Another well-made Tamron lens, the 20mm f/2.8 Di III OSD M1:2, $349 ($379), is available only for Sony's full frame mirrorless cameras. The quality is much higher than expected for its price and it comes with weather protection. Although there are some issues, such as an autofocus that isn't as quiet as one would expect, it's still a great prime lens.
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Both short and squat
The prime lens is the 20mm f/2.8, which is short and squat. The 2.5x2.9 inch (HD) lens measures 7.8 ounces and can support 67mm front filters. Tamron also includes front and rear caps, as well as a petal-style, reversible lens hood. This hood is only available for Sony mirrorless cameras and works with full-frame and APS-C sensor models.
Polycarbonate is used to make the barrel. This is a strong composite and not the less durable plastic Rokinon used in their 18mm F2.8AF ($399) also for Sony. The front element is coated with anti-smudge fluorine and dust protection.
It occupies a large part of the barrel and is recognized by its manual focus ring. The manual focus ring is made of the same material but with ridges to make it easier to find and turn, even while 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. It is a good thing for imaging but not for video, which requires repeatable, linear focus pulls.
Autofocus is a common feature for photographers. This is why the 20mm F/2.8 isn’t as up-to-date as it should be. Instead of using an optical design that uses an internal focus group, the focal point is changed by moving all the elements.
However, there are some limitations to this approach. It can make noise when adjusting focus and take a while to go from close to infinite to faraway. It's unlikely that you'll use the 20mm lens for action photos, so this is less important.
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.
This is how the lens achieves its macro focus ability of 1:2. 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.
Although there is no optical stabilizer built-in, don't be discouraged. Many Sony cameras have the option to include optical stabilization in their bodies. This feature is good enough for smoothing out jitters in handheld video, and creating crisp handheld images at long exposure times. With the a7R IV, I was able to get good results even with long exposure times.
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In-Camera Corrections Required
The 20mm f/2.8 was tested along with the 60MP Sony a7R IV. This is Sony's current highest-resolution body camera. Although the images at f/2.8 aren't sharp enough to be considered a masterpiece, they are within the acceptable range of the 60MP Sony sensor. They show about 3,600 lines in an Imatest.
Our very high resolution range is reached at f/4 (4.220 lines), but it's excellent at f/5.6 (4.600 lines), 4600 lines, and 5/6 (4.600 lines) and 11 (4.450 lines). A loss in clarity is seen at f/16 (3850 lines) as well as f/22 (2600 lines). This is due to diffraction which all lenses have to deal with.
The distortion is quite severe. Images shot without in-camera adjustment show a noticeable barrel effect. If distortion correction is turned off, curvilinear lines will be automatically straightened.
Although there isn't yet an Adobe Raw profile that supports automatic Lightroom adjustments, it will soon be available. It's possible to eliminate the barrel effect using manual tools. I used this technique for the corrected image above.
Images taken at f/2.8 or f/4 will show a slightly vignette effect. For JPG photographers this is almost eliminated by in-camera adjustments. Raw photographers can fix distortion just like the ones for JPG.
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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. Even when used with high-resolution images sensors, the 20mm f/2.8 Di III OSD M1:2 captures excellent images.
Although there is some distortion, in-camera adjustments or Raw adjustments can quickly correct it. However, the real problem is the focus system. It is slower and noisier that modern lenses but is less of a problem for photographers than videographers.
You have other options. Tamron makes the 24mm F/2.8 as well, but it has a narrower view angle. Rokinon also makes a 18mm F2.8AF, with a slightly wider view angle but isn’t as high-quality in optics and construction. You might consider the Tamron 17-28mm F/2.8 Di IIIRXD zoom if you have more money. Although it's expensive at $900 due to its zoom capabilities, small build and strong autofocus system, you may find the price worth it. The 20mm f/2.8 is a great value at $350.