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.A7R II test data on DxO Mark SL (Typ 601) test data on DxO Mark
Superb image quality; Very high resolution; Surprisingly good high ISO performance; Fast autofocus; Compact, comfortable body with lots of customization potential; Bright, roomy and clear viewfinder; Tilting LCD display; Five-axis stabilization; Intuitive Wi-Fi / NFC connectivity
Movie button is poorly located; No touch screen; Not as fast to start up or shoot photos as a similarly-priced SLR; Rather slow buffer clearing; No built-in flash; Single card slot; Not weather-sealed to the same degree as some rivals
Superb build quality; Amazing EVF; Great image quality from RAW files; Good high ISO performance; Very good dynamic range; Very low shutter lag; Fast AF on stationary objects; Class-leading ~11 fps burst mode; 4K DCI and UHD video; Full HD up to 120p. Built-in Wi-Fi and GPS; Dual card slots; Decent battery life for its class.
Big for a full-frame mirrorless camera; Very slow buffer clearing; Contrast-detect AF system struggles with moving subjects; In-camera JPEGs are muted and dull; Very limited native lens selection (but supports many other Leica lenses via adapters); Expensive.