Video and the Death of Focus

This sounds simple and for a hundred years it was. Then we started making movies with video. Suddenly, nearly everything on the screen seemed to be in focus. The viewers were free to let their eyes wander all over the screen and become distracted by every little thing. Sometimes at the cost of losing the storyline.

Why is this happening? As in any photographic image capture, there is only one subject-to-lens distance that can be in focus at any time. While it is true that there is only one plane of focus on any given image, the Dept of Field (DOF), or area of acceptable focus, will vary. The focal length and the aperture of the taking lens have a lot to do with DOF. That's because both the focal length and the aperture affect the Circles of Confusion. Circles of Confusion are formed when light passes through a lens. The smaller the Circle of Confusion, the sharper, or more in focus the image appears. The iris opening is directly proportional to the size of the circles of confusion. So, a larger aperture will result in larger circles of confusion. The focal length of a lens comes into play because of the way it magnifies the image. The more magnification, the larger the circles of confusion. Here are some important considerations regarding Depth of Field and focal length:

• The longer the focal length of the lens the shorter the Depth of Field. The transverse is also true.**
• The shorter the focal length, or the wider the lens, the longer the area of acceptable focus. The other factor that comes into play is the iris setting.**
• The larger the aperture the shorter the DOF. As the iris is closed down the range of focus appears to grow.

There is another important element that needs to be factored in. The size of the recording media, in film, or imaging device, in video, makes a huge difference in the range of acceptable focus. The larger the physical size of the image produced, the shorter the DOF. So, when an image is recorded on a frame of 35mm film, it has a shorter (more shallow) DOF for any given lens magnification/aperture combination than an image recorded onto a frame of Super8 film, or video.

Since the imaging devices, or chips, in a video camera are considerably smaller than a frame of 35mm film, the DOF is increased. This is true in the full sized 2/3" CCD broadcast cameras. It is especially true in the 1/3" camcorders that have become so popular with many indie filmmakers. The conventional way of dealing with the extremely long Depth of Field IN VIDEO is to zoom in as far as you can and add neutral density filters, if necessary, to get the aperture as wide open as possible. This approach is somewhat effective for closeups, but the background is sill not out of focus enough for many filmmakers in a wide range of situations. Also zooming in all the way removes a lot of the freedom and flexibility of a varying length focal lens. In short, it is not the ideal solution for achieving shallow DOF.

**The complete and technically correct information is that longer focal length lenses have shorter depth of field than shorter focal length lenses if they are the same distance from the subject.