In the days of my old Nikon F, I had a few lenses each with a different purpose. I had my favorite 105mm telephoto lens, my 43 to 86mm zoom lens which I often doubled with a doubling ring, and a few more. I couldn't rally afford a macro lens, so with a reverse ring by Vivitar, I was able to attach my telephoto lens of 105mm backwords, thus allowing me to take awesome macro pictures of flowers, close- ups of raindrops on leaves etc..The raindrop shots were so close, that the water acted as a magnifying glass on the leaf. Awesome!
Understanding the true function of a lens is relatively simple. It all has to do with focal length and field of view. Whether your camera be a digital one with a fixed lens, or one that allows you to switch lens, most often you're using a zoom lens. 'Focul length' is better defined as follows;
The 'focul length' simply determines how much of a scene it will capture. A 'normal' focul length will capture a horizontal field of about 45 degrees for example. While a wide angle lens captures a wider field, such as 90 degrees and more. Telephoto lens are great for close-ups because, it captures a much smaller field of view. Actually any background as you focus on your main subject will appear blurry, or out of focus, which makes for a really nice effect.
Most people understand how a zoom lens works. You zoom out to get more of a scene, while you zoom in to get less of a scene. But since the advent of digital cameras, in comparison to our old 35mm cameras, the relationship between focal length and field of view has changed somewhat because the dimension of the sensors in the cameras are now different, and much smaller. Confusing? Sure is!
What this means for you is that unless you're a real photography nut, and really want to know what each of your lens will do , you shouldn't concern yourself with all of this stuff. But let's assume you really need to know for whatever reason.
In the old days of the 35mm with film, If you owned a wide angle lens, you could count on the fact that it would deliver an end result (a photo) almost exactly how you saw it in your camera viewer. But today, using the same lens on a digital camera will give you a complete different result.
For example, I'm holding in my hand my wife's HP digicam with a 7-21mm zoom lens. But because it's a digital, this small lens will give me a result compared to and up to my 105mm telephoto lens, because of the size of the sensor. Still confused? Don't blame you. Bottom line is that most of you have given up on old 35mm cameras, and only use digital anyway. And as is with any camera, just as it was in the 35mm era, you still need to test in many different situations to get any good at it.
For example, I can take a sunset scene and within 5 minutes get a whole bunch of different effects just by using different aperture openings. You don`t need a photography course to be a good photographer. You just need to read up on what all these numbers mean and do, and as you experiment take notes.
Here's a trick I learned very early in my photograpy years; every time you take a shot, make a note on the lens you used, the apeture opening, the scene, and the shot # you're at. In this way, you can generally repeat the process for your favorite shoot. Anyway, if you plan on entering any photo contest, as you submit your entry, you usually have to inform them of the type of lens you used, the aperture opening, the ASA/ISO rating etc., hence the importance of notes when it comes to serious photography, digital or not.