Tuesday, June 27, 2006

Depth of Field

Today's Topic: The Basics

Disclaimer: This post is based on a true story. Names have been changed to protect the innocent.

Last year, the nameless church that I attend showed a video. Though the video was professional in many ways, one part of it was like the visual equivalent of fingernails on a chalkboard for me.

The video included a series of interviews in different locations. The settings were somewhat visually interesting and also somewhat indicative of the people being interviewed. However, in each case, the background was in focus. And the interviewees were all out of focus.

Now, to be fair, I'll bet most people didn't even notice, since "out of focus" has gradients and these were only slightly out of focus. Somebody had focused on the background and forgotten the basic principles of focusing and depth of field.

I'm going to give a quick overview of depth of field here that hopefully will introduce you to the concept. If you would like more information, applications, and the mathematics behind depth of field, check out the wikipedia entry.

Depth of field in a camera image is the distance in front of and behind a subject which are in acceptable focus. In actuality, there is only one plane in front of a lens where the subject will be in perfectly sharp focus, but the sharpness of the focus gradually goes down as you move closer to the lens or further from it. That's why I refer to the depth of field as the range of acceptable focus.

What affects depth of field? What control over it do we have on our cameras? Unfortunately, there aren't a lot of options for adjusting depth of field. Basically, you have two options: lens focal length and lens aperture.

Lens Focal Length. Lens focal length is essentially how "zoomed in" or "zoomed out" your camera is. Focal length affects depth of field in an inverse way, meaning that as you increase your focal length (zoom in), you decrease your depth of field, making more of the foreground and background out of focus. As you decrease your focal length (zoom out), you increase your depth of field, making more of the fore- and backgrounds in focus.

How would this knowledge have helped our videographers in the earlier situation? They could have used depth of field cues to make sure they were truly in focus on their subject. How? If they had zoomed in on their subject, the depth of field would decrease. More of the "foreground" and "background" would have been out of focus and they could see exactly what was actually in focus. Then they could have adjusted their focus to pull the person into sharp focus rather than the books or picture on the wall.

Lens Aperture. Aperture is the setting of the camera's lens "iris" which determines how much light is allowed into the camera. If you increase the aperture, the picture will get brighter. If you decrease the aperture, the picture will get darker. Similar to focal length, lens aperture affects depth of field in an inverse way. If you increase the aperture, you decrease the depth of field and vice versa.

When you are taking a picture or shooting a video, however, you need to assure that the image is exposed properly by setting the light entry amount with the aperture to the best level for your film. You can't just decrease the aperture (make the picture darker) to increase the depth of field.

Therefore, in order to use aperture to adjust depth of field, you actually have to change the lighting. Turn more lights on to decrease the aperture and then increase the depth of field. Or turn the lights down or off so that you have to increase the aperture and decrease the depth of field. Had our example videographers had more lighting for their interviews, they may have had in-focus subjects because the depth of field would have been such that the subject would have been in acceptable focus.

Don't let depth of field problems distract from the depth of your message.


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