You ought to be in the movies: Achieving a film look with selective focus aka Shallow Depth of Field
When everything is in sharp focus, no one thing stands out. And that’s no way to tell a story. Selective focus is just the opposite. It allows the videographer to guide the viewer to a specific place or person on screen by placing everything else, usually the background, out of focus. This article will discuss how selective focus, depth of field and focal length work together in film – and using new technology, can work in video as well.
The selective focus technique is also popular in still photography, particularly fashion, portraits and sports. A telephoto lens, with shallow depth of field, puts the subject in dramatically sharp focus. Everything else becomes a blur, or “Circle of Confusion.”
Feature films use this approach too. 35mm cinema lenses (wide angles excepted) can isolate the subject against a background, drawing the viewer right in, letting the subject stand out in a busy crowd.
Film budgets versus video budgets
Most of us are familiar with the relative cost of film versus video. With video vastly more economical (and easier to use than film), it was inevitable that the movie industry would turn to video to cut costs. The so-called low budget film is not usually a film, it’s actually video. And one of the major differences is that with video, virtually everything on screen seems in focus Depth of Field is so deep, that nothing stands out.
Depth of Field
In practice, the Depth of Field (DOF), or area of acceptable focus, varies. It’s governed by the focal length and aperture of the taking lens. 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 considerations regarding Depth of Field and focal length:
- The longer the focal length of the lens, the shorter the Depth of Field. And vice versa: 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.
- 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.
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 full-sized 2/3″ CCD broadcast cameras, and it’s especially true in the 1/3″ camcorders popular with many indie filmmakers.
One way to achieve narrow Depth of Field in video is to zoom in as far as you can and, if necessary, add neutral density filters to get the aperture as wide open as possible. It can work for closeups, but the background is still not sufficiently out of focus 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. It’s not ideal for achieving shallow DOF.
Solving the Depth-of-Field Challenge with 35mm Lenses
Unless you’re working with a seriously large budget that will support the cost of a film crew, film stock and film processing, how do you get around the selective focus issue?
One of the most popular solutions is to use a lens made for a 35mm camera (motion picture or still) and project the image onto a ground glass. The image is then captured by the video camera’s lens and recorded. This combination results in the selective focus usually only available on 35mm film and the cost effectiveness of shooting on video.
The new crop of relatively inexpensive HD and HDV cameras suffer from the same long DOF as their SD counterparts because of their relatively small CCDs. However, the 35mm image projected onto the ground glass solution works for them as well.
An Unexpected Additional Benefit
The images created in this way are a bit softer than images captured directly through the camera’s lens. Many filmmakers find this to be beneficial because it saves them from having to add a pro mist or other diffusion filter to eliminate the video edge that is associated with many electronically captured images.
The video below demonstrates the difference between 35mm lenses and regular video to achieve selective depth of field. The first segment shows a young man shot on video with stock Fujinon video lens. The background is relatively sharp. The next sequences makes use of the Redrock M2 Encore cinema lens adapter with a variety of Nikon 35mm lenses set at wide apertures. The man in the foreground receives significantly more emphasis, with the background pleasantly blurred.