From GSM Arena:
The telephoto itself uses a 12MP 1/2.9" Sony sensor with f/2.3 or f/2.8 aperture depending on the focal length you use and optical image stabilization. Digital zoom can now reach the equivalent of 300mm, compared to 200mm on the Xperia 1 II and 5 II and Sony calls it AI super resolution zoom. The main camera uses the good old 12MP 1/1.7" sensor coupled with an f/1.7 OIS lens while the ultrawide snapper has a 12MP 1/2.6" imager, f/2.2 aperture and autofocus, which is not that common on ultrawide units even among flagships.
For optimal performance on those precious cameras, Sony has put ZEISS lenses with T coating. Additional software features include real-time Eye AF on top of the already existing Eye AF that works on animals and humans, Optical SteadyShot and 4K recording at 120fps for those crisp slow motion takes.
With the cameras mostly identical it's the screens that set the Xperia 1 III and Xperia 5 III apart. The larger Xperia 1 III uses a 6.5-inch 4K OLED panel with a cinematic 21:9 aspect ratio and is capable of running at 120Hz and displaying HDR content. The Xperia 5 III also moves to 120Hz refresh rate, but on a smaller 6.1-inch OLED panel of Full HD resolution with the same 21:9 aspect ratio and again complete with HDR support.
Both phones are powered by the flagship Snapdragon 888 chipset with the Xperia 1 III having 12GB of RAM and 256GB or 512GB storage. The Xperia 5 III gets the slightly more conservative 8GB/128GB and 8GB/256GB versions. Both phones have hybrid dual SIM slots so you can easily expand the storage through a microSD card if you need to.
Sony says both phones will be available at the start of 'summer 2021', with pricing to be confirmed later on. The biggest miss for the smaller Xperia 5 III is again the absence of Qi (wireless) charging - this is now table stakes in the smartphone world so I've no idea why Sony has omitted this yet again.
Watch this space for coverage and comparisons.
PS. I've added some of the 1 III and 5 III promo videos below, plus - at the bottom - the full announcement keynote, for your convenience: