Thread: F-ratio myth
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Old 22-02-2018, 01:36 PM
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skysurfer
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Quote:
Originally Posted by Peter Ward View Post
I'd expect a 40cm dob to just that!

Try forming a solar image with a 50mm aperture F50 lens and then a 50mm F1.0 lens and let me know how is goes
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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