The Tamron 24mm f/2.8 Di III OSD M1:2 ($349) is one of three low-cost prime lenses, all with identical housings and 1:2 macro focusing, designed and sold exclusively for use with Sony mirrorless cameras. Despite an asking price that's a downright bargain compared with others, the lens is sturdily built, with dust and splash protection, and has a very sound optical design. Autofocus isn't as speedy or as quiet as pricier alternatives, but if you're shopping on a budget, you won't do much better near this price.

The Wide Standard

With more and more photographers cutting their teeth with smartphone cameras, what's been thought of as a standard angle prime has been redefined. For photographers who feel limited by the confines of the traditional 50mm standard prime, a good wide prime can be your everyday lens.

The 24mm focal length here is just a smidge wider than the main lens on your iPhone, and matches the widest setting on the de facto standard pro zoom design, the 24-70mm f/2.8. It's smaller than a zoom, of course.

The lens measures 2.5 by 2.9 inches (HW), weighs 7.6 ounces, and supports 67mm front filters. Tamron includes front and rear caps, as well as a reversible hood. The barrel is sturdy polycarbonate, and incorporates dust and splash protection. There's also an anti-smudge fluorine coat on the front element.

The focus ring is the same composite material as the rest of the barrel, with raised ridges to improve the grip. Manual focus response scales—if you turn the ring quickly changes are more dramatic than with slow turns. This is a benefit for photographers, as it gives the option for sudden or precise adjustments based on speed.

But it's not as good a system for video, where a linear response for manual focus is desired so that it's better repeatable from take to take. Autofocus is an option too, of course. For stills the autofocus performance is decent, although it does take about a half-second to drive from close distance to infinity or vice versa. There's some noise, and the angle of view changes as focus is adjusted.

The changing angle helps the lens net better macro results. Because the angle of view is a little tighter when focused close, the lens is able to net 1:2 life-size reproduction at its minimum 4.7-inch focal distance.

There's no image stabilization, which isn't a big deal as most compatible cameras offer the feature in-body. With the a7R IV, half-second handheld exposures aren't out of the question.

Excellent Resolution, Some Distortion

I tested the 24mm f/2.8 along with the 60MP a7R IV, Sony's highest-resolution full-frame camera at press time. Imatest shows that it's an excellent performer in terms of sharpness wide open (4,750 lines). And, while the edges don't show quite as much contrast, they're still very good (4,000 lines).

See How We Test Digital Cameras

There's a slight improvement all around at f/4 (4,850 lines), and the lens delivers outstanding resolution at f/5.6 (5,125 lines) and f/8 (4,955 lines). Diffraction cuts into detail starting at f/11 (4,520 lines), but doesn't noticeably soften shots until you narrow to f/16 (3,710 lines) and f/22 (2,475 lines).

There is some visible barrel distortion, about 4.5 percent. It's less extreme than you get with the wider-angle 20mm f/2.8 Di III OSD, but can still be noticeable in images of subjects with perfectly straight lines. In-camera corrections are available to remove it from JPG images and video; Raw photographers are able to compensate for the effect with software, like the industry-leading Adobe Lightroom app. At press time there's no one-click lens profile available from Adobe to remove distortion, but I expect it to be included in Lightroom's next update.

Likewise, in-camera corrections remove the visible vignette cast over the corners and edges of the frame at f/2.8. Stopping down lessens it naturally, but you're unlikely to notice it in real-world shots if you leave your camera settings untouched. Photographers working in Raw format have the option to remove or lessen it during the normal editing workflow.

Great Images, Some Drawbacks

Tamron is doing a wonderful job delivering quality lenses for the Sony full-frame mirrorless system, and it's doing so at prices far less than first-party options. The 24mm f/2.8 Di III OSD M1:2 delivers outstanding resolution and strong build quality, complete with dust and splash protection to match your camera.

There are certainly some drawbacks. The autofocus system, which visibly moves the optics forward and back to acquire focus, makes noise and takes some time to move across its range, is the big one. It's also an f/2.8 lens, so it doesn't capture as much light as pricier alternatives like the Zeiss Batis 2/25 or the Sony FE 24mm F1.4 GM.

If you're into video, you may prefer to spend a little bit more and get the Sony FE 28mm F2 instead. We don't like its lack of close-up focus ability, but quieter autofocus and brighter aperture may be worth it to you. Rokinon also sells a smaller 24mm F2.8 for about $50 more—we've not yet completed our review, but can report that it's not as well built, nor as good optically in lab testing.

Zoom lenses cost more, are generally bigger, and take you out of the prime-lens mindset when making images. But we should note that Tamron has a pair of zooms that almost straddle the 24mm in coverage, the wide 17-28mm F2.8 and the standard 28-75mm F2.8, both available for under $1,000. And for a little bit more, the Sigma 24-70mm F2.8 DG DN Art is also an option. For photographers looking for a light, inexpensive, wide-angle prime, though, the 24mm f/2.8 Di III OSD M1:2 is a compelling option.