Large Full frame (35.6 x 23.8 mm) sensor Large sensors provide shallower depth of field and a nice blurry background for your portrait shots.
Very High Resolution Sensor: 24.0MP Sony A9's 24.0MP sensor will provide larger room for cropping, and you will be able print your portrait shots larger with more fine detail.
Image Stabilization Image stabilization reduces the effects of camera shake at long exposures and long focal lengths thus ensuring sharper portraits
Electronic Built-in Viewfinder During a portrait shoot, using a viewfinder allows the photographer to stabilize the camera and reduce the camera shake since it is closer to the body. Besides, it comes handy where LCD screens are very hard to read under bright sunlight.
Good Ergonomics&Handling Good handling and existence of well placed physical controls on the camera body is very important during portrait shooting. It is faster and more intuitive to change the camera settings with dedicated buttons and knobs instead delving into the menus on the LCD.