Infinity focus is the focus setting at which objects effectively infinitely far away, such as distant mountains, the horizon, or the stars, render sharp. On a lens focus scale it is marked with the infinity symbol, and at this setting parallel rays of light from a distant point converge precisely on the sensor.
In ordinary daylight, achieving infinity focus is trivial, since autofocus locks onto a distant subject instantly. The concept becomes important when autofocus fails, most notably in astrophotography and dark landscape scenes, where there is too little light or contrast for the autofocus system to find anything to grab, forcing a switch to manual focus.
A common trap is assuming you can simply turn the lens to the infinity mark and trust it. Most modern autofocus lenses are designed to focus slightly past infinity, partly to guarantee they can reach it across temperature changes and manufacturing tolerances, so the hard stop and even the printed mark do not correspond exactly to true infinity. Setting the lens to the stop will often produce soft stars, and the focus point can even shift as the lens cools through a cold night.
The reliable method is to use the camera’s magnified live view, point at a bright star or distant light, and turn the focus ring until that point becomes as small and sharp as possible. Once set, many photographers tape the focus ring in place so it does not drift during a long session, and avoid switching back to autofocus, which would undo the careful setting.
A lens with a long focus throw makes this fine adjustment easier, since a small turn moves focus only a little, while a short throw makes precise infinity hard to nail. Some photographers prefocus at home on a distant object in daylight and mark the exact infinity position with a piece of tape for use later in the dark.
Infinity focus connects to depth of field through the idea of hyperfocal distance. Focusing at the hyperfocal distance rather than at infinity keeps the far scene acceptably sharp while extending sharpness much closer to the camera, which is why landscape photographers often focus short of infinity to get both a near foreground and distant peaks sharp in a single frame.
The reassuring news is that once you have found and marked true infinity for a given lens, it does not move from shot to shot, only slowly with large temperature swings. Confirm it on the first frame of a night session by magnifying a bright star in playback, and you can shoot the rest of the night with confidence. Live view magnification is far more trustworthy than the distance scale here, and a strip of gaffer tape on the focus ring keeps the setting from drifting as you work.