Check this article out on wikipedia - http://en.wikipedia.org/wiki/F_stop
- It should help you understand it a little better.
F Stops, numbers, aperture, are all the same thing; the size of the space in which light can enter the lens, and are measured in F-Stops/Numbers. The larger the Aperture, the smaller the F-Stop Number.
F/1.8 is a LARGE or FAST Aperture as it allows a lot of light in to the lens, but 1.8 (in numerical terms) is a small number compared to an Aperture of F/22 which is a SMALL or SLOW Aperture, and 22 is a larger number than 1.8.
So Small (F-Stop) Number = Large/Fast Aperture & Large (F-Stop) number = Small/Slow Aperture.
A Larger Aperture gives a smaller depth of field than a Smaller Aperture does. A better explanation than I can give of this effect can be found on Wikipedia and over at [url="http://www.dslrtips.com"]DSLR Tips[/url]
Quality of the Glass:
Better quality lenses are usually bigger, heavier and more well built on the outside than their lesser counterparts, but this is not always true. Reviews are always a good thing to check. Obviously Cameralabs and DSLR Tips do reviews on lenses but any that they don't have, I believe Thomas recommends [url="http://www.photozone.de"]PhotoZone[/url] for reviews.
Different Lens or One Lens:
Generally it is better to get different lenses, this usually minimizes distortion across the focal lengths, but there are exceptions such as the Nikkor 18-200mm which does a pretty good job across its focal length, but of course nothing can beat 2 or 3 good quality zooms or primes.
G Lens Vs. Carl Zeiss Lens:
I'm afraid I don't know enough about these lenses to pass a verdict, but like above, look for reviews of the lenses.
I'm not 100% but I think Sigma and Tamron offer lenses for the Sony mount, but you would have to double check that. (or hopefully someone else here can confirm/deny that) But you won't be able to use Canon or Nikkor lenses. (at least without a converter, but those are best avoided)
A Tele-converter is a "lens" which goes between the camera and the lens you are using and multiplies its focal length by usually 1.4x or 2.0x at the expensive of image quality and maximum aperture.
Macro & Fixed Focal Length:
Macro lenses are lenses that can focus to a distance which makes an object 1:1 - Life size or larger. Life size as in Life size on the cameras sensor, and when on a print or viewed on a computer screen, it becomes much larger than life size!
Fixed Focal Length lenses, also referred to as Primes, are lenses which can not zoom. They are generally much shaper and produce more accurate colours etc. than zoom lenses, but nowadays zoom lenses, especially good quality ones, aren't far behind in the sharpness and colour reproduction departments!
The Lens you are thinking about:
If you only want one lens, then from the face of it, should fit your needs. Depending on the lens you have just now, the 75-300 might be a better choice if you are willing to lug a second lens about.
Hope that helped
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever