Magnifying Glass Method - Fire Lighting

The magnifying glass method is a method of fire lighting that involves focusing the energy in sunlight onto flammable material, to raise its temperature and cause it to ignite. Contrary to common belief, the focal length or power of the lens (magnifying glass) used does not affect how much heat is generated, only the size of the lens matters.

Method

You should first be familiar with the article on fire lighting to understand the basic principles and terminology of this method.
Assemble a pile of tinder.

The quality needed for the tinder will vary greatly on how much energy (power) you are going to collect from the magnifying glass. For good conditions with a large lens, especially near the equator for example, the tinder may not need to be very good (very dry fine kindling will do). For less suitable conditions (for example, most of the year in Britain) the tinder will need to be of a much better quality. Darker coloured tinder will light more easily, so you may want to make the tinder black with a marker pen if one is avaliable. Generally darker tinder (char cloth for example) will work substantially better than light tinder (tissue paper for example).
Hold the magnifying glass so that it is pointing directly at the sun.

CARE: do not look at the sun at any time, never look at or near the sun through the magnifying glass or any lens; permanent complete or partial blindness is the most likely outcome.

You will have to judge whether the glass is pointing at the sun through indirect methods, for example, looking at shadows. Hold the lens so that it points at a light object (a rock or some paper for example), pointing at the sun on the other side. Move the lens closer and farther from the object so that all the bright 'shine' from the lens focuses into a small bright dot. Now move the lens over your tinder, so the dot is on the tinder. Move and rotate the lens slightly to make the dot as small as possible. Now you must hold the lens still and wait. If the conditions are favourable enough the tinder under bright dot will start to smoulder, and will get hotter and hotter. With practice at this method and general fire lighting skills you will be able to light a fire from this heat.
Factors

The following factors affect how easy (eg how well you will need to focus the light, how fast the fire will start and whether it will at all) it will be to light a fire using this method.
General firelighting factors

These factors vary the ease of lighting any fire, and will also do so here: tinder quality, ambient temperature, temperature of tinder, moisture, wind speed.
Sunny day' factors

A warm day makes lighting any fire easier, but here the heat from the sun is of special importance: general 'sunniness', distance from the equator (being further will make it harder) and cloud cover are factors of this type. If you can feel the sun is warm on your skin this is often the best indication. This method will not work at all in the shade, even on a hot day.
Size of the lens

Having a large lens is probably the most important factor you can vary. For example, starting a fire with the lens from a pair of glasses would usually be impossible anywhere cooler than the tropics, no matter how 'good' the glasses are. A large magnifying glass is much better.
Shape of the lens

If the lens does not focus the light down to a very small spot it will make the method far harder.
Position of the lens

Holding the lens in the right place can make a major difference, this aspect requires practice.
Colour of the tinder

A dark tinder really will light much more easily.
Pros

* This is one of the few methods that you really can use as many times as you like without ever having to restock your materials, as long as you are careful with the lens.
* In challenging conditions, you will not waste matches or erode your striker, you just have to wait longer and keep trying. If the method does not succeed you have lost nothing.
* The method is very quick and fairly easy if you do have all the factors in your favour, and doesn't require any physical effort like the friction methods.
* For the above reasons, this can be a very relaxing and satisfying technique on an warm, pleasant day.

Cons

* If the conditions are against you it will be very hard and often impossible to get this method to work regardless of your skill or effort.
* The method is very dependant on factors that can rapidly change, for example a cloud moving over the sun is enough to undo all your hard work at trying to light a fire.
* It can be easy to damage a lens, and the lens need to be clean for the method to work.
* The required conditions for the method are very often not those present when you need a fire, for example at night, when it is cold or damp, making this method much more useful either as an alternative to be used when possible, or with a backup to ensure that you can light a fire somehow.