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.D5500 test data on DxO Mark K-5 test data on DxO Mark
Excellent high ISO performance, one of the best APS-C sensors yet; 6.5 fps burst mode has very good buffer depths; Weather, dust, and cold resistant; In-body Shake Reduction corrects not only horizontal / vertical, but also rotational motion.
Default saturation and contrast a bit high; Auto white balance too warm in tungsten light; Fewer AF points than main competitors; Only a single card slot, and no UHS-I card support (slow buffer clearing).
Amazingly small for a DSLR; Great image quality for its class, including superb low light and high ISO performance; Generous JPEG buffer depth; High resolution.
Buffer depths are shallow when shooting RAW; Lacks GPS from the predecessor; No AA filter means it's susceptible to aliasing artifacts.