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A few days ago, on a sunny day, I found myself hiking in the woods and wanting to help fellow picknickers to light a fire with no matches or lighter.

However, I had my 17-55 f2.8 Nikon on my camera, so I thought: it's basically a telescope, why don't I concentrate the rays onto some dry paper to start a fire? I put the lens into manual, focused on the infinity, manually opened the iris (there is a handle to do that), and... nothing happened. The paper did not light up, the focused beam (a disk few milimeters across) did not even hurt when pointed on my hand.

What was wrong? A semi-decent magnifying glass starts a fire in no time. Is the lens not focusing the rays enough?...

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closed as off-topic by dpollitt, mattdm, Philip Kendall, NickM, MikeW Jun 29 '14 at 0:11

  • This question does not appear to be about photography within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

9  
This question appears to be off-topic because it is about starting a fire. –  dpollitt Jun 28 '14 at 15:51
    
Well yes, but it's using one of the key elements of a photographic camera, thereby learning more about its inner workings. Considering it unlikely that there could be a SE site on the topic of fire, what better place to ask this? –  vektor Jun 28 '14 at 16:08
2  
Not having another SE site where this would be on topic does not make it on topic here, sorry. What about Physics? –  dpollitt Jun 28 '14 at 16:17
    
physics.stackexchange.com –  Kevin Panko Jun 28 '14 at 16:24
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@dpollitt Yes but there's no fire.stackexchange.com :-) –  user4894 Jun 28 '14 at 21:01

1 Answer 1

You need a much longer focal length than 55mm to do this with a camera lens. The sun only occupies about a 1/2° arc of sky, yet at 55mm your lens is collecting light from about a 70° arc - the sun is occupying well less than 1% of your lens' field of view! And the entrance pupil (effective aperture) at 55mm and f/2.8 is only about 20mm. A cheap magnifying glass, on the other hand, is typically around 100mm wide and focuses the light from a much smaller angle of view.

Be aware that trying this with very long focal length lenses can damage the internal coatings of the lens if you allow the internal temperature of the lens to get too high. With lenses that use fluorite elements (currently Canon is the only major mass marketer of camera lenses that uses fluorite in their high end telephoto lenses) the heat can actually cause the elements to crack.

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Doesn't it also have to do with the size of the front element? If you have two lenses with the same focal length but one has a larger front element, that one collects more energy from the sun than the smaller one and has more paper-burning potential. –  Justin Jun 28 '14 at 15:42
    
@Justin Yes you are correct. Both the collection aperture and the focal length matter, the first because of the total captured energy and the latter because of the focussed spot size. –  Carl Witthoft Jun 28 '14 at 17:26
    
I'd assume the aperture matters much more than the focal length. The aperture determines how much power is collected (I guess I recall that the power of the sun is something like 1 kW/m²) and the focal length determines how big the image of the sun disk is - longer focal length = bigger image = less energy density; a shorter focal length produces a smaller image = higher energy density = more heat, but is harder to focus (you are not looking through the SLR but rather play around with a gleamy spot on some wood), so one may guess that the hot spots are about the same size. –  Hagen von Eitzen Jun 28 '14 at 18:32
    
Well, the only light that matters is the light coming directly from the sun, so the most important factor is going to be the surface area of the collection element; i.e. the bigger the lens cap is, the more likely you're going to be able to start a fire with the lens that it caps. –  Robert Harvey Jun 28 '14 at 19:13
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@HagenvonEitzen You are correct that it is the aperture, or more accurately the entrance pupil (effective aperture), size that matters. But it just so happens that most telephoto lenses have larger entrance pupils than most wider angle lenses because the entrance pupil of a longer focal length lens must be proportionally larger to achieve the same f-number as a shorter focal length lens. To get f/4 at 400mm requires an entrance pupil 100mm (a little over 4 inches) in diameter. To get f/4 at 40mm requires an entrance pupil 10mm (about 0.4 inches) in diameter. –  Michael Clark Jun 28 '14 at 20:16

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