The 20mm f/2.8 Di III OSD M1:2 ($349) is another affordable, well made Tamron lens available exclusively for Sony's full-frame mirrorless camera system. It's much better than you'd expect for the price, and complete with weather protection. There are some drawbacks, including an autofocus system that's not as quiet as we'd expect, but it's a very good prime lens, especially when you take its very wide angle of view and low price into account.

Short and Squat

The 20mm f/2.8 is a short, squat prime lens. It measures 2.5 by 2.9 inches (HD), weighs 7.8 ounces, and supports 67mm front filters. Tamron includes a reversible, petal-style lens hood, as well as front and rear caps. It's only sold for Sony mirrorless cameras, and is compatible with both APS-C and full-frame sensor models.

The barrel is made from polycarbonate. It's a sturdy composite, not the cheaper-feeling plastic used by Rokinon in its competing 18mm F2.8 AF ($399), also available for Sony systems. It also includes dust and splash protection, and an anti-smudge fluorine coating on the front element.

The manual focus ring is prominent, occupying much of the barrel. It's finished in the same composite material as the rest of the barrel, but this time with ridges so you can find it by feel and more easily turn it, even when wearing gloves.

Focus is electronic—you can only adjust it manually when the camera is powered on—and the response isn't linear, so adjustments are smaller with slower turns of the barrel. This is beneficial for imaging, but a downside for video work, where linear, repeatable focus pulls are desired.

Most photographers will use autofocus, which is one area where the 20mm f/2.8 isn't quite as modern as we'd like. Focus is adjusted by moving the optical elements in their entirety, rather than using an internal focusing group as part of the optical design.

There are some drawbacks to the approach. The lens makes noise while adjusting focus, and can take some time, about a third of a second, to drive from close distance to infinity or vice versa. You're not likely to use a 20mm for action photography, so it's less of an issue.

The focusing method does introduce quite a bit of breathing—changes in the angle of view as you move from close focus to distance—which isn't a concern for still shooters, but is one for videographers.

It's also how the lens nets its 1:2 macro focus capability. It locks on to to subjects very close to the front element—about 4.3 inches from the sensor plane—which certainly helps, but the angle of view is also narrower when focusing close, effectively increasing magnification.

There's no optical stabilization built in, but don't let that worry you. Most Sony cameras offer the feature in-body. It's good enough to smooth jitters from handheld video and net crisp, handheld shots at longer exposure times. I got good results with exposure times as long as a half-second with the a7R IV.

In-Camera Corrections Required

I tested the 20mm f/2.8 along with the 60MP a7R IV, currently Sony's highest-resolution camera body. At f/2.8 its images aren't razor sharp, but are still in the good range for the a7R IV's 60MP sensor, showing about 3,600 lines in an Imatest evaluation.

The resolution enters our very good range at f/4 (4,220 lines), and is excellent at f/5.6 (4,600 lines), f/8 (4,600 lines), and f/11 (4,450 lines). There's a loss of clarity at f/16 (3,850 lines) and f/22 (2,600 lines), an expected result caused by diffraction, something that all lenses must deal with.

There's quite a bit of distortion. We see a strong barrel effect in images shot without the aid of in-camera correction. However, if you leave distortion correction turned on, curved lines are straightened out automatically.

There's not yet a Raw profile for automatic Lightroom corrections, but it's only a matter of time before Adobe adds support for one-click corrections. In the meantime, it's easy enough to remove the barrel effect with manual tools, as I did for the image below, a corrected version of the shot above.

There is a slight vignette effect when making images at f/2.8 and f/4. Again, in-camera corrections come to the rescue here for JPG photographers, all but removing it. For Raw photographers the fix is as easy as the one for distortion.

Good Doesn't Have to Mean Costly

Tamron is filling a need that Sigma, Sony, and others making lenses for the E-mount system aren't meeting—a desire for quality lenses at prices that won't stretch your budget too far. The 20mm f/2.8 Di III OSD M1:2 nabs images with excellent detail, even when paired with a high-resolution image sensor.

There is some distortion, but in-camera corrections or Raw adjustments make short work of it. The real drawback is the focus system, which is a bit slower and noisier than more modern lenses, but this is more a concern for videographers than photographers.

There are some alternative options to consider. Tamron also makes a 24mm f/2.8, which delivers the same level of quality with a slightly narrower angle of view. And there's Rokinon's 18mm F2.8 AF, which covers a wider angle, but isn't as well made, both in terms of construction and optics. If you can stretch your budget, you may want to consider a Tamron zoom, the 17-28mm f/2.8 Di III RXD instead. It's priced at $900, but its ability to zoom, compact build, and stronger autofocus system may prove worth it to you. For $350, however, it's tough to argue with what the 20mm f/2.8 delivers.

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• Oops! $13K Camera Lens Sells for $95 Due to Prime Day Error