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 NX1 test data on DxO Mark
Very good, high-resolution image quality; Excellent high ISO performance with RAW files; Very good dynamic range; Blazing fast 15fps full-res burst mode; Deep buffer; Great build quality & comfortable ergonomics; Lots of physical controls; Great battery life; Fast single-shot AF.
Continuous AF performance still not as robust as competing DSLRs; Aggressive JPEG noise reduction at high ISOs; JPEG colors and saturation less pleasing at higher ISOs; H.265 video codec can be difficult to work with.
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.