One of the challenges faced by serious videographers/cinematographers is the ability to "land" the camera's focus ring on the right spot when shifting focus between two onscreen objects. If you're shifting between a person in the background and a flower in the foreground, for instance, it can often take several tries before getting a take where you don't focus right past the flower, or overcompensate by slowly creeping up to it. Professionals use a device called a follow focus to avoid this problem, but they can often be prohibitively expensive for amateurs and low-budget film-makers. Fortunately, however, those people now have an alternative - the DSLR Follow Focus.
Made by Idaho company DSLR Solutions, the device (as its name implies) is designed for use on digital SLRs.
Unlike electronic systems such as the wire-attached Okii or the Bluetooth Redrock, the DSLR device attaches straight to the lens, and is operated by hand. Essentially just a steel circular spring with a handle, it simply grips the focus ring's finger grooves, allowing the ring to be turned by raising or lowering the handle. This does introduce a "human element" to its use, as shakes or hesitations in the videographer or focus-puller's hand will be transmitted directly into the camera.
It's still claimed to be better than simply grabbing the focus ring directly with one's fingers, however, for a few reasons.
For one, it places the hand in a more ergonomically-friendly position, in which smooth movement should be easier. The long handle also increases the physical distance that the hand is required to move between focus points, so users won't be struggling to modulate their finger movements when twisting the focus ring just a few millimeters.
Additionally, stop and start points can be placed on the lens. This is done by applying the supplied focus marker strap adjacent to the focus ring, and clipping the protruding focus arrow onto the Follow Focus. Raised markers are then placed on the marker strap, in line with the user-determined start and stop focus points on the ring. When users proceed to move the focus ring via the Follow Focus, its range of movement will be limited by the focus arrow running into the two focus markers. This means that a videographer could simply move it to one marker, start recording, then gently pull the Follow Focus' handle until they feel it lightly touch the other marker - no going past the final focus point, or wondering if they haven't reached it yet.
The DSLR Follow Focus is available on the DSLR Solutions website, and comes in two sizes, to fit different types of lenses. The original and large size each cost US$59.95, or both can be purchased for $89.95.
The video below explains more about how the device is used.