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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