Quote:
Originally Posted by skysurfer
Simple calculation: both burn the same. Same amount of energy.
(pi/4)*(0.050)2 * 1000 = 2W of energy, on 50mm distance or 2.5m distance. In the latter case you are (slightly) right: the air cone of 2.5m long which is heated by the two watts results in slightly less burn at the focal point, but it is the same power being captured.
But when used as an objective lens for light and the air is clear inside the 2.5m tube of the 50mm f/50 lens it does not make any noticeable difference.
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Let's just look at the focal length.
At 24mm a lens will produce a solar image .22 mm across
At 2000mm the solar image will be 18mm across
Sure, the same power/flux is being captured by the same aperture, but in the short FL case all the flux is going into an area of just 0.4 mm, versus 254 sq mm in the long FL case.