When looking at lenses you will see that they always have numbers with them. Chances are that your camera came with a lens that says something like 18-55mm. What does this mean, and how can we use this when comparing lenses?
The number is the focal length of the lens. Unfortunately there is a pretty big disagreement on what numbers to use. It comes down to how big the CCD is. More often than not the number given is for a 35mm film. The lower end of digital SLR's have a smaller CCD than a 35mm frame, so the numbers must be adjusted. For my camera (a Canon Eos-400D) the numbers need to be multiplied with 1.8 to give the 'true' focal length. To make matters worse, Canon has a series of lenses called EFS lenses. These lenses are made for the smaller CCD and therefore no multiplication is needed. In general the numbers given are for 35mm frames. When dealing with SLR's they are always 35mm or '35mm equivalent' meaning you can actually compare the numbers and get an idea about how the lens will perform. For smaller, cheaper cameras, the numbers are no longer normalized to 35mm and thus you must figure out for yourself how they must be adjusted for comparison.
The actual number is the distance from the lens to the focal point, i.e your film or CCD. This is optics as it was taught in your physics class and I will not dive much further into it but I do recommend you read the links at the end of this series if you are interested. One thing you will notice though is that if you take out your ruler and measure the lens it will not be the exact number given on the lens. This is because the lens is not made up of a single lens but rather a set of lenses so it will always be shorter than the stated number. As a general rule the higher the number the more tele, or zoom, the lens has. Generally lenses are grouped as follows.
|Focal length for 35 mm film.|
|less than 20 mm||Ultra wide angle|
|21 mm to 35 mm||Wide angle|
|36 mm to 55 mm||Normal angle|
|56 mm to 300 mm||Tele angle|
|over 300 mm||Super tele angle|
As you have noticed your SLR will not let you just press a button to zoom. This is of course because it is the job of the lens to perform the zoom. If you've just started with SLR's you may still find it strange (and possibly a bit annoying) that the lenses do not say "X times optical zoom", so it is hard to tell how good a lens is at zooming. Once you start to understand how zooming works, you will soon realize what a bad measure the "times optical zoom" really is, and how little it actually convey to you. Anyway if you want to work out how many times optical zoom your lens provides you divide the larger focal length with the lower one. For example I have a 75-300 mm zoom lens, so to work out how many times optical zoom it has I simply divide 300 by 75, giving me the answer 4. Now 4 does not sound very much, BUT that is because 75mm is already quite a zoom! My 75-300mm zoom lens has much much better zoom than any pocket camera. Usually they start at 25-30mm(converted to 35 mm numbers, so it is not the numbers printed at the lens) and then up to whatever zoom they provide. So see here what happens 75/30 is 2.5 times zoom, and it is 10 times zoom to reach the same maximum level. My ultra wide angle lens, on the other hand is 10-22 mm, giving me an optical zoom of 2.2 times, but it's maximum zoom is still wider than the widest your normal pocket camera goes. All of a sudden you see how easy it is to compare lenses with the focal length instead! You can tell right away that a 10-22 mm lens is an ultra wide angle lens, and that the 75-300 mm is a zoom lens, you can compare the two, knowing that if you are after zooming lens the last one will zoom much better and take you closer than the 10-22 mm lens ever could. "Times optical zoom" is marketing and in general will not tell you anything about the lens.