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.EOS M5 test data on DxO Mark E-M5 II test data on DxO Mark
Excellent handling in a compact, weather-sealed body; Crisp, roomy electronic viewfinder; Image quality that can take the fight to APS-C DSLRs; Unique high-res mode lets it bat far above its weight for static scenes; Extremely fast 10 fps burst shooting, Excellent video feature set
Control dials are easily bumped; Below-average battery life at default settings; Buffer depths for raw shooters are a bit limited; 16-megapixel sensor resolution feels dated; Weak bundled flash strobe
Enthusiast-oriented compact camera body; Good image quality in most situations; Built-in electronic viewfinder; Impressive Dual Pixel CMOS AF; Very good overall performance.
High ISO image quality and dynamic range aren't quite up to par with some of its competition; No 4K video recording; Native EF-M lens selection is still limited; Below average battery life.