Have you noticed a scene in a movie where the focus changes smoothly from the background to the foreground? Usually you'll see this effect used to change the point of interest from one character to another, or from a scenic view to a main character. Changing the focus is known as racking or pulling focus. In order to get that smooth change in focus filmmakers use what's known as a follow focus.
A follow focus is a control mechanism that allows you to easily make changes on the focus ring of your camera lens. It's important to realize that a follow focus doesn't change any of the functionality of your camera, it's an ergonomic modification that allows you to more smoothly move the focus ring. You can spend a lot of money on a high quality follow focus or you can try making one yourself for about the price of a sandwich.
In the video above, AndrÃ© Desroches takes a metal clamp, a nut and bolt, and a few other pieces you can find in a hardware store for a few dollars. Just by assembling them around the focus ring you get a lot more leverage and that helps you move that focus ring with more accuracy. It's worth noting the rubber band, just by placing this on the barrel of the lens and putting some small pointer, maybe a paper clip under the ring clamp, you can mark where your focus points are. This is really helpful when you're shooting and you need to move from one focus point to another accurately.
To get an idea of the type of shots you can get with a follow focus like this, see the example below by AndrÃ©.
Notice how easily he's able to change from a focused background to a focused foreground. That's a really useful technique when you're shooting your video to get the viewer to give their attention to specific section of your image frame.
There are plenty of other ways to make yourself a follow focus with just a few parts. Here's one by the folks at DVRebellion made from the plastic gears typically used in radio controlled helicopters.
With a bit of ingenuity and some spare parts you can make your own follow focus to give your videos a more cinematic look. Give it a shot!
[challenge: Make yourself a follow focus and post a video showing either how you made it or some footage you took using it, share with us!,url: http://vimeo.com/groups/74235]