Sunwayfoto's DRH-60 Focusing Handle adjusts and safely locks onto the focusing rings of lenses with 58 - 66mm diameters to smoothly adjust focus. Diameter is the measurement of the distance across a circle. In cases that measuring the diameter is difficult, an alternative is measuring the circumference of the circle and then converting that measurement to the diameter equivalence. Circumference is the distance around a circle. Its conversion to diameter is its value divided by pi, by 3.142.