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.D4S test data on DxO Mark S (Typ 006) test data on DxO Mark
Excellent, tank-like build quality; First-rate ergonomics a pro would expect; Superb performance with confident autofocus; Great image quality; Big, bright viewfinder with 100% coverage; Excellent battery life; Doubles as a powerful video capture tool
Fairly minor upgrade over the D4; Image quality improvements apply only to JPEG; XQD card format isn't broadly accepted; Consumer-friendly default JPEG processing may turn off some pros; Body is large and hefty (but that's normal for pro gear)