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.5D Mark IV test data on DxO Mark D90 test data on DxO Mark
Superb image quality from RAW files; Improved dynamic range; Excellent high ISO performance; Fast 7fps burst rate with unlimited JPEG buffer; High-quality cinema 4K video; Built-in Wi-Fi; Dual Pixel CMOS AF is very good.
JPEG files look soft at default settings; 1/200s flash sync; 4K video codec is cumbersome; 29:59 continuous video recording limit.