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.EOS M100 test data on DxO Mark G3X test data on DxO Mark
Good overall optical performance from the 24-600mm eq. lens; Very good image quality and high ISO performance for this class, especially with RAW; Lots of external controls; Comfortable grip; Shoots 1080/60p video; Includes both mic and headphone jack, Focus peaking for video; Clean HDMI.
No built-in EVF; Mediocre AF speeds; Sluggish burst rate with RAW/RAW+JPG and with servo AF; Below average battery life; No 4K video unlike competitors.