The choice of lens is something that I feel should be given more importance. Everybody seems to be going on and on about the latest cameras, how mirrorless cameras are going to kill DSLRs and so on. You don't hear much talk about buying the right lenses and that can be confusing for beginners. Before discussing buying photo gear, it’s important to keep one thing in mind: the lens is the most decisive factor when it comes to image quality. It’s not about the megapixels and the low noise capabilities. It’s about the glass and it that’s the way it’s always been.
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With that said, there are a lot of choices out there today. Zoom lenses, prime lenses, fixed and variable apertures and so on. How do you know which one to buy? Well the first thing I would recommend to any beginner is to spend some time shooting with a kit lens. You know, those cheap, plastic lenses that come supplied with most DSLRs/mirrorless cameras today. Sure they're far from delivering any good quality, but using one will allow you to figure out your needs in terms of focal range.
The kind of photography you do should dictate lens choice. You probably don’t need a 300mm lens if you want to shot landscapes and a fisheye is totally unnecessary for a fashion photographer.
After you figure out your needs, you have to decide whether you want to use zooms or primes. Zooms make shooting easier for obvious reasons. However only pro zooms, the really expensive ones, come close or perform similarly to fixed lenses.
Speaking of fixed lenses, while they do offer better image quality and often faster autofocus speeds, they will also set some limits. Changing perspective has to be done by walking and that’s just fine if you have enough room and time to experiment. But if you're shooting in a crowded room full of people at a wedding, a fixed portrait lens might not be the best choice.
You also need to look out for maximum aperture. Most photographers go for f2.8 lenses because they believe they’re going to need to shoot at f2.8 all the time. I find an aperture of f4 a lot better for normal lighting conditions, and luckily there are a lot of great quality f4 lenses on the market. Again, it’s all up to individual needs. While the “holy trinity”, composed of an ultra-wide zoom, a standard zoom and a telephoto zoom is the arsenal of many pros around the world, others use primes only.
Spend some time thinking about what you need because it’s the best way to make the right investments.
Here’s a very helpful video from B&H with Canon USA’s Rudy Winston giving valuable insight on lens purchasing.