Focus Lock
Holding the current focus distance so that further movement of the camera or subject does not trigger refocusing.
In depth
Locking is essential wherever the camera would otherwise choose wrongly — shooting through glass, foliage or a crowd, or composing with the subject far from any focus point. It can be achieved by holding the shutter halfway in single-servo mode, by releasing a back button, or by a dedicated AF-lock control.
Examples in practice
Photographing an animal through a fence, focus is locked on a clear gap and then held while the composition is adjusted.