What is a Prime Lens? Why use one?

Posted by David Peterson on 03 Nov 2011 as Tips
If you own a digital SLR camera, or are thinking of purchasing one, you may have heard of prime lenses. Unlike zoom lenses, prime lenses don’t allow you to zoom in or out while taking pictures. This has led many to wonder what they’re good for. If they do less, why purchase one? I’d like to take a few moments to clear up some of the confusion surrounding prime lenses.



Two different 50mm prime lenses for a Nikon camera, side by side.
Photo By Cary and Kacey Jordan

How prime lenses differ from most lenses

These days, most lenses in circulation have some kind of zoom capability. To zoom, you either twist the lens or press some buttons on the back panel of the camera. The camera then adjusts the focal length of its lens, allowing you to see things that are further away with better clarity.
Prime lenses are different. They only have one focal length. You can’t zoom in or out. You get one viewing distance, and that’s it.
The most standard prime lens is the 50mm “normal lens.” Its focal length roughly matches where your eye focuses, giving you an image that’s fairly close to what’s in front of you. There are other prime lenses too that come in all sorts of different focal lengths.

What is the advantage of using a prime lens?

If you can’t zoom in or out, why purchase a 50mm prime lens when you can easily purchase a 50-200mm telephoto zoom lens for about the same price? There a number of reasons for that.
To get higher quality images. Because prime lenses aren’t built to zoom, they have a much simpler design. This leads to fewer visual defects and aberrations. In short, you get a much less distorted and more technically correct image when you shoot with a prime lens. For example, look at the edges of the text in the image above, and you’ll notice that they have a reddish-yellow fringe. Prime lenses can help to eliminate this defect.
To get a better aperture at a lower cost. Wide apertures (low F-numbers) have their cost, especially when you add them to zoom lenses. Have you ever seen a giant zoom lens sports photographers use (like the Canon 500mm lens below) and wondered why it needs to be so big? The reason is the aperture. To get the lens to zoom with a big aperture, you need a much bigger lens. Making the lens really really big also makes it really really expensive. That’s why they cost so much too.

If you just stick to one focal length, the cost of making the aperture a lot wider isn’t all that much. It’s a simpler design with less moving parts.

Some of you are probably wondering why it’s important to get a lens that can handle wide apertures like F1.8. Wider apertures help you get more light out of dimly lit situations while isolating your subject from the background. They’re ideal for indoor portraits too. Many beginning photographers purchase prime lenses for the better aperture, usually intending to upgrade to nicer zoom at some point. Prime lenses help fill the gaps until they can afford more gear.
What about quality? How much better is it?

It is true that you will get better image quality from a prime lens compared to a zoom lens, but it really depends on how you’re using the zoom lens. If you’re shooting in the middle of the zoom lens’ focal range, you’ll create a less distorted image. However, if you’re shooting at the extremes of the zoom lens, you’re more likely to get an image with a little bit of distortion.

For example, a 50-200mm zoom lens at 125mm performs about as good as a 125mm prime lens. It does not perform as well at the 50mm and 200mm ends. If you want perfect quality all the time (no color fringing, no image distortion), you should always shoot with a prime lens. If you’re willing to sacrifice some image quality for the convenience of zooming, go for a zoom. A well-seasoned photographer is likely to have both kinds of lenses at several different focal lengths.

Prime lenses are a great place for any budding photographer to begin experimenting with new lenses because they force you into a new and unfamiliar place. Suddenly, instead of simply zooming, you need to get more creative and walk up to your subject to frame it correctly. You’ll also learn how to create new effects using the wider apertures readily available. If you can afford a 50mm F1.8 lens for your bag, you’ve made a good start.