What Is a Prime Lens? Everything You Need to Know

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Photography of the front element of a prime lens

petekarici/Getty Images

A prime lens is a fixed-length photographic lens, meaning it can't "zoom in" on a subject. Compare this to a zoom lens, which can adjust its 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.

What Is a Prime Lens?

The name "prime" comes from the sole focal length, with only one measurement. A prime lens can be wide-angle, normal, or telephoto. The differences between prime and zoom lenses isn't about focal length, but about the fixed glass.

A standard zoom lens might include all the infinite gradations of focal length between 24mm and 70mm, but a fixed prime lens would have only one measurement, say 50mm. While the zoom lens can zoom out to capture more of the scene or zoom in to focus on detail, a prime lens cannot. So why would people want prime lenses?

Prime Lenses vs. Zoom Lenses

Despite the flexibility of zoom lenses, prime lenses remain popular and common. Turns out, there are several benefits to using prime lenses: improved robustness and durability, better low-light performance, lower weight, and lower cost.

Diagram of the Vivitar Series 1 70-210mm f/3.5 zoom lens
Paul Chin [CC BY-SA 3.0]

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

Simple, robust design

Prime lenses can use simple designs, including fewer glass elements and groups than a zoom lens. Because no adjustment is required, no complicated physical mechanisms need to be included, meaning the lens has fewer areas where flaws can creep in over time. The less complex a device is, the more robustly it can be built and maintained.

Photographers can also clean prime lenses more easily, keeping dirt and grit outside the lens and where it belongs. This improves the reliability of prime lenses when compared to similarly-priced zooms. It can also help reduce manufacturing costs, allowing lens makers to offer high-quality optics at a lower price point, depending on what the market will bear.

Better Low-Light Performance

Additionally, prime lenses can offer better low-light performance than zoom lenses. Thanks to the simpler design, 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.

Photograph comparing aperture sizes and f-stop numbers
KoeppiK

Thanks to their simpler optical path and lens construction, it's easier to make prime lenses fast. 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. The cost to manufacture a similarly-fast zoom lens would be prohibitive, and the weight of such a lens enormous. It's not impossible, just impractical.

As a result, manufacturers have often made prime lenses the fastest lens in their lineup, providing exceptionally low-light performance in a lightweight and often less expensive package.

Better sharpness?

Some insist prime lenses must be sharper than zooms because they contain less glass, but that's not exactly how optics work today.

With modern lens design techniques and fabrication methods, additional glass need not be 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.

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

Compare the images captured with a professional zoom lens and a professional prime lens at a mid-range aperture. While the viewer's impression of the images might differ, there's no objective, visually-detectable difference in sharpness. Zoom lenses and prime lens are capable of the same highly-precise resolution.

Nevertheless, prime lenses have a reputation for sharpness, especially when compared to lower-quality "kit" lenses included with most DSLR cameras. However, that's based on the quality of the lenses, not an inherent difference in quality between zoom lenses and prime lenses. As ever, the cost is the chief determinator of lens quality.

Aesthetic differences

While relative sharpness might be a wash, 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 produce a visual impression indicating a prime lens is sharper since the variation between in-focus and out-of-focus is immediately apparent. The contrast creates an impression of a greater maximum sharpness.

Do You Need A Prime Lens?

Every photographer should have at least one normal prime lens in their bag that reproduces 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.

For serious photographers, a fast 50mm lens should be considered required equipment. An experienced photographer will always pick the right tool for the job, whether you need the speed and depth of field of a fast prime lens, or the flexibility and capability of a zoom lens. Consider your subject and shooting conditions when you pick your equipment, rather than abstract ideas of whether prime or zoom lenses are "better" in some amorphous way.