Auto ISO is a camera setting that lets the camera choose the ISO automatically while you keep control of aperture, shutter speed, or both. Instead of dialing sensitivity by hand every time the light changes, you set boundaries once and let the camera fill in the third leg of the exposure triangle. It is one of the most useful features for shooting in changing conditions, where stopping to adjust ISO manually would mean missing the moment.
The setting becomes powerful when you configure its limits. The two that matter most are the minimum shutter speed, which tells the camera how slow it is allowed to let the shutter speed drop before it starts raising ISO, and the maximum ISO ceiling, which caps how much sensitivity the camera will add before it lets the shutter slow further. Set the minimum shutter speed fast enough to freeze your subject or beat camera shake, and set the ceiling at the highest ISO whose noise you still find acceptable on your sensor.
Many cameras offer an Auto setting for the minimum shutter speed that ties it to focal length, following the reciprocal rule so that the threshold becomes faster as you zoom in and slower as you zoom out. Better bodies let you bias that Auto threshold faster or slower, which is the single most useful adjustment for action, since it forces the camera to hold a quicker shutter before it reaches for higher ISO.
Auto ISO behaves differently in each shooting mode. In aperture priority you pick the f-stop, the camera picks the shutter speed first, and only raises ISO once it hits your minimum shutter speed. In shutter priority you fix the shutter speed and the camera trades aperture and ISO. In manual mode with Auto ISO active, you set both aperture and shutter speed and the camera floats ISO to hold a correct exposure, which gives full creative control while still adapting to light.
A common point of confusion is exposure adjustment. Even with Auto ISO running, exposure compensation still works on most cameras, nudging the metered target brighter or darker by shifting whichever value is being automated. This means you keep the ability to deliberately overexpose or underexpose while the camera handles the routine work, although on some bodies the compensation control is mapped to a different dial when Auto ISO is engaged.
Auto ISO suits event, wedding, street, and wildlife photography, where light shifts as you move between sun and shade or follow a subject indoors and out. The trade-off is that the camera may push sensitivity higher than you would choose in a tight spot, so a sensible ceiling and an awareness of your camera’s high-ISO behavior matter. Photographers who shoot static scenes from a tripod, by contrast, usually turn Auto ISO off and select the lowest native ISO for maximum image quality.