When it comes to photogrpahy, choosing your lens is often more difficult than choosing a camera. A lens will last serveral times longer than our cameras. I still have the telephoto lens I bought way back when I was using my first SLR - a Canon EOS Rebel. The Tamron zoom lens I bought while I was snapping with my Canon EOS Digital Rebel is now on my 50D whenever I need to travel light (and need a large zoom range). My point is, a good lens will be an investment that you can use for a longer period of time (if handled correctly).
Another thing to think about is that all the light that reaches the sensor needs to pass through the lens. A low quality lens on a high quality camera will result in a bad image quality, but a good lens on a lower quality camera can still produce good results (* by lower quality camera, I mean the company's "entry level" D-SLR camera).
It is important to realize that different lenses can distort and compress the view. Wide angle lenses can make the distances seem greater than they really are. In contrast, a telephoto lens can compress the view and make far away objects seem closer than they are.
We call them normal lenses because they have the focal length that resembles the view of human eye, thus creating a natural view, unlike wide-angle that distort and telephoto that compresses the view. These lenses usually have a very low f-number, making them ideal for low light conditions. These usually are 50mm prime lenses with aperture of f/1.2 to f/1.8.
Personally, I would recommend everyone to go out and get a 50mm prime lens, even if you have a zoom lens that cover the focal length. The 50mm primes on the market today are often reasonably cheap but with exceptionally good optics for the price.
These lenses are great for landscapes, architecture and indoor photography - but be aware of the distortion they create. The closer you are to your object, the more distorted it will be. The distortion is visible the most in the corners.
With such short focal lengths, they can be useful in low-light situations, both because they can take in light from wider angle and because a little camera shake is not as visible as it is on longer focal lengths.
These lenses have a narrow field of view and a long focal length. Telephoto lenses are great for wildlife and sports photography, and can be good to use for portrait when we want to isolate the model from the background.
Telephoto lenses with their longer focal length require better light conditions or the use of a tripod. Some teletphoto lenses, like the EF 400mm f/2.8, are very fast but are often very expensive and out of reach when it comes to most amateurs.
Last decade, many companies have started to produce these high end telephoto lenses with Image Stabilizer to make them more usable without tripods. Lately, this feature has been made available in more and more low-end lenses.
Macro photography usually refers to close-up photography, but we need to realize that not every photo of an insect or flower is a macro. True macro photography is at the scale of 1:1 (or greater) - meaning the object you are photographing should be the same size or larger on the sensor.
Most macro lenses have a focal length between 50mm to 200mm, and usually have a large maximum aperture that gives them both the ability to be fast as well as totally isolate the subject.
Many modern macro lenses can focus to infinity and are prime lenses which can make them ideal when it comes to portrait photography. Just because it is a macro lens does not mean it can only be used for that type of photography.
Prime Lenses vs. Zoom Lenses
A prime lens is a lens that has a fixed focal length - these lenses come in all shapes and sizes and price classes. Zoom lenses are more versatile - a zoom lens can be a wide-angle lens, a normal lens and a telephoto lens - all in one - where as a prime can only be what it is. High-end telephoto lenses as well as macro lenses are almost always primes.
Most primes are considerably sharper than the zooms in the same price class, even when you go to the very high-end lenses the primes are sharper. Not only are primes sharper but they often have a larger maximum aperture which makes them faster and ideal in low light situations.
So why choose a zoom lens? The answer usually is "convenience." We can carry one zoom lens that cover the focal range of many primes. It is much lighter to carry a single zoom lens instead of carrying a bag full of primes.
Quick advice on buying a new lens
Please consider this:
- How much are you willing to spend?
- What do you need it for (sports, landscape, portraits, etc.)
- What lenses do you already own
- Prime or Zoom
- Image Stabilizer or not
The budget question is rather obvious - don't buy lenses you can't afford. Well, obvious to most, but I still dream about these lenses that I can't afford. What you need your lens for is another very important factor when it comes to focal length and speed. For instance, there are many telephoto lenses on the market but many of them are not suitable for action sports due to the fact that they are a bit too slow - and with slow I mean that their largest aperture isn't letting enough light through to freeze action. Many action sport situations require a lens that has an aperture of f/2.8 or so (unless it is well lit or flashes are allowed).
It is also important to consider what lenses you already have in your collection and what a new lens will add. Sometimes, you purchase a new lens as an upgrade from your previous lens, sometimes it's for a focal length that you do not have. Don't worry about small gaps in the focal length in your collection. For example, it's not a problem to have a 17-55mm and a 70-200mm zooms - sure you don't have 56-69mm but those are not big gaps and buying extra lenses to fill such gaps is not something I advise you do.
When it comes to upgrading, my personal opinion is that the upgrade should be more than just better image quality, a worthwhile upgrade you should get a faster lens, or a feature such as image stabilizing. The choice between prime and zoom lenses was described earlier in this page, and there is no right or wrong here, just personal preferences.