If you would like to get a feeling how different focal length looks like without having a real lense for testing you can take a closer look at the website from Tamron. It is a company that produces lenses, also for Canon cameras. I do not know what camera you are going to get. If it is a DSLR with an APS-C sized sensor, like the 4xxD, 5xxD, 60D or 7D choose "digital camera". If it is a full frame camera like the 5D or 7D than choose "35mm film camera". Under the picture is a slider that can be used to simulate different focal length settings.
http://www.tamron.eu/uk/lenses/focal...omparison.html
I suggest trying the lense before buying it if it's possible.
Robert