Marc Alhadeff from SonyAlpha.Blog posted the world’s first review of the new Tamron 20mm f/2.8 FE lens:

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The Tamron 20mm F2.8 Di III OSD M 1:2 (450 euros) is an excellent wide angle lens

It is part a set of new Prime lenses with all a short focusing distance , a 1:2 magnification ratio, a F2.8 aperture,  same length, same weight

If you compare it with the 24mm F2.8 (code F051) & the 35mm F2.8 (code F053) on A7RIV you maybe a bit disappointed as performances are less stellar

But if you compare it with all other primes around 18-21mm , the Tamron is one of the best available in term of sharpness, on par with a few others  but at a fraction of the price and its very short focusing distance allow it to provide the best bokeh/background blur among all of them

If you only have a A7III with only 24Mpix and have already the  Tamron 17-28mm F2.8 you  won’t see a lot the difference in sharpness, if you have a 42Mpix or 61Mpix , the upgrade is worth it

Highly recommended for wide angle shooter

Pros

• Excellent sharpness on the entire frame as of F5.6
• Very good sharpness in the centre wide open
• Very good background blur and color rendition
• Very short focusing distance allowing to reach 1:2 magnification
• Very compact and light : discrete and going well for video on a Gimbal
• Moderate price
• Very Good resistance to flare
• Very low chromatic aberrations

Average

• Good build quality (but much lower than GM)
• Vignetting is moderate (to be corrected in a future Lightroom Lens profile)
• Corners are only good at F2.8 and require to be closed down to F5.6 to reach their max sharpness
• Fast and reliable AF in photo but sometimes hesitation in AFC in the corners

Cons

• Lot of distorsion (to be corrected in a future Lightroom Lens profile)
• Noisy AF in Video and not regular

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All links to the new Tamron FE primes:
Tamron 20mm  f/2.8 at Adorama, FocusCamera and BHphoto.
Tamron 24mm  f/2.8 at Adorama, FocusCamera and BHphoto.
Tamron 35mm  f/2.8 at Adorama, FocusCamera and BHphoto.