First off, another name for a fixed-length lens is a prime lens, but don’t let that throw you off…. If you want a sharp picture you must use a fixed lens. End of story.
Just kidding. I probably got a few people in a huff there. But you will come across people who swear by fixed lenses and those who swear by zoom lenses. I own and use both. It really comes down to personal preference. My personal preference changes depending on what I’m photographing and what mood I’m in.
In the “old days”, prime lenses were sharper than zoom lenses. But technology has evolved so much that zoom lenses can be just as sharp as fixed lenses. Of course, there are cheap, low-quality zooms, and those are still not as sharp as a fixed. But a quality zoom lens, especially one that can stay wide-open all the way through, should have stellar results just like a prime lens.
So which should you use? To answer this, I’ll go through the not-so-technical differences.
If you use a zoom lens, you can stand in one spot and change your view of the subject. Basically, you can zoom in and out (real technical talk here).
If you use a fixed lens, you have to move.
What do we get from this? Use a zoom lens if you’re lazy. Want some exercise? Use a fixed lens.
If I’m taking pictures of kids or a sporting event, I use a zoom lens. Also, a zoom is my pick for wedding ceremonies, parties, or other events. It offers me a lot more versatility in the types of pictures I can take without having to move all over the place.
A quality fixed lens is generally easier to make than a quality zoom lens. Therefore, zoom lenses cost more than fixed lenses.
I also have a 28-70mm zoom that is not super-quality and a 50mm f/1.8. The 50mm was under $100 and the 28-70mm was close to $400. I get sharper images with my 50mm even though it was less-expensive! It is simply better quality glass.
Also due to all that technical mirrors and gear “stuff”, fixed lenses can be made to open wider than zoom lenses. I guess this one technically is a technical difference – if you want to get technical about it. When shooting in low-light, I reach for a prime lens.
Size & Weight:
Prime lenses are smaller than comparable quality zoom lenses. If you want to look cool, the zoom is the way to go. If you want to save your arms some strain, go for the prime.
I have an 85mm f/1.8 prime and a 70-200mm f/2.8 zoom. When shooting portraits, I mostly use the prime. Why? I have bad wrists. My 70-200mm is really heavy and big – and I’m not the steadiest-of-hand. So in this case, I get sharper images with the prime lens (but it’s not due to lens quality, it’s just me).
It all boils down to personal preference. But understanding the benefits – even the not-so-technical ones – helps you figure out what will work best for you.