What Is a Prime Lens? Everything You Need to Know

Prime lens definition and uses

A prime lens is a fixed-length photographic lens, which means it can't zoom in or out. Learn the exact definition of a prime lens and how they compare to zoom lenses.

What Is a Prime Lens?

With zoom lenses, you can adjust the focal length by moving glass lenses inside the lens body, typically by using a rotating or sliding sleeve on the external barrel of the lens. A prime lens, on the other hand, has a fixed focal length. The name "prime" comes from the sole focal length, with only one measurement. A prime lens can be wide-angle, normal, or telephoto.

A standard zoom lens might include infinite gradations of focal length between 24mm and 70mm, but a fixed prime lens would have only one measurement, such as 50mm. While a zoom lens can be adjusted to capture more of a scene or to focus on up-close details, a prime lens cannot. So why would you want a prime lens?

Photograph depicting reflections on a camera lens attached to a Canon DSLR
Ben Schaub/EyeEm/Getty Images

Prime Lenses vs. Zoom Lenses

Despite the flexibility of zoom lenses, prime lenses remain popular thanks to their durability and lower price. Prime lenses also perform better in low-light conditions, and they are more light-weight.

Some photographers insist that prime lenses produce sharper images as well, but evidence shows the best camera lenses can achieve essentially the same absolute measurements of visual sharpness whether they're prime or zoom lenses.

How Are Prime Lenses Designed?

Prime lenses come in simple designs, including fewer glass elements and groups than a zoom lens. Because no adjustment is required, no complicated physical mechanisms are needed, meaning the lens has fewer parts to break. This also helps reduce manufacturing costs, allowing lens makers to offer high-quality optics at a lower price point. It's also generally easier to clean prime lenses than zoom lenses,

Prime Lenses in Low-Light Performance

Thanks to the simpler design of prime lenses, lens makers can include comparatively larger maximum apertures, increasing the amount of light that can reach the sensor or film. Lenses with wide maximum apertures are often described as fast lenses by photography aficionados because they allow for shorter shutter speeds in low light, providing sharpness, clarity, and flexibility to the photographer.

While a zoom lens might be considered "fast" with a maximum aperture of f/2.8, professional 50mm prime lenses routinely provide f/1.2 maximum aperture. This allows more than double the amount of light to reach the film or sensor. Zoom lenses simply aren't built to achieve such wide apertures.

Camera lenses with different aperture sizes and f-stop numbers

Do Prime Lenses Improve Sharpness?

Some people claim that prime lenses must be sharper than zoom lenses because they contain less glass, but that's not exactly how optics work today. With modern lens design techniques and fabrication methods, additional glass isn't an image-degrading liability. Thanks to the impressive precision in design and manufacturing, as well as optically clean antiglare and antireflective coatings, additional lens elements no longer blur the image.

Fast prime lenses give artists different tools for creating images. A wide aperture produces a shallow depth of field, blurring backgrounds to create a satisfying bokeh effect emphasizing the in-focus subject. This can result in photos looking sharper since the variation between in-focus and out-of-focus is immediately apparent.

If you compare images captured with a professional zoom lens and a professional prime lens at a mid-range aperture, there's no objective, visually-detectable difference in sharpness. Nevertheless, prime lenses are associated with sharpness, especially when compared to lower-quality lenses included with most DSLR cameras. However, that reputation is based on the quality of individual lenses, not an inherent difference between zoom and prime lenses.

Do You Need A Prime Lens?

Every photographer should have at least one normal prime lens in their bag to capture the "natural" human field of view. For a 35mm camera, a normal lens is about 50mm. If you have a digital camera with a crop sensor, be sure to consider the lens' equivalent field of view when attached to your crop body. With a fast prime lens, you can capture images in darker scenes without lighting, flashes, or noisy high ISO ratings.