There is a significant variety of choices when it comes to choosing lenses for DSLR cameras. From wide angle to telephoto, zoom to prime lenses, fish eye, fast lenses, wide aperture lenses, the choice seems to be impossible. It's not really. What it comes down to is asking yourself a simple question: What do I want to shoot?
What do I want to shoot?
If you are just beginning in photography, chances are you are still experimenting and finding out what you like to shoot. You might shoot a few family portraits one day and landscapes from your holidays the next. On the other hand, you may have decided right from the start that you love taking photos of wild animals and this is all you want to do. Either way, the lenses required to get the best out of these subjects differ greatly. To fit an expansive landscape image into your viewfinder, you would need a wide angle or ultra wide angle (UWA) lens. However, trying to take a portrait with the same lens would result in a completely different (and possibly worse) image than using a lens designed for portraits (e.g. 85mm). While trying to take a picture of a wild animal from 100 or more meters away is just often impossible to get close, in which case you need a longer focal length (e.g. 150-600mm). In a perfect world you would have 3 different lenses for each of these subjects. But in a perfect world you'd also be a millionaire and be able to afford them all. So the thing to do is to decide what type of photography interests you and choose your lenses accordingly.
Should I go wide on fixed prime, or Zoom lens?
There are benefits to using both zoom lenses and prime (fixed or non zoom) lenses. On one hand, zoom lenses are versatile, and reduce the need for a whole bag full of lenses that you need to change and change again while you are out shooting. On the other hand, a good quality prime lens can be gold. Prime lenses, if they are well built, generally produce a crisper, better quality image. This is because they have fewer pieces of glass and moveable parts. Therefore the light coming in doesn't need to pass through as many objects and so is less diffused. The other great advantage of prime lenses is that because of this, they tend to be “faster” than zoom lenses. Practically, this means that you can use slower shutter speeds as the lens needs less light to create a correct exposure. This is especially useful if you want to take portraits with available light.
What Aperture will I need?
Another important factor to consider when choosing your lens is its maximum aperture. This is indicated in the description by an f symbol. Eg. f/2.8. The lower this number, the wider your aperture choices. For example, if you want to take a portrait with only your subject's facial features in focus, you would use a wide aperture. If you want to take a sweeping landscape where everything needs to be in focus you would use a narrow (high number) aperture. Selecting a lens with a wider aperture gives you more options when out shooting.
It is well known that lenses can cost as much, or more, than cameras themselves. It is also worth noting that with lenses you get what you pay for. While no piece of equipment can singularly make the difference between a good photo and a bad one, a well built lens using quality glass, can lead to sharper pictures. Therefore it is worth considering the lenses you buy carefully and investing in the best quality you can afford. Knowing what sort of photography you want to pursue can make this process a whole lot less daunting and more cost effective.