Maximum effective ISO is an estimate of the highest sensitivity at which a camera can capture excellent quality photos.
Cameras with higher effective ISO will be better choices for indoor photography, night shooting, and indoor sports photography, especially if you intend to make large prints.
You can learn more at our glossary entry.
Maximum effective ISO test data courtesy of DxO Mark.A99 II test data on DxO Mark S (Typ 006) test data on DxO Mark
Superb image quality at low, moderate ISOs; Improved high ISO performance; Fast continuous shooting speeds; Very impressive hybrid autofocus; High-quality 4K UHD video; Rugged camera body.
Multi-selector joystick can be awkward to use; No EVF live view at fastest burst rate; Buffer clearing is slow with no UHS-II support; 4K UHD video isn't as full-featured as E-mount counterparts.