Why won't a 17-55mm f/2.8 camera lens light paper like a magnifying glass?
Asked 6/28/2014
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I tried to use a Nikon 17-55mm f/2.8 lens to start a fire on a sunny day by focusing sunlight onto dry paper. I set the lens to manual focus at infinity and opened the aperture, but the focused spot was a few millimeters across and didn't get hot enough to burn anything. Why doesn't a camera lens work like a magnifying glass for this? Is the focal length or aperture too small?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
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A camera lens can focus sunlight, but your 17-55mm isn’t very effective for starting a fire.
At 55mm and f/2.8, the lens’s effective aperture is only about 20mm wide. That means it collects far less sunlight than a typical magnifying glass, which may be around 100mm across. Also, a 55mm lens has a relatively wide angle of view, while the sun itself is only about 0.5° across, so only a tiny fraction of what the lens “sees” is actually the sun.
A magnifying glass works better because it usually has a much larger diameter and is designed simply to concentrate light from a narrow source area.
So the main issue is not that the lens can’t focus enough, but that it doesn’t gather enough solar energy to create a very hot spot. Much longer focal lengths and larger front elements would work better, but attempting this with camera lenses can overheat and damage internal coatings or special glass elements.
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