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.5D Mark IV test data on DxO Mark 80D test data on DxO Mark
Very good image quality; Better RAW high ISO performance than predecessor; Improved buffer depth; Very fast AF; Dual Pixel CMOS AF is excellent; f/8 AF support; 1080/60p video; Headphone jack; Very good battery life.
New kit lens isn't very sharp; No 4K video; No clean HDMI out; 29:59 minute video recording limit; Phase detect AF not as sensitive in low-light as expected (Dual Pixel AF better in low-light).
Superb image quality from RAW files; Improved dynamic range; Excellent high ISO performance; Fast 7fps burst rate with unlimited JPEG buffer; High-quality cinema 4K video; Built-in Wi-Fi; Dual Pixel CMOS AF is very good.
JPEG files look soft at default settings; 1/200s flash sync; 4K video codec is cumbersome; 29:59 continuous video recording limit.