Often we hear the term “Infinity Focus” being used while shooting landscapes or during astro photography. What it actually means is, the focus plane to be such that maximum light rays which focus on the sensor shall be in parallel plane.
While shooting landscape photos, we focus at a plane, where the infinity of the lens is within the hyperfocal distance; but while shooting in astro photography, the focus is at infinity. This post elaborates more on the point.
In practice, sometimes, due to ambient conditions, the camera may not be able to focus correctly and give results of the following kind.
This is where the infinity focussing is useful. Since the object is far away from the lens, focussing in a manner where the subject is in the hyperfocal infinity, yields a better result.
The infinity markings on the lens are found in the following ways
Go out there and shoot beautiful landscapes and amazing star trails and milky ways and never worry about soft focus any more.
Last time, I was for ISO.