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.K-3 II test data on DxO Mark D90 test data on DxO Mark
Top-notch image quality; Spectacular detail from Pixel Shift Resolution; Better performance than K-3; Geotag images with no accessories; Great ergonomics in a compact body; Clever on-demand low-pass filtering; Broad sensitivity range; Large and bright viewfinder for an APS-C camera; Dual card slots
No built-in flash strobe; No wireless flash support out of the box; AstroTracer function can be finicky; AE Lock button is poorly located; Below-average battery life for a prosumer DSLR; Requires a different battery grip accessory than its predecessors