I just started on photography and was comparing lens available to go with the DSLR. From what I have been reading, the two key characteristics of a lens are its (a) range of aperture size (denoted by f, the smaller the number, the bigger the aperture); and (b) the focal length (which controls the zooming ability of the lens). Other characteristics include image stabilisation and auto focus capability.
Other than the above characteristics, what else differentiates one lens from another (other than the obvious different brands - presumably some manufacturers are better than others)? Say if you bought a lens whose focal lenght is 18mm - 200mm, would you still need any other lens which have focal lengths between 18mm - 200mm?
Thanks in advance for any help!