Magnification from your telescope and eyepiece focal lengths, with Barlow, exit pupil, and the useful magnification range for your aperture.
Magnification = telescope focal length ÷ eyepiece focal length, times any Barlow. The practical ceiling is about 2× the aperture in mm (atmosphere and optics limit it); below about aperture ÷ 7 the exit pupil is larger than a dark-adapted eye and light is wasted.
Divide the telescope focal length by the eyepiece focal length: a 1200 mm scope with a 10 mm eyepiece gives 120x. A 2x Barlow doubles that to 240x; a focal reducer works the other way.
About 2x the aperture in millimetres (roughly 50x per inch), so a 150 mm scope tops out around 300x. Beyond that the image gets bigger but not sharper, and most nights the atmosphere limits you to 150-250x regardless of the scope.
Yes. Below about aperture ÷ 7 the exit pupil exceeds the ~7 mm of a dark-adapted pupil and some of the light misses your eye. For a 150 mm scope that is around 21x.