Depth of Field - DoF
By Bill Pryor

In video and film production it seems like everybody wants shallow depth of field. The more inexperienced the filmmaker, the more he desires shallow depth of field, or so it seems. Orson Welles worked hard to get the opposite. He wanted deep depth of field in many of his shots.

If you watch movies and pay attention to the individual shots, you'll notice that every shot does not have a shallow depth of field. Typically closeups do, and so do medium shots, though to a lesser degree. Normally, wide angle shots rarely blur the background much. Sometimes shallow depth of field is used to "pop" the subject, to separate it from the background. This is a useful technique if the background is "busy" or otherwise distracting. In shooting interviews, a blurry background can hide the fact that a person walking by looks at the camera.

Often in TV shows you see shallow depth of field being used as a transition between speakers. The typical shot is a fairly tight 2-shot, and the person closest to the lens will be talking; then they shift focus to the other person as he speaks. This is, I think, OK if you do it once. But on some network TV shows they go back and forth with the focus shift, and that is about as annoying as a shakeycam zoom.

Unfortunately for people who seek shallow depth of field, you can't violate the laws of physics. The main property that yields a shallow depth of field in photography is the size of the imager--ie., the chip or the negative. It's fairly common on some of the discussion boards to see people saying, "I want film type depth of field." Film has nothing to do with it. The size of the negative behind the shutter has everything to do with it. The bigger the negative, or chip, the shallower the depth of field.

The really high end HD cameras today, including the much-ballyhooed Red, have chips that are about the same size as a 35mm negative. Their main reason for doing that is not so much for shallow depth of field as it is for matching focal lengths of lenses. A 35 mm cine lens on a video camera than has a 35mm-size chip will have the same focal length as the same lens on a 35mm film camera. In other words, a 50 mm lens will act like a 50 mm lens.

On a 2/3" chip video camera, or a 16 mm film camera, you need a 20-something mm lens if you want the same focal length as a 50 mm lens in the bigger chip/negative world. In a 1/3" chip camera, a 7 or 7 mm lens gets you about the same effect as that 50 mm lens in the 35mm world. This is one reason most of the small "handycam" type cameras don't have wide enough lenses. To get the same field of view as a 2/3" chip camera with an 8.5 mm lens in a 1/3" chip camera, for example, you need about a 4.5 or wider lens. When you get down to 1/4" chips, it will take a wide angle lens adapter stuck onto the end of your camera's lens to get that wide. It would cost too much to make a decent lens wide enough to give you what most people think of as a wide angle on a 1/4" chip camera. Even the above-mentioned 4.5 mm lens is only about 32 mm in 35 mm lens terms--hardly a "real" wide angle.

So, the smaller the chips, the deeper your depth of field and the more difficult it is to make it shallow. The other factors involved in depth of field are the distance of subject from the lens, distance of subject from background, and aperture. If you want the shallowest depth of field you shoot tight and wide open and get the subject as far from the background as possible.

These three screen grabs from an interview shoot show what you can expect, regarding depth of field, out of a 1/3" chip camcorder (in this case a Canon XH A1. These are fairly low resolution screen grabs, not indicative of the camera's HDV quality. They're only to illustrate depth of field and lighting.

Bar Picture.

Bar 1 Close Up.

Geese in background are out of focus to make the subject the center of attention

The first one, Bar (it was shot in a bar), shows a very slightly soft background. I let the background go darker to help pop the subject, and because I liked that look. Instruments used were Lowel Caselight fluorescents with daylight lamps. The camera's 5600 preset was used, as well as an internal ND filter. The standard -3db setting was moved to zero db; otherwise the ND would have been too much.

One Caselight 4 was used for the subject but with only 2 of its lamps on. Fill was with a Caselight 2 backed way off and with one lamp on. The background was lighted by the indirect daylight coming in plus a third Caselight with one lamp on.

I like this level of background softness--not enough so you can't tell those are wine bottles, but soft enough so the foreground is nicely separated without the need for "Rembrandt" lighting, which often can look too staged and artificial in a documentary style film.

In the second photo, Bar 1, I zoomed in to a tighter shot, which automatically softened the background even more. Some people get confused over focal lengths and think a longer lens gives you a shallower depth of field. It doesn't if the image size remains the same. In other words, I used the zoom to get closer to the subject. I could have done the same thing by leaving the focal length the same and physically moving the whole camera and tripod closer until the subject reached the same size in the frame--and the depth of field would have been the same. When doing interviews, it's easier to zoom in or out during the pauses or questions than it is to move the camera. I don't use the zooms, but if I change the angle of view enough between the longer shot and the closer one, I can often make an action cut that works. In most cases there will be cutaways to cover the stops and starts in the interview, but it's nice to shoot it so you could use it with no cutaway if you really had! to.

The shallowness of depth of field in the Bar 1 photo is about all you'll get under average situations in an interview with a 1/3" chip camera, unless you go in for an ECU.

Take a look at the photo labeled Ducks. It's about the same image area as Bar 1, but is in very bright outdoor light, with heavy ND and stopped down to around a 5.6-8. This is the kind of situation where an external ND filter would be nice, but I didn't have one with me at the time. Still, the background is blurred enough to give a nice look to the shot. Why is that? Because it's far away. You can open your aperture or move away from the background to achieve a shallower depth of field, or move in closer to the subject. Or any combination of those three.

So if you want a shallow depth of field from a small chip camera, shoot tight, shoot with the background far away when possible, and, using a combination of lighting and ND filters, open the aperture as wide as you can.