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magnifying glasses

B

Bumppo

Joined
Jan 27, 2006
Messages
1,290
Does the magnification strength of a magnifying glass affect how fast it will catch a given material on fire? Does the diameter matter? Can ( practical) light sources other than the sun be strong enough to cause fire?
 
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Sheesh--these are dang good questions! As much as I know about lenses, I have to confess I might be wrong here...

The diameter will affect the distance you hold the lens from the source material: with a big lens you might be able to start a fire from a foot away...with a smaller lens you may need to be a few inches up.

I'm not sure what you mean by practical...a super-white halogen can start a fire, but (a) those aren't commonly laying around and (b) if you have one handy, you can start a fire without a lens using either the hot bulb or using the light's electrical supply.

The mag power...dunno, I think that's similar to the distance of the lens to the source material.

Anyone else who can correct me? Please??
 
My vast and overwhelming experince with optics being that I wear glasses and own a pair of binoculars...

I believe size does matter in terms of diameter as you are collecting and focusing more light with a larger diameter lense. The focal length changes with the power of the lense but the heat you can concentrate is a function of the amount of light being collected and the optical quality of how closely it can be focused. A poor quality lense will give you a diffused focal point that makes it tough to heat your tinder up to the burning point. FWIW that's how I understand it.

Making fire with a lense is much easier of you start with charcloth or relight a piece of a coal then proceed as usual with a coal in a tinderball. Mac
 
I have posted this before but I thought you might be interested.

firewithSAK2-forphotobucket.jpg


firewithSAK3-forphotobucket.jpg


I'm concentrating the focal point on True Tinder fungus (Inonotus obliquus). The magnifying glass is on a Victorinox SAK Champ.

Doc
 
DOC-CANADA said:
I have posted this before but I thought you might be interested.
...
I'm concentrating the focal point on True Tinder fungus (Inonotus obliquus). The magnifying glass is on a Victorinox SAK Champ.

Doc
Click to expand...

Sweet… about how long did you hold the magnifying glass like that before it started to burn?

-sh00ter
 
if you look in my Becker thread I carry one,it is a flat magnifying lens card ,it is lay'n on a info sheet in the pic lower left. never hurts to carry a extra firestarter....Great Example Doc....you tha' man !!! I love my Swiss champ
 
sh00ter01 said:
Sweet… about how long did you hold the magnifying glass like that before it started to burn?

-sh00ter
Click to expand...

I don't remember exactly. The picture was taking a couple of years ago, but I don't remember it taking a long time. The secret when using a magnifying glass is to focus on dark coloured material.

Try this experiment. Find a cigarette and try to burn a hole through the white paper. Then focus on the brown, tobacco end. You will see what I mean.

Doc
 
Bumppo said:
Does the magnification strength of a magnifying glass affect how fast it will catch a given material on fire? Does the diameter matter? Can ( practical) light sources other than the sun be strong enough to cause fire?
Click to expand...

No. Yes. No, not that I know of, unless you mean lasers. Lasers kick ash!

Magnification is inversely proportional to focal length. Spot size has a theoretical minimum proportional to wavelength, and since sunlight is sunlight is sunlight...you can only make a focus just so sharp. And AFAIK only sunlight works for starting fires, but if you're DOC-CANADA, you can use your Superman X-ray vision to make it look like you're starting a fire with a SAK magnifrying glass when your actually doing it with your super powerful eyeballs.:) I agree with DOC that focusing on a dark spot vs. a light spot is important too. Sometimes I'll dirty up a piece of paper with a pencil mark or a little bit of dirt, because the light absorbtion capability of the tinder really makes a difference.

There are several pretty good wikis on optics. If anyone's interested, this is one:
http://en.wikipedia.org/wiki/Magnification
 
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The magnification is not significant if you can get a focal point. The diameter is very significant. This is because it collects and focuses more area of light into the same size small point. The greater the area of the glass, the more light (and heat) you focus into the same spot.
 
I was super surprised by the power of starting fire with magnification by the way. I got one of those super cheap (less than a dollar) Fresnel credit card sized plastic lenses - I flamed up a paper towel with no practice in less than a minute! I am sure with practice and good tinder - you may be able to do it even quicker.

TF
 
Grab a full page magnifier from an office supply store. It's an 8x11 1-2mm thick plastic fresnel, and will light up just about anything you want it to in a matter of seconds as long as the sun cooperates.
 
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