What is a st op?

A lot of references are made to ‘stops’ with ND grads and NDs. A ‘stop’ equates to a change in either the shutter speed, aperture or ISO, and denotes halving or doubling of light levels. If you halve the shutter speed (going from 1/60 to 1/30 sec, for example), that equates to a stop. Most D-SLRs will allow smaller increments, so you may need to move the dial two or three ‘clicks’ before achieving a full stop.