Hope this helps to clarify things: as eyepiece magnification is increased the viewing apperture, through which the light passes, is reduced in diameter. So a low magnification eyepiece (such as the popular standard ones) will have a relatively large apperture, hence provide a well illuminated image, especially in poor light.
Zooms tend to be attractive because they offer high magnification, but almost without exception push the limits of the scope they are fitted to. The higher magnifications are only likely to provide a decent image in very bright light conditions, which are also those in which heat haze and other atmospheric abberations occur.
As a rule of thumb an apperture of 4mm provides a well illuminated image in most conditions, as per a 10x40 binocular. So a 65mm scope will give good results in most conditions with around 16x magnification, a 77mm with around 19x, etc (divide objective lens diameter in mm by the magnification of the eyepiece). In very good light a 2.5mm apperture should be OK, i.e. 24x and 30x respectively. A 60x mag with a 65mm scope gives barely 1mm apperture, which is likely to give a very poorly illuminated image.
To see the apperture size, hold the scope or binoculars away from your face, but up to a light or window, when in the eyepiece you can see a black disc with a small neat hole in it. It is not a real hole, but a virtual one, which is a function of the optical components. :h?:
Now with my 100mm binoculars 4mm apperture equates to 25x and 2.5mm provides 45x - most satisfactory!
Graham.