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Thanks Nik - I did kind of assume the diameter of the front element was the determining factor - just like a telescope really, but the amount of light captured mist still be affected by the focal ratio because f/1.4 still needs less exposure time that f/5.6 irrespective of the size of the front element in camera lens at least.
Also fast lenses do have far bigger front elements than their slower counterparts. Zoom lenses are a further complication.
Coming back to my question then, there is no fixed answer as to which size camera telephoto lens is the equivalent of my Takahashi fsq-85 f/5.3 refractor?
Chris
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