Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 10/08/2011 18:08:49

Title: What sort of camera lens would be best for starting a fire?
Post by: thedoc on 10/08/2011 18:08:49

Hi,

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?

Thanks

Martin
Asked by Martin Fennell


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Title: What sort of camera lens would be best for starting a fire?
Post by: syhprum on 09/08/2011 21:33:01

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.

Title: What sort of camera lens would be best for starting a fire?
Post by: imatfaal on 10/08/2011 10:15:49

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

Title: What sort of camera lens would be best for starting a fire?
Post by: peppercorn on 10/08/2011 11:18:33

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.

Title: What sort of camera lens would be best for starting a fire?
Post by: imatfaal on 10/08/2011 14:23:33

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...

Title: What sort of camera lens would be best for starting a fire?
Post by: Soul Surfer on 10/08/2011 19:13:54

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.