Science Questions

What sort of camera lens would be best for starting a fire?

Sat, 6th Aug 2011

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Martin Fennell asked:



I just wondered if I was stranded somewhere and I wanted to start a fire using camera lenses, would it be best to use a telephoto or a wide angle lens, assuming they had the same aperture?






Dave -  I think the biggest effect is essentially just the aperture of the lens and how big the lens is to start with.  Its basically to do with how much light you can focus in onto that spot of the sun, so you want essentially something which will work in the dark as much as possible - so as low an F-stop as possible.  A wide angle lens will work better and get more heat in that small space and cause ignition.  But the amount of energy you can get in is limited by the radius of your lens so a big lens is better.

The biggest effect I would say is you want to use a cheap lens because the focal length of these lenses - designed to focus onto a sheet of film at the back of your camera - that's probably only 2 or 3 centimetres away from the back of the lens.  And so, any fire you start is going to be very, very close to your lovely camera lens, so use as cheap a lens as possible.


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the F number (the ratio of the focal length to the diameter) is the most important factor the lower the F number the higher the temperature of the focused spot.
Of course the larger the diameter the lens the more power is captured from the Suns radiation. syhprum, Tue, 9th Aug 2011

I would go along with Syhprum. 

I have looked-up the details on the net of my lens for my old nikon - cannot find the exact details of the objective lens - but from memory they are about the same.  but the minimum f-stop is 1.8 for the 50mm lens, 2.8 for the wide-angle and 3.5 for the telephoto - these are all fairly cheap but good lenses.  You could pay more (a lot more) and reduce the min f-stops.  On this basis you go for neither the wide-angle nor the telephoto - you use the 50mm imatfaal, Wed, 10th Aug 2011

Would the focal length affect the difficulty of keeping the dot of light as small as possible? - This might be a practical issue especially if you are focusing by hand. peppercorn, Wed, 10th Aug 2011

Depth of focus comes with high f-stops - that's a potential problem with the hypothesis above.  I haven't listened to the podcast yet so I dont know what the expert pronounces.  I have a camera with three different lenses here at the office - so if the sun comes out I will give it a bash.  Initial thoughts are that at a practical level the most difficult part will be keeping the diaphragm open without damaging/touching the optics.

(50mm f1.7 / 24-70mm f3.5-f5.6 / 75-200mm f2.8)  The large zoom though has an enormous objective lens at least twice that of the others.  I reckon the zoom - but the chances of hold it steady...
imatfaal, Wed, 10th Aug 2011

There are two factors involved in this.  The F no defines the energy density that can be achieved and the aperture defines the total energy that can be collected.  The end results therefore depend on the sort of tinder used to start the fire and the energy density that it needs to ignite properly. Soul Surfer, Wed, 10th Aug 2011

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