The Sony RX10 line combines the higher resolution of a 1" sensor with a much bigger zoom than the small RX100 line can muster. That just expanded even more with the Sony RX10 Mark III. With the previous camera offering about an 8x zoom, the latest version offers a huge 25x zoom. While there are super zoom cameras with bigger zooms, the RX10 offers a larger 1" sensor that makes that big 25x zoom much harder to attain.
Besides the zoom, there isn't much else different about the camera from the RX10 II, but the big zoom increase is plenty of reason for an update. The camera still sports a 20.2 megapixel 1" stacked sensor, which is much larger than what's inside your typical super zoom, but smaller than a DSLRs. Burst speed is an excellent 14 fps, making the camera a good option for sports for those who don't want to get into a DSLR system. Despite expanding the zoom reach, the camera can still focus as close as three centimeters from the front of the lens in macro mode at the widest angle. The Sony RX10 III also has optical image stabilization and 4K video, like its predecessor.
The lens update does change a few other specifications though. The maximum aperture of the new lens is f/2.4-4, where the previous version had a constant f/2.8 capability. While the aperture isn't as good, it's very good for a lens with that kind of reach. The expanded lens also comes with an electronic shutter, which expands the traditional shutter's 1/2000 speed to 1/3200. The larger lens also adds almost a pound to the camera, so there is a noticeable weight difference between the two.
The Sony RX10 III, based on our previous experience with the line, should produce excellent images, and that bigger zoom will be a big advantage. But, I'm a bit hung up on the price (just like with the previous models). The Sony RX10 III costs $1,499. You can get a nice DSLR with that kind of change, though you can't get both a DSLR and 600mm zoom lens for that price. Still, the 4K capable 16x zoom Panasonic FZ1000 is half the price and while there is less zoom, is that really worth another $800?