[Wavytone]

Liz,

Ignore focal length - bear in mind there are many scopes far bigger than amateur ones.

What matters is the focal ratio f = F/D where F = focal length, D = aperture.

f/3-f/5 are short, suitable for wide fields, low to medium magnification, but not suited to high magnification.

f/6 to f/9 are medium focal ratios, these scopes can give high power up to the diffraction limit with "reasonable" eyepieces, yet also give fine wide field views. IMHO f/7 is optimal for visual observing, and I'll explain why, below*.

f/10 is the beginning of "long", f/15 scopes are definitely best suited for high powered observations only (lunar and planetary) because a low power eyepiece is impracticably large.

There are a few rules of thumb for eyepieces:

a) for high power with reflectors, the highest useful magnification is x1.2 per millimetre of aperture. This is accomplished by an eyepiece of focal length (in mm) equal to F/1.2.
b) for high power with refractors, the highest useful magnification is x1.5 per millimetre of aperture. This is accomplished by an eyepiece of focal length (in mm) equal to F/1.5.
c) the lowest magnification is set by the eye pupil; where the pupil is X in mm the longest eyepiece focal length for low power is f x X.

For an f/7 refractor with good optics the highest power eyepiece is about a 5mm, while the lowest power eyepiece you can usefully use is 42mm (for a 6mm pupil)**. If you look at the commercially available quality eyepieces, 5mm is about the shortest and 42mm is about the longest. This means an f/7 scope is capable of the widest possible range of magnification with good eyepieces.

**I'm deliberately excluding eyepieces in the range 2.5mm (Vixen) to 4mm as in my view they're intolerably uncomfortable, and Barlows, which IMHO degrade the image to the extent you're better off without them.

By way of example I have an f/7 refractor precisely for this reason, it can be used with my entire set of eyepieces which span 5 to 50mm. At 50mm it has a whopping field of view, while at 5mm the Airy disks around stars are easily seen.

I also have an f/15 Maksutov specifically for lunar & planetary; a 2" widefield 30mm eyepiece gives 90X for starters and its really best suited to 150 - 250X which require very comfortable 13 and 8mm eyepieces. But for low power, forget it. For the Mak to deliver low power I'd need a 75mm widefield eyepiece, fitted in a 75mm diameter focusser. No-one makes such a beast, for good reason - an eyepiece this big would weigh half a tonne, and cost the Australian GDP to make.

For imaging the priorities are different - f/7 down to f/1.