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 M test data on DxO Mark GM5 test data on DxO Mark
Excellent image quality similar to Rebel T4i, T5i and SL1 DSLRs, with 18-megapixel APS-C-type sensor delivering considerable resolution for a mirrorless camera; Solid build and sleek design; Bright, high-resolution 3-inch LCD touchscreen monitor; Full 1080p HD video recording that's virtually silent with an STM lens.
Barely acceptable autofocus speed still lags far behind most mirrorless cameras despite firmware update fix; Limited physical controls and buttons, including no Program, Priority, or Manual on Mode dial; Lacks built-in flash or electronic viewfinder option; Only two compact EF-M mount lenses currently available.
Great image quality for a tiny camera; Excellent high ISO for its class; Built-in EVF; Built-in hot-shoe; Improved ergonomics; Improved image quality for long exposures; Improved burst rate with hybrid shutter; 1080/60p video.
Slightly bigger than GM1 (but not by much); No built-in flash; Shallow buffers with RAW files; Dynamic range not quite as good as some competing models; Below average battery life.