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Old 12-02-2020, 04:18 AM
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skysurfer
Dark sky rules !

skysurfer is offline
 
Join Date: Dec 2011
Location: 33S 150E (AU holiday)
Posts: 1,181
It is a bit of a comparison between a Nissan Micra and a Toyota Avensis. Different specs.
A 15x70 captures twice the light of a 7x50, and goes a lot deeper than a 7x50, but a narrower FOV. A 7x50 shows the whole Southern Cross in a single field (7º), while the 15x70 can just show Orion's belt (4º).


And most cheap binos have too small prisms so the aperture is capped: I had a 15x70 which was actually 65mm and an 10x50 which was actually 44mm.
You can check that by holding a flashlight (a cellphone's builtin flashlight can also be used) behind the center of the eyepiece and holding a flat surface (e.g. a white wall) a few cm in front of the objective which shows a circle with the diameter of the real aperture.
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