The Sony a7 was a landmark camera, bringing a full frame sensor to the mirrorless arena for the first time, though a slow burst speed kept it from joining the ranks of sports, wildlife and action cameras. That's changing with the Sony a9, a full frame mirrorless camera boasting a 20 fps burst speed and 693 autofocus points.
The a9 brings Sony's stacked sensor of the RX100 V to full frame for the first time. The stacked design isn't to enhance image quality, but to enhance speed by dedicating an entire layer of the sensor to memory and another to processing. Paired with the BIONZ X processor, that brings the a9's speed to an incredible 20 fps burst. To achieve that speed though, the sensor doesn't have quite the resolution as the a7R II's 42.2 megapixels, however, dropping back down to 24.2 in order to spit out files at such a fast rate. Sony has paired that fast speed with a 693 point autofocus system designed to draw in action photographers.
On the outside, the a9 carries over much of the design of the a7 with a few notable exceptions. The body is fairly compact considering the full frame sensor inside, but Sony has managed to fit two SD card slots inside, as well as a battery with twice the life of the a7 series. The 480 shots on one charge isn't close to a DSLR, but brings up one of the a7's weaknesses to something much easier to work with. The electronic viewfinder has also been enhanced -- during those 20 fps bursts, the screen doesn't black out while an image is being taken, allowing you to follow the action.
The Sony a9 is a technological feat and a landmark camera that boasts several firsts. While the camera costs $4,500, that's still less than Nikon and Canon's fastest shooters, the D5 and EOS-1DX Mark II. The camera is designed for capturing action and motion and a camera that should be on the shortlist for sports, wildlife and even event photographers. When it comes to resolution though, the a9 doesn't have the same level as the a7R II, which might still be the better choice for subjects that don't require much speed, such as product photography. While there's a few negatives like the still low but improved battery life, the a9 marks a big step forward for camera technology. Like with any interchangeable lens system, consider Sony's compatible lens options before you buy -- while the selection is certainly growing, the lens options are often less diverse than DSLR brands.