DSLR Video – Learning from Focus and Depth of Field Mistakes at Monday Night Acoustic Jam
September 15, 2011 Leave a Comment
I like to go back and watch videos I’ve made. I love Doug and Mike’s singing on this one, but my camera work is embarrassingly bad, particularly the focus. (I’ll cut myself some slack because this was my first time trying to film live music.) Part of the problem was that I was naively using autofocus, which doesn’t work well with video on DSLRs in poor light, but that isn’t what I want to talk about right now.
My big mistake was that I was using a 70-200mm F/2.8 telephoto lens at 12 feet. With long focal lengths, wide apertures, and short distances you don’t get much depth of field. It’s just the laws of optics.
(If you aren’t familiar with depth of field, here’s a quick example. Imagine you’re on the stage of a theater looking out at 100 rows of seats. You focus your camera on a person in the 50th row. Obviously that person will be in focus, but how many rows in front of or behind them will be in focus? That’s what depth of field calculations determine. You get more depth of field with shorter focal lengths, smaller apertures (bigger F numbers) and greater distances.)
Here are the depth of field values for a few focal lengths from the DoF calculator at DOFMaster. The other variables I plugged into the calculator were an aperture of F/2.8, a distance of 12 feet, and a Nikon D7000 as the camera. Those were the variables in the video above, except that I think I was using autoexposure and the camera was (thankfully) using a smaller aperture). Notice how quickly depth of field (the part of the picture that’s in focus) falls off a cliff as the focal length increases, from 10 feet to a little over an inch.
|Lens Focal Length||Depth of Field|
Let’s say I was using that 70-200mm lens at 100mm. The DoF would have been just six inches. But wait, it’s worse than that. DoF doesn’t start at the point of focus. It’s centered at the point of focus. Roughly half of the DoF is in front of the point of focus and half is behind.
If I focused on a person’s nose at those settings I would have 3 inches of depth of field in front of his nose (in thin air, basically) and three inches behind his nose. That would barely reach far enough back to get his eye in focus. With the way people move when they sing and play that’s a hopeless situation for trying to maintain focus in a video.
There’s a reason 24mm and 35mm lenses were so popular with photojournalists and street photographers before the invention of autofocus. Those wideangles give plenty of room for error when you’re manually focusing in a hurry.
Needless to say, DoF is even more critical for video. With still pictures you only have to maintain focus for a small fraction of a second. With video you have to maintain focus for seconds and minutes while people move around.
My favorite lens for video is now a Nikon 35mm F/1.8 that I focus manually. It has a big aperture for low light. For shows in low light I often stop it down from F/1.8 to F/2.8 to get a usable combination of light-gathering ability and depth of field – about 4 feet DoF at 12 feet distance. When there’s more light I can stop down to F/4 and get 6 feet of DoF, or 9 feet at F/5.6.