in real life, and is not applicable to your situation. There's a famous fictional example of making an improvised magnifying glass to start a fire from water between two hemispherical glass covers of pocket watches, with clay at the edges. Jules Verne, L’Île mystérieuse, ch. 1.10, fr.wikisource.org/w?curid=5735 , English translation at en.wikisource.org/w?curid=70091 = gutenberg.org/files/1268/1268-h/1268-h.htm#link2HCH0010 , other trans. gutenberg.org/ebooks/8993 .
    – b_jonas
    Commented Sep 26, 2018 at 14:11

8 Answers 8


It's possible to do it with ice that has been shaped into a convex lens. The ice needs to be clear and fairly large like 5 inches across. Obviously this won't work in the summer, and the cold temperatures won't help either, but it does look like it can be done.


All of the above links are for starting a fire with a magnifying glass made from ice, would be interesting to see if there are other materials that can do this as well.


(Taking into account that we do have an "ice" answer already)

The one magnifying lens that is fast and easy to get and that does have an optical quality that actually helps vision as a magnifying glass are in fact drops of water. But that's for the "home-made field microscope" application of a magnifying glass.

If you consider starting fire by sunlight, you expect conditions where it may be easier to get directions during day via the sun rather than during the night via stars. I.e. either find south if you still have (local) time, or east-west from shadow if you don't have local time (slower - and precision depending on how much time you are willing to spend for this).

For starting a fire, depending on latitude and weather conditions, the quality of the lens may be far lower. And it may be an option to have a rough mirror: it may be far easier to collect sun from a much larger area by aluminum/rescue foil.

  • 1
    +1 for the mirror. You could probably make a frame of a good shape from flexible wood like willow, under tension like a bow (or rather several bows)
    – Chris H
    Commented Sep 25, 2018 at 11:42
  • @ChrisH and cbeleites: That is a good idea. I was not thinking about reflecting light when I wrote the question, only focusing it as by a lens, though I have heard people claim it's possible to reflect enough to start a fire. In the spirit of the question, can you think of any natural materials that would be reflective enough for this task and which could be applied here?
    – Loduwijk
    Commented Sep 25, 2018 at 18:40

As has been mentioned in other answers, for vision, a pinhole device, similar to what is used for pinhole cameras, is your best bet. Short of water (which you have already ruled out), that's about the only natural option you have.

For fire though, you have multiple options:

  • Don't discount making your own glass. Yes, it's hard, but you don't need high quality glass for a simple focusing lense for starting a fire, it just has to be reasonably clear.
  • Ice has been mentioned already. This is pretty much your best option in really cold climates. Shaping a usable lens out of ice is not trivial, but it's not super difficult either.
  • Mirrors are also an option. If you have some aluminum foil or, better yet, a mylar survival blanket, you can put together a convex reflector to focus the light. You could also just make a solar oven though if you only need the fire for cooking.
  • This will probably sound utterly stupid, but you could make a lens out of sugar glass. Same general principals as for regular glass, just lower temperatures and therefore slightly easier.
  • Don't discount other methods of creating a fire. Friction-based fire making is covered in survival training for a reason, it needs almost nothing and it works reliably (if you practice).
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    Actually not sure sugar glass is viable Because sugar glass is hygroscopic, it must be used soon after preparation, or it will soften and lose its brittle quality. a banked fire would probably be a better solution. Commented Sep 25, 2018 at 14:22
  • @JamesJenkins A banked fire is almost always going to be the best solution if you already had a fire. As far as the sugar glass, it could still work in really arid environments (It can last for quite a while in a temperate desert like the Gobi), and it's low enough temperatures that a solar oven can be use instead of a fire with forced ventilation. Commented Sep 25, 2018 at 14:59
  • Interesting ideas. For making glass: I've seen hobby chemists better than I try on youtube with laboratory style gadgets that I assume work better than I could make in the wild, and they barely got something translucent that you could barely see anything through. Maybe that could work, but I assumed it was blocking out enough light that its magnifying effect was cancelled out: that is, I assumed the net energy at the focal point was not reasonably higher than without the magnifier, because of the light blocked out by the lens itself. I actually hope I am wrong. Do you have more info on that?
    – Loduwijk
    Commented Sep 25, 2018 at 18:58
  • @Aaron Think about what was used historically for making glass. They didn't have fancy equipment and still got decent glass. The trick here is quantity though. If you have an array of a dozen little lenses, all focused on the same point, no one lens has to be perfect, they just have to let enough light through and focus it well enough to contribute. Commented Sep 25, 2018 at 19:13
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    I am not sure what the minimum amount of tools required for glass making was. The most primitive instances of glass making I have seen, even where old techniques were used, still used a well made forge, bellows, glass blowing tools, and some other tools specific to the task they were doing. If there are instances of glass making which are even more primitive, that would make for some great material.
    – Loduwijk
    Commented Sep 25, 2018 at 23:02

A clear mineral crystal can be worked to a lens shape with enough time and effort. Quartz, sapphire and ruby are likely candidates. Ruby lenses were common on wand style barcode reader pens.

Even some salt crystals are routinely used as lenses and optical windows in certain applications (lasers, IR and UV systems). Many of these salts and those you may crystallise or find will likely have a problem with moisture.

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    I'm not sure how easily I could find a sapphire or ruby this the wilderness, but some kind of quartz or quartz-like mineral might be an option; I would have to study that and see how feasible it is to find such a clear mineral and work it.
    – Loduwijk
    Commented Sep 25, 2018 at 18:52
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    I'm excited about your claims of salt lenses and windows. You might have hit on the easiest to find resource here! Can this be done with conventional salt? We must look into this further.
    – Loduwijk
    Commented Sep 25, 2018 at 18:52
  • @Aaron: you can use conventitonal salt. It does crystallize in clear cubes and does have a suitable refractive index and isn't too hard. The drawbacks are water solubility (and it does often come with hygroscopic impurities) and that it takes a long time to grow large crystals (own experiments as school kid: you get nice large crystals if you let them grow over a few months). The smaller the crystals, the more intensity you loose as stray light. Commented Sep 26, 2018 at 12:01
  • @Aaron There are deposits in the mountains in for example Switzerland, I remember finding fairly clear minerals (which I vaguely recall to indeed be "quartz") in the mountains. It was clearly an excavation area for tourists, but it's definitely in nature, and I'm sure you'd be able to find a quartz crystal if you tried looking for it in a similar environment for long enough.
    – Azeirah
    Commented Sep 26, 2018 at 15:04
  • @cbeleites I was thinking more along the lines of extracting salt from water (by boiling) and then burning the salt. I have heard of liquid salt being used for heat storage, but I don't recall what the temperature needed for that is, and I'm not sure if it would solidify into one solid piece when cooled.
    – Loduwijk
    Commented Sep 26, 2018 at 20:10

If you can recover a source of glass, you should be able to melt it using a charcoal fire with bellows to force air and increase the temperature. You'd need a mould, for which clay would be good. This can be worked into shape, smoothed, and dried before casting the glass.

This requires real glass, not plastic as is commonly used in spectacles , and not laminated (many car windows). Thermoplastics have a lower melting point but would be very easy to burn.

The resulting lens should be able to start a fire, but getting anything like the right form or surface quality for use as a magnifier or corrective lens is probably too much - for that you'd need to polish it (though jeweller's rouge is made from finely ground rust, so it's not completely impossible to improvise a polish).

  • 3
    Plausibility check: we need a good fire to make a fire-starting tool... As for vision, well, it's not going to be easy to get a lens (either magnifying or diverging) of a quality that actually helps vision that is "barely too bad to make out the north star easily". Commented Sep 25, 2018 at 10:30
  • @cbeleites conceptually you could use your last match to get a fire going then want a portable fire starter. The vision aspect I think is impossible to improvise, whether in the field or with a well equipped general workshop
    – Chris H
    Commented Sep 25, 2018 at 11:27
  • 1
    sure. (I'm less concerned about your answer but about an aspect that I'd locate more with the question: being "stuck in the wilderness", I'd have lots of things to do much higher up my priority list (with the last-match fire) than setting up a glass workshop from scratch... I.e. I'd think it more important to focus on tasks/problems that need to be solved rather than particular techniques). I did not upvote your answer because right now I cannot imagine that it will actually work: where/how do you keep the molten glass? Ash contains a lot of potassium (and sodium) oxide, and you'd have to... Commented Sep 25, 2018 at 11:49
  • @cbeleites I was thinking of a lens-shaped crucible from clay, so no need to store the molten glass. Once you've got a fire going it's easy to keep one or more fires going in place but harder to transport (though not impossible), so in the hypothetical case that you were preparing to trek back to civilisation from an established camp, you might want something portable
    – Chris H
    Commented Sep 25, 2018 at 12:24
  • Oh, maybe store was not the correct word. I was thinking about the crucible to melt the glass in (or tongs to hold it as you don't need to get it truly liquid). In any case, ashes would mess up the glass (local change of melting point) and grinding even starting with a roughly correct raw shape will be substantial work and things not avaiable when stuck in the wilderness. Even though it is apparently perfectly doable on a "home workshop" basis: stellafane.org/tm/atm/index.html. Maybe still easier to use the clay to make a fire pot and then carry the fire with you... Commented Sep 25, 2018 at 13:25

Though, this probably won't help making a fire, but ...

As I'm short sighted by myself, I'm using this simple technique to focus my eyesight, if I'm out of my glasses. It works pretty good at night, but you need to have a general clue where to look before you apply it.

The one I use, is creating a kind of resizable diamond by putting thumb and index fingers together, then connect both hands (kind of 3mm-by-3mm square hole)

The one used in video, is a small hole created by one single finger closed to the palm itself.

And, in fact, any small hole would help, but the fingers are readily available and, more than that, are adjustable.

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    Sorry, this does not count as an answer on SE sites, because users have to go elsewhere to read it and link rot may happen. Can you edit the essentials into your text?
    – user15958
    Commented Sep 25, 2018 at 11:38
  • @JanDoggen I think the edit makes the question stand alone without the link. I just read it and understand without the link. Still room to improve though. Commented Sep 25, 2018 at 13:25
  • @J... Actually, if I understand this answer correctly, it is not even that. It is a frame challenge, treating my question as an XY question. It assumes that no lens is needed at all. I have heard of this technique before and forgotten about it. Apparently, just looking through a tiny hole somehow helps your eye to focus and see more than it otherwise could within the very narrow field.
    – Loduwijk
    Commented Sep 25, 2018 at 19:01
  • ... but the narrow field of view means that it won't help for the purposes in the question: for starting a fire, you want to concentrate the energy of sunlight collected over a larger area onto a spot. The aperture in this answer is pretty much the opposite: shutting out all but a tiny fraction of the light. For vision, there's a tradeoff here as well: the aperture helps in the same way the pinhole in a pinhole camera (or the aperture in "normal" photography) works: the smaller the pinhole, the larger the depth of field. But it comes at the cost of reduced intensity, which is certainly a ... Commented Sep 26, 2018 at 12:09
  • ... consideration for star light navigation. Plus the field of view question is whether it helps at all in the sense that I suspect you may need to know north direction (plus altitude of the north star) with an accuracy (to get it into FOV) that would already answer the practical navigation question. Commented Sep 26, 2018 at 12:15

Similar to demigod's answer: grab a chunk of cardboard, or a sheet of bark, and punch two small holes spaced at the separation between your pupils. Wear this as though they were goggles. Due to the magic of optics :-) , you won't have a great field of view but what you can see will be in near-perfect focus.

As to firestarting, forget about using solar heating unless you can find a big chunk of obsidian and a near-spherical rock to grind a concave mirror. Use direct friction techniques to start your fire. See for example, this bow & peg tool.


It's also possible to start a fire using a plastic bag filled with water.

If by any chance you stumble open some waste of people that left it behind, or you happen te have kept that one sandwich bag of yours, you could use the lens-like properties and start a fire.

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    The OP wrote All of my clear bags have long since been punctured
    – user15958
    Commented Sep 25, 2018 at 17:49
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    @JanDoggen I agree this answer missed my point in my question, but it does still provide something that brings it back on track: "If by any change you stumble upon some waste of people that left it behind". Human trash covers a significant portion of the planet. Even on the survival show "Alone", the contestants find (and are allowed to use) useful trash in remote wilderness areas of the world. Finding a trash clear water bottle might be helpful. I do think this would be possible, but unlikely and should not be counted on.
    – Loduwijk
    Commented Sep 25, 2018 at 19:05
  • Ezgi, as @JanDoggen has quoted, the question states no such items are left. However, I think your answer has some good to it and would be even better if you emphasized the part about finding trash. You can borrow from my other comment above if you want. Also, if you included any statistics about how common human trash is even in remote areas, you would probably get more up-votes.
    – Loduwijk
    Commented Sep 25, 2018 at 19:08
  • A condom also works. Commented Sep 26, 2018 at 16:42

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