Dot Line's Quick Focus Handle, in black, is a very affordable alternative to a follow-focus. It easily wraps around your lens and provides a neat way of adjusting focus or zoom when you're shooting. In fact, you can wrap one around the focus ring and one around the zoom ring, if your lens is big enough to support both. And they come in black and red, so you can have one of each attached to differentiate between focus and zoom.
The strap is flexible and features a knurled interior that securely grabs the lens ring. It's adjustable from 70mm to 89mm in diameter and compatible with most DSLR lenses.
When you're shooting on a tripod, and you try to adjust focus with just your fingers, you'll at least slightly budge the camera, but with this handle, you can just gently push the tip up or down and highly reduce camera movement.