For example, if you want to capture a particular field of view using a camcorder with a 2?3-inch image sensor, you might select a focal length of 50mm. If you swap out the camcorder with a Canon EOS 5D Mark II that has a much larger image sensor, you’ll need to change focal lengths in order to capture that same field of view—assuming that your subject distance remains the same. In the Canon example, you’ll need something close to a 270mm lens. And it’s this lens change that enters into the depth-of-field equation.
In addition to changes in lens focal length, with larger image sensors come larger (wider) apertures. On any given lens-camera setup, a wider aperture narrows the depth of field and a smaller aperture increases it.
It should be noted that the sensor-size differences among cameras can be dramatic. For example, a 2?3-inch image sensor (8.8×6.6mm) is smaller than 16mm film (10.26×7.49mm). And 35mm cinema film is sized around 21.95x16mm, while Super 35 is 24.89×18.66mm. Digital cinema cameras like the ARRI Alexa or the Red size up around the 35mm/Super 35mm range.
But SLRs come in various sizes. The 5D Mark II has the largest sensor of the bunch at 24x36mm, which surpasses even Super 35. This means it has the potential for more depth-of-field control. I say "potential" because it really depends on what lens apertures are available to you.
Smaller than the full-sized sensors are the APS-sized sensors from several manufacturers that range from 28.7x19mm to 22.2×14.8mm. Smaller still is the Four Thirds System image-sensor format used by Olympus and Panasonic (including the recently released Panasonic AG-AF100), which is sized at 17.3x13mm. The new Sony cameras like the F3 use a Super 35-sized sensor.
Unfortunately, depth of field can be a double-edged sword. If you use an extremely narrow depth of field, focus is even more critical. If you typically rely on deep depth of field in a 2/3-inch camcorder to handle minor focus misses, you may have developed bad habits—or may not have noticed a problem. You won’t get away with them with an SLR.
As an example, say you’re using an 85mm lens rated with a maximum aperture of ƒ/1.2 on a Canon EOS 5D Mark II and you’re eight feet from your subject. Since you want that narrow depth of field, you shoot wide open. The depth of field is only about two inches.
If your subject is moving, you’re going to struggle to keep things in focus. And by moving, I mean even slightly moving. Two inches is pretty narrow. An interview subject in a chair will move enough to have his or her eyes go in and out of focus.
Remember, however, that depth-of-field selection should be a creative choice. At the end of one of my presentations, a person walked up to me and went on about how sick and tired he was of 5D shots. At first, I didn’t understand what he meant by a "5D shot." Was it something in the compression that he could pick out? Was it the dynamic range that he noticed? Or was it some sort of artifact like rolling shutter? After a little conversation, I realized that he was reacting to "shallow depth of field, just for shallow depth of field’s sake."