Fire with magnifying glass...How much magnification?

I used to light my cigarets with the tiny magnifier on my SAK.

When I was a kid, we set leaves on fire with a small magnifier, certainly less than 5x.
Once you have leaves in flames, a few twigs will give you the basis of a good fire.
 
The ease of lighting a fire with a magnifying glass is the size and not the magnification. The bigger the lens, the more light it gathers and focuses. The magnification just changes the focal point.

Anyway, as Esav said, it's possible using the tiny magnifier on a swiss army knife. It's easy with a big old 3" diameter lens used for map reading.
 
I burnt the s--- out of my finger in about 2 seconds with just the magnifier on my compass (1" diameter) out in Joshua Tree. Had no idea it would heat up that fast.
 
Since a thread without pictures is useless:

firewithSAK2-forphotobucket.jpg


firewithSAK3-forphotobucket.jpg


Part of the success lies in focusing on a dark object, not a light one. True Tinder fungus (Inonotus obliquus) used in the pictures.

Doc
 
I feel that magnification is not so important as is the area of the lens. You should see what an A4 fresnel lens is capable of on a sunny day!
G
 
about four years ago i tried different reading glasses and magnifying lenses to see what would work, the 1.50 reading glasses were poor and i had a very hard time wiht them, the higher power the lens was the better it worked. the best magnifying glass was the large hang around the neck lens that the needle point folks use, it was the cats meow. so the bigger the better and the higher power the better.

alex
 
I have a fresnel in my hudson's bay box that is about 1.5 in diameter and ( I think ) 8x magnification. Once focused it takes about 3 seconds to start burning.

David
 
Since you asked:

Lens magnification is not relevant per se.

The two important factors are:
* lens size
* lens spherical aberration

A larger lense will gather more sunlight
A lense with little spherical aberration will be able to focus it on a tiny powerful spot.

Spherical_aberration_2.svg


see:
http://en.wikipedia.org/wiki/File:Spherical_aberration_2.svg

http://en.wikipedia.org/wiki/Spherical_aberration

Also add material quality and thickness: the more transparent and the slimmest the less power lost "inside" lens.

So best shot would be large lens, good manufacture and material (Frenesl would work too).

Of course weather and material flammability and dryness are paramount.
 
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Echo4v said:
I have a fresnel in my hudson's bay box that is about 1.5 in diameter and ( I think ) 8x magnification. Once focused it takes about 3 seconds to start burning.

David
Click to expand...
I keep a couple of the credit card sized fresnel lenses. One in my wallet and one in an altoids tin that I use for a tinder box. They weight nothing and take up no space.

Plus they're fun to play with.
 
i used to do this too when i was a kid!!! :D fun thread...
if you have a broken camera you should be able to get the lenses out of there too!!!
 
I got a piece of char cloth glowing in about 15 seconds using a small magnified makeup mirror. To date I haven't had any luck with the smaller camera lenses. HD, I'll bet those reenactor tins work great, always wanted to play with one of those.
 
Of course the shortcoming is that it typically isn't a bright sunny day when you really need to make a fire. (captain obvious signing off)
 
baldtaco-II said:
I think he was pinching from the paper size - 210*297 mm
Click to expand...

Thanks for picking that up. I got a cheap plastic fresnel lens that was A4 sized in a newsagent shop. I keep it stowed in the back of my daypack, although it tends to pick up a lot of scratches, which has slowed the firemaking down a bit.
G
 
if i understand my physics correctly, another factor is your latitude. the closer you are to the equator, the more energy there is per area (e.g. square inch), so the smaller lens you'll need to gather the requisite sunlight.

however, seeing as how doc-canada can light stuff seemingly quite easily in hamilton, and i've done it with a fresnel lens in vancouver, the other factors of lens size and darkness of the tinder are probably more significant.

speaking of tinder darkness, have you ever played with a laser pointer? notice how the dot gets smaller on a black surface and is bigger on a light surface. if the difference between light absorption is that noticeable, i'd say that light/dark surfaces is quite a significant factor.

i've also noticed when experimenting that, while we typically want our tinder to have a big surface area (e.g. you fluff up your PJ cotton balls to expose the fibres), sometimes it's harder to get something with a big surface area to start smoking because you can't focus the light with the lens on one specific point. parts of the surface might be slightly closer and other parts might be slightly further away than the focal point. so it can be a bit of a balancing act, at least depending on what tinder you use.

all in all though, the most important thing is probably just a visible sun and some dry tinder... the rest is patience.
 
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