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 M-P (Typ 240) 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.