Focal Length Module

The focal length is the measurement (in millimeters) from the optical center of the lens to the imaging plane (the film itself or, if you’re using a video camera, the CCD). Zooming is the process of changing the focal length, wherein zooming out shortens the focal length and zooming in lengthens the focal length. Remember that prime lenses have fixed focal lengths.

Most people equate zooming with getting closer or farther from the subject.  While this is one function of changing the focal length, there are a number of other qualities of the image that change, each of which are powerful tools in drawing your audience’s attention to what you want them to look at.

WHAT HAPPENS WHEN YOU CHANGE THE FOCAL LENGTH?

Shorter focal length (zoomed out) Longer focal length (zoomed in)

Farther from subject Closer to subject

Deeper depth of field Shallower depth of field

More, exaggerated depth Flatter image

Perception of faster time Perception of slower time

• Shot size: Changing the focal length has a significant impact on the look of your image. The most obvious is the difference in shot size. Longer focal lengths bring the audience closer to the subject, whereas shorter focal lengths give us a very wide angle. Believe it or not, the change in shot size isn’t the primary reason most DPs change the focal length of the lens.

• Depth of field: The depth of field is the zone in front of the lens in which objects are in focus. If a woman is standing in the depth of field, she’s in focus. If she’s standing outside the depth of field, she’s out of focus – and the farther outside the depth of field she stands, the more out of focus she is.







Changing the focal length, the exposure, and the distance of the subject from the lens affect the depth of field. Lengthening the focal length (zooming in), opening the iris, and moving the subjects close to the camera lens are three ways to create a shallow, or small, depth of field. Shortening the focal length (zooming out), closing the iris, and moving the subjects away from the camera lens are all ways to create a deep depth of field.







Whereas the focal length determines how big the depth of field is, the focus determines where the depth of field is. Moving the depth of field from one subject in the foreground to a subject in the background is called racking or pulling focus. The farther apart the subjects are and the shallower the depth of field, the more effective the technique.

• Depth: The third change in the image when shortening or lengthening the focal length is depth perception. A short lens (short focal length) will exaggerate the distance between objects in the foreground and background. Increasing the focal length will flatten the image and the viewer won’t be able to determine how far apart the foreground subject is from the background image.

• Time: One effect of using a long lens to flatten out a shot is the illusion of slowing time. If an actor were to start walking toward the camera from a distance of 200 feet, and you were to shoot him or her with a long lens, the resulting image would be so flat that it would appear as if the actor wasn’t moving forward at all. This creates the illusion of time slowing down, because the character doesn’t seem to be making any forward progress. Fight scenes, on the other hand, are usually shot with short lenses, because the increase in depth adds to the intensity and exaggerates the motion of the actors.