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Message: <blockquote data-quote="Binastro" data-source="post: 3174827" data-attributes="member: 111403">. Mark, I did wonder about that, whether using a 50 mm binocular with a newspaper to the rear of the eye lens, if the beam is still concentrated enough to set fire to the newspaper.Is the field stop of the eyepiece the only place where the beam is concentrated, or as the beam travels through the eyepiece, are there other positions where the beam is concentrated enough to start a fire.It is known that a 4 inch refractor or even a 3 inch refractor can crack the awful sun filters that were supplied to go on the rear of eyepieces. Whether the heat at this position is enough to set a newspaper on fire I don't know.. Does it it depend on the size of the exit pupil? For instance, with a high magnification, the exit pupil might only be one millimetre across. If you place the newspaper here, I presume it can be set alight.</blockquote>