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.D5 test data on DxO Mark S (Typ 006) test data on DxO Mark
Excellent image quality; Excellent high ISO performance; Fast 153-point autofocus system; Fast continuous shooting speeds; 4K video recording; Pro-quality camera body; Outstanding battery life.
Large and heavy body might not be right for all; Loud shutter; Underutilized touchscreen; Low ISO dynamic range not as good as predecessor; Extended high ISOs not very useful; Sluggish Live View AF.