What does an 18 - 50 mm lens mean? Are those numbers the first focal length and second focal length? How do you use this parameter to judge what kind of lens to buy? Does more equal better?
Focal length, in and of itself, has nothing to do with lens quality. Focal length is just a way of measuring how much a lens bends light by expressing it as the distance behind the front of the lens(more or less) where colimated light is brought to focus.
When a lens has a dash between two numbers it means that you are looking at a zoom lens. The first number is the focal length when the lens is at its widest setting, the second number is the focal length when the lens is at its longest setting.
Having said that, there are some general tendencies about lenses on the market that allow you to read between the lines a little.
- Lenses with only one focal length, often referred to as prime lenses, can usually be expected to perform better optically than comparably priced zoom lenses that include the prime lens' focal length in the range of the zoom lens' focal lengths. An 85mm prime lens will generally outperform a similarly priced 70-300mm lens set at 85mm. A 24mm Prime lens will usually outperform a similarly priced 17-40mm or 17-50/55mm lens set at 24mm.
- Zoom lenses with a higher multiple between the widest and longest focal length tend to be of lower optical quality than similarly priced zoom lenses with a lower multiple. An 18-200mm zoom lens, with a multiple of 11.1x, will generally be of significantly lower optical quality than a similarly priced 70-200mm zoom lens with a multiple of 2.86x. A 70-300mm lens, at 4.29x, will usually perform somewhere between the two when priced similarly.
- Lenses with focal lengths closer to the diagonal measurement of the camera's imaging sensor (or film plane) can be made simpler and cheaper than lenses with focal lengths further from the diagonal. A 35mm film camera or Full Frame digital camera, for example, has a diagonal of about 43mm. Lenses in the 40-50mm range can be made for these cameras much cheaper and with very good optical performance when compared to lenses in the 20mm or 200mm range.
- Lenses with a wider maximum aperture/lower f-number will generally outperform lenses of similar focal length with a narrower aperture/higher f-number. They will also usually cost quite a bit more, or include fewer features, such as autofocus. This is especially true of zoom lenses. There are a few notable exceptions to this rule of thumb.
Take all of these generalizations together and you get one of the comparisons I love to use to illustrate them: The Canon EF 24-70mm f/2.8 L II vs. the Canon EF 50mm f/1.8 II. The 24-70 "II" costs around $2,300. The Nifty-Fifty will set you back around $125 from an authorized Canon reseller. At apertures of f/2.8 and above, the $125 prime lens performs as well as the $2300 zoom when set to 50mm and the same aperture!