How to Focus While Shooting Video With a DSLR
Focusing versatility is another area in which video-capable DSLRs have changed the game for cinematographers. It's a whole new experience for both professional videographers and consumers: For years, cinematographers have had access to really fast glass that's incredibly expensive, while the consumer video market has been stuck with slow, cheap, fixed-lens cameras with digital zooms and autofocus-only controls. With video-capable DSLRs, that high-quality glass is more affordable for pro shooters, and casual users have more freedom to experiment with depth-of-field effects and manual focusing.
You'll find cameras out there that will autofocus while in movie mode, but the vast majority of them don't. To keep an object in focus that's moving toward or away from the lens, you'll need to get skilled at "pulling focus," or adjusting the focus distance of the lens while you're filming so that the subject is always sharp.
And that's no easy feat. Professional focus pullers spend years learning the craft of getting a shot in focus; on a film shoot, it is one of the most technically demanding positions on the crew. DSLR owners, however, are expected to get the camera out of the box and start shooting with everything in focus straight away. You'll need quite a bit of patience, and these focusing tips should help.
1. Use a loupe.
You can't use a DSLR's optical viewfinder while shooting video, because the mirror box necessary for the optical view is flipped upward while you're filming video. That means you need to use the Live View feed on the camera's LCD as a viewfinder. A loupe is an eyepiece attached to a hood that lets you get an eye-level display of what you're filming; many available loupes will attach to the back of a DSLR's LCD viewfinder. Varying in price from $50 to $500, they magnify your screen and shield it from the sun so that you can clearly see the image.
2. Use a smaller aperture when you can.
With some lenses having apertures as wide as f/1.2 (and even faster), the focal plane is so small that you can literally focus on specific parts of a hair. Unless you're looking for an extremely shallow depth of field, this freedom of focus can turn a beautifully composed shot into an unusable, blurred mess. If you don't need the dramatic depth of field, and you're not shooting in low light, push the aperture up. The higher you go, the easier you will find the focus; it's much easier to get a well-focused shot at f/5.6 than it is at f/1.2. Stop it down!
3. Focus on the key point of interest.
Look at what you're filming, and focus on the most important part. If it's a person or an animal, the eyes are where it's at.
4. Find points of reference for your focus distance.
When you're framing the shot, look at the entire image and find static points that are about the same distance away from the subject you want to keep in focus. Before you start shooting, you can focus on these static objects to make sure that any moving subjects at the same distance are in focus as you start shooting. It makes life a lot easier. You can even pull a tape measure out and work back to your lens to have total certainty of the distance of the focus point.
5. Get a follow-focus system if you can afford it.
A follow-focus system is a small rig that wraps around the lens and lets you control focus by adjusting a knob instead of having to put your hand on the lens. These systems generally give you more-accurate, fine-tunable control over the focus, and there's less potential for jostling when your hand is removed from the lens itself. These rigs can be expensive--they vary in price from around $600 to $8000--but they are great tools for adjusting focus more comfortably.