Using a lens to start a fire

Something has been troubling me for a while -- a lot of people advocate including a Fresnel or magnifying lens in their fire kit for starting fires. That's great, I wholeheartedly agree especially since the benefit-to-weight ratio is so high.

But so often I see vendors advertising higher magnification lenses and I see on Dave Canterbuy's videos and others advocate high magnification type lenses. I scratch my head and say "WHY?" the magnification has absolutely nothing to do with its ability to start a fire. You are simply taking the light gathering ability of the surface of your lens and concentrating it into a dot of high intensity light. Simply put, the larger the surface of the lens the greater then intensity of the dot of light when focused. A 2" dia. lens will gather 4x the light as a 1" dia. lens and concentrate 4x the energy making it easier to start a fire. A 4" lens will gather 16x the light into a point and so on. A really good quality lens with little spherical aberration is better because of its ability to generate a tighter focus point is worth the money but a higher magnification isn't. Similarly, a non-scratched lens will work better than one clouded with scratches.

The magnification affects the focal point not the ability to gather light. Am I missing something about simple physics here? Do these survival gurus know something I don't? Are you paying more for a high magnification lens that does nothing for you? Do we need snake-oil salesmen who trick you into spending more than you need to?

Actually, a hand lens for starting fire is not a prominent part of my plans. You need fairly robust sunlight to be successful, and that is not likely to be a situation where fire will be critical. Or to put it another way, when i have really needed a fire, it was after sunset, overcast, cold, etc.

Still, it is a worthwhile option, presuming one can plan ahead. It's been a long time since high school physics, but I daresay you are correct.

I tried it last summer with a small hand lens.

Fail.

Even at 9,000' on a sunny day it failed. Atmosphere wasn't very clear due to forest fire smoke and I had difficulty holding it steady enough long enough.

More practice would be helpful I'm sure but it just doesn't seem worth the effort.

A second Bic lighter is a better back-up IMO.

I keep a credit-card-sized fresnel lens in most of my kits. As hikermor points out when you most need a fire it will least likely to work but I have used them to remove splinters, read small print on medicine bottles, etc. A fresnel is great since it's light, flat, flexible and very durable.

A short focal length/high magnification lens has the slight advantage that it needs to be closer to the tinder.
In most circumstances it is easier to hold the lens steady and at a short distance than at a longer distance.
The actual light gathered is indeed in proportion to the area and not the magnification.

I have started fires in very marginal conditions by use of a large Fresnel lens, the smaller credit card sized ones only work reliably in ideal conditions IME.

As has already been posted, a fire is most needed in conditions when a lens is useless. Still worth carrying though IMHO, each fire started thus is one more match left for less favourable conditions.

A large Fresnel lens, say A4 sized will start fires in unfavourable conditions and is worth stocking in a long term TEOTWAWKI kit, but is unlikely to worth carry in a backpack.

In good sunlight an A4 Fresnel lens will light a fire of large sticks without any need to procure tinder.

Failing eyesight is the main reason I keep one in my kits too, Phaedrus. They are fun for firecraft practice but I wouldn't want to depend on that method in an emergency.

Sidenote: a Fresnel lens and a pencil sharpener are a fun way to introduce kiddos to fire-making.

Sidenote: a Fresnel lens and a pencil sharpener are a fun way to introduce kiddos to fire-making. wink

Jacque, I just put together 4 survival themed geocaches. I never thought about a magnifying hlass but included a pencil sharpener for the enclosed pencils (graphite won't run if it gets wet) and as fire starter material.

Nice, Wildman! Geocaching is something I'd like to try. I have to figure out my GPS first though.

My son and his friends never have any trouble getting stuff to burn with magnifying glasses in the summer. After their initial success, my son and my Cub Scouts once suggested ditching their lighters and ferro rods in lieue of magnifying glasses. So... we made them try it that night. Lesson learned.

I've never thought of it that way, Adam. I like your thinking. Thanks!

Very true. I do have some A4 Frensel lenses around but I don't carry them as part of my survival gear. As you say, probably good as long term preparedness items but to me a little bulk and impractical to carry vs the likelihood of use. When I most need a fire there often isn't any sun to begin with.

I will say though that fire making is one of the areas where I have the most gear on me. When I'm out on hikes and camping trips I will generally have at least two firesteels, a minimum of two lighters (one peanut-style and one Bic, and I'll note that I don't smoke), and UCO Stormproof matches. Very often I'll also have a Spark-lite and I always have a variety of tinder/fuel items (including Hexamine tablets, fatwood and Tinder-Quik tabs).

No pencil sharpener? They make great tinder.

I don't think I've done this since I was a child; I had no trouble getting a leaf to burn by simply using my glasses.

Jeanette Isabelle

Believe it or not, I have been able to start a fire indoors with a 1.5" dia magnifying glass and a lamp 2' away. (Using one lens of a triplex magnifier set).

It helped that the TinderQuick was already charred, ordinary TinderQuick would not do this. It took me by surprise as I was just goofing around one day. After seeing this, I charred a few more pieces and put them in my fire kit - anything that worked that quickly deserves a corner of my EDC survival kit.