First you need to consider your budget – how much are you willing to invest into your next lens?
When you have decided on the money you can start thinking about the available lenses within that price range. The lenses today have a lot of different features so ask yourself these questions to find out what it is you’re looking for.
APS-C or Full-frame?
Most manufacturers offer both APS-C and full-frame cameras in their ranges. But this makes buying lenses even more confusing.
Let’s say you own an APS-C camera. But at the back of your mind you think you might one day buy a full-frame body. That raises the question – do you buy a lens that works on APS-C bodies only (the advantage being that it is probably smaller and lighter than a lens that would fit a full-frame camera) or one that fits a full-frame body as well (which will probably be larger and more expensive)?
Tricky question to answer, isn’t it? And that’s not even taking into account the difference that sensor makes to the lens’s angle of view.
Ideally, you should decide when you buy your first camera body whether it should be APS-C or full-frame, then stick to the same sensor size in the future. It greatly simplifies the lens buying process, and eliminates a lot of confusion.
Should you buy a zoom or a prime lens?
The benefit of zoom lenses is convenience. If you are a wedding photographer it is much easier to zoom from a wide-angle to a telephoto when you need to, than it is to change lenses. If you are a landscape photographer it is easier to use a wide-angle zoom to frame the scene precisely, than it is to change prime lenses (or it may not be possible to stand where you need to get the shot).
The benefits of prime lenses are image quality and wider apertures. Compare an 18-55mm kit lens (typical maximum aperture f/5.6) with a 50mm prime with maximum aperture of f/1.4. There’s a four stop difference 16 times more light) between f/1.4 and f5.6, which helps you take photos with blurred backgrounds, and also to shoot in low light conditions, without raising the ISO too much. That’s why a 50mm prime is a better portrait lens than the 18-55mm kit lens (taking us back to the point about lens choice being driven by subject, not focal length).
Some photographers prefer primes, others zooms – thinking about your priorities will help you decide which is best for you.
For example, if you are a landscape photographer who always shoots at f/8, f/11, or f/16 then the wider apertures that prime lenses have are of little use to you, and a zoom may be a better choice.
Brand name or third party?
You can often save money by buying a third-party lens for your camera, but in my opinion it is best to buy a lens made by your camera’s manufacturer whenever possible. Unless you have a specific reason to buy a third-party lens (usually because the type of lens you need isn’t made by your camera’s manufacturer) then stick with OEM (Original Equipment Manufacturer) lenses. They hold their value better, and autofocus performance is usually superior.
Source: Digital photography School