Buying yourself a new lens - particularly if this is your first purchase away from the kit lens - can be an overwhelming experience! It's an important decision for you to make too, since your lens choice plays a critical role in making sure your images look the way you want them to! Unfortunately, there is no one single lens that will suit everybody, all of the time, so which lens is best definitely comes down to personal choice.
With that in mind, I've come up with 8 questions that you can ask yourself before you buy a lens to determine which is the best lens choice for YOU, and help you narrow down all the options that are out there.
Before we dig in, if you are brand new to the world of lenses, then you might want to check out this post on understanding the writing on the lens - this breaks down things like focal length and understanding the aperture value, and also how a lens might perform differently on your camera if you have a crop sensor and so on. If you are not sure of all the terms and exactly what they mean, I suggest you start there first to get a good overview, then move onto the questions.
OK, here we go!
1) What is my main subject going to be?
When buying a lens, your first consideration should be what you intend photographing MOST OF THE TIME with that lens. This will help you determine the ideal focal length for your new lens. For example, if you are mainly shooting documentary / lifestyle images of your children you are going to want a 35mm or 50mm lens - both thse focal lengths are ideal for allowing you to shoot indoors, get in more of the environment, and allow you to stay close to your kids. However, if your goal is to capture portraits, then an 85mm or 135mm lens would be better choices.
Here is a rough guide:
Street Photography - 35mm / 50mm
Landscape / Architecture - 16mm - 35mm
Portraits - 85mm or 135mm
Sports - 70 - 200mm
LIfestyle / Documentary - 35mm or 50mm
Everyday Macro - 100mm
Wildlife Macro - 200mm
That does not mean that you can't shoot landscapes with a 200mm lens, or that you can't use your 135mm for shooting lifestyle - far from it. Use this as your starting point though, to make sure you can get a lens that will be best suited to what you want to photograph.
If you are not sure what the differences are between each one, I have a post here that compares the different focal lengths. so you can see exactly the different field of view each one creates.
2) Do I prefer to be up close and personal with my subjects?
It's also important not only to consider what is considered an "ideal" focal length for what you want to shoot, but also your personal shooting style. When using lenses with longer focal lengths (100mm +) you will get that wonderful background blur and compression that many photographers love to use. The downside of this style is that you need to be further back from your subjects, and sometimes therefore you are simply not close enough to interact with them. For example, say you want to get happy, smiling images of young kids looking at the camera - then you are going to probably want to interact with them to get those smiles and therefore the "look" you are going for, so a focal length on the shorter "ideal" focal length would be best. If you prefer to capture unposed, natural images, then you are probably going to want to be farther back.
3) What Minimum aperture do I need?
As you are probably aware, your aperture determines the amount of light let you let into your camera. All lenses have different minimum or maximum aperture sizes, for example, a 50mm prime lens may stop down to F1.4, but a 24 - 70mm zoom will only stop down to F2.8 or so. That doesn't sound like a lot, but it can make quite a difference to the other settings - for example, you would have the option of shooting with faster shutter speed or with a lower ISO just by the difference in light that those larger F numbers allow. Lenses that stop down to low apertures are commonly referred to as "fast" lenses - I've written previously about what a difference this can make in real terms and you can read all about it here.
Generally speaking, if you are shooting indoors, you will want F2.8 or wider, If you shoot sports indoors, I would say F2.8 as a absolute minimum, ideally somewhere closer to F2.0. If you shoot mainly outdoors, then F4.0 is probably fine. However, there is also the issue with how you want your images to look - a wider aperture is going to help give you those blurry backgrounds and beautiful bokeh so if that is the look you are going for, I'd suggest a lens that stops down to F2.8 or less.
4) Is weight a consideration?
If you want to one lens to cover a range of focal lengths, then of course a zoom is a great idea. However, there is one drawback, and that is that these tend to be bulkier and heavier than their prime (one focal length) counterpart. If you prefer something smaller and lighter, go with a couple of primes (each in a different focal length) or you don't mind the extra size and weight, then investigate a zoom.
5) Do I need Image Stabilisation?
Many big heavy lenses offer Image Stabilisation (sometimes also referred to as Vibration Reduction) and what this does is reduce the amount of blur you get from camera shake. So, if you are hand holding a heavy lens, you are likely to get motion blur at lower shutter speeds just by your motion from holding the lens! That's why if you have a particularly heavy lens, IS is a good idea as it counteracts that movement and allows you to shoot with a lower shutter speed. That said, if you spend your days running after children, you are likely to need to use a high shutter speed to capture THEIR movement, so IS becomes a bit of a moot point :)
6) How much can you afford to spend?
Now you know what you want from your lens, it's time to look at how much you can afford to spend. Most lenses have at least a couple of variations - for example, with Canon and the 50mm focal length, you can buy at 50mm F1.8 for around $125, a 50mm F1.4 for around $350 and a 50mm F1.2 for a whopping $1300! Obviously there are differences between these - generally speaking, more expensive lenses will be sharper, noticeably so when stopped down, allow you to work with lower aperture numbers, and be quicker to find focus. That's not to say you can't get a great lens for less money - the Canon 85mm F1.8 lens is a superb lens, and is only $350, and the Canon 50mm F1.8 is actually a very decent little lens, perfect for starting out with.
7) Are you happy to go off brand?
You need to get a lens to fit the mount of your camera - for example, a Canon or Nikon mount. Although Canon and Nikon lenses are fabulous, there are some other lesser known brands that provide equally good lenses. Two names to note are Tamron and Sigma. For example, my Sigma 35mm lens is an absolute dream of a lens, but considerably cheaper that the Canon counterpart, and one of my first lenses, the Tamron 28 - 75, was a great lens for the price. Just remember to get the correct mount to fit your camera.
8) Do I really need this lens?
To which the answer is Yes. Always.
I'm joking (a bit!)- it's tempting to get enough lenses to cover every eventuality (I know - I have them, and am now looking at having to sell some of them off!) but many times you really don't NEED that amount of lenses. Make sure that any lens you are buying will fill a gap in your lens arsenal, and that you know exactly WHY you need that lens over another.
If you want to see some example of images taken with the lenses I own, then this post on what's in my camera bag shows plenty of examples and what you can expect from each lens. Enjoy your lens shopping!