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 definition of a prime lens and how these lenses 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 24 mm and 70 mm. A fixed prime lens has only one measurement, such as 50 mm. 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?

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 because of their durability and lower price. Prime lenses also perform better in low-light conditions and are lightweight.

Some photographers insist that prime lenses produce sharper images as well. However, 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. The lens has fewer parts to break. This also reduces 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

With 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. These lenses 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 50 mm 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 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 the lens contains less glass. However, that's not how optics work today. With modern lens design techniques and fabrication methods, additional glass isn't an image-degrading liability. With the precision in design and manufacturing, as well as optically clean antiglare and antireflective coatings, additional lens elements don't 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 pleasing 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. 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 35 mm camera, a normal lens is about 50 mm. If you have a digital camera with a crop sensor, consider the lens' equivalent field of view when attached to the crop body. With a fast prime lens, you can capture images in darker scenes without lighting, flashes, or noisy high ISO ratings.

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