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.5DS R test data on DxO Mark K-3 II test data on DxO Mark
Phenomenal resolution and sharpness; Fine Detail Picture Style is like in-camera Unsharp Mask; Excellent print quality; Decent high ISO performance; Improved normalized dynamic range compared to 5D Mark III; Decent burst speed with good buffer depths; Built-in intervalometer & timelapse movie mode.
AF speed tested closer to consumer-level DSLRs; Dynamic range lower than competing models from Nikon & Sony; Lack of OLPF means subject to more aliasing artifacts with some subjects; No 60p video frame rate; No headphone jack; Timelapse video is only 1080p.
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