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Zoom is not a word I'd use as it means something very different.
Think of it this way.
Focal length = Aperture of telescope in mm multiplied by focal ratio.
if you have a 100mm f/12 and a 200mm f/6 a 254mm f/4.7, they will all have a focal length of or very near to 1200mm.
if you put a 15mm eye piece in each telescope, you would see the same amount of sky in the eyepiece, however the 254mm and 200mm would show a brighter images than the 100mm This is because focal ratio, is essentially 'how fast' a telescope gathers light.
a 65mm f/6.4 scope has a focal length of 420mm, so does a 150mm f/2.8... However, the difference in light gathering power between a 150mm aperture and a 65mm aperture is quite immense, this is why you typically see 'faster' telescopes and camera lenses (f/2 ~ f/3.5) quite a bit more expensive than an f/6 of the same optical design...
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