Creating a shallow depth of field
by, 02-07-2009 at 09:30 AM (7239 Views)
It's relatively easy to achieve the fabled film look. We've featured various post production tips to get the look and feel of a Hollywood production, but it's not just film treatment that gets results. Long before hitting post production, camera technique goes a long way to giving your footage a film look. So rather than rely on expensive plugins, why not learn a few tricks (and in the process dispel a few myths) to give your footage impact without post production wizadry? This first in our series of camera trickery looks at achieving a 'shallow depth of field'. This helps to highlight your subject or 'point of interest', but also adds that professional touch to your footage.
Achieving a shallow depth of field isn't easy with a consumer camcorder, but if it's got manual controls you'll get the desired results. Here's the quick and dirty guide without the technical explanation (the techie bit follows!):
Step 1: Get in as close as possible to your main pont on interest in your frame (and don't worry about focus for now) but leaving the maximum distance between this point of interest and the background to the frame.
Step 2: Now switch to manual focus and focus the camera on your point of interest. You'll see that the subject becomes crisp whilst the background blurs.
Step 3: Lets really get those manual settings out in the open now. First leave the camera's auto exposure on, but increase the noise detection filter to maximum. If there's enough light to shoot the scene in, you're all set to film. But if the light's not perfect, change the noise detection filter and exposure to the the next lowest. Repeat this until there's enough light to film and you're ready to shoot.
Below is a video that puts are technique into practice:
And now the more techie bit from our resident guru on all things video related. We asked him whether we'd be able to reduce the depth of field with a longer lens. He has a lot to say.
Firstly, a lens is only sharp at the precise point of focus. A few millimeters either side and the unsharpness begins. The question is: at what point does "a bit unsharp" become "out of focus?" Which is like asking "how long is a piece of string?". So we say that once the point becomes larger than "X" it's unsharp. We come up with this figure depending on the number of pixels, HD or SD transmission and so on. This is important. Look at a picture on a 3" screen and it all looks sharp, look at a 42" plasma and it all looks out of focus! In other words, Depth Of field is variable, depending on all sorts of circumstances.
What we're calling Depth Of Field is normally thought of as the distance in front of the focussed mark plus the distance behind the focussed mark which "appears" to be still in focus. The main three criteria which affect video DoF are the aperture, sensor size, and distance. So, let's take an interview as an example a 25mm lens, at F2.8 focussed on 10 feet has a theoretical Dof (for Video) of four feet. To get the same size image from a 50mm lens, you need to be 20 feet away and, at f2.8 it has a DoF of...4 feet.
However, due to the foreshortening effect of a lens at the long end of the zoom, the focus appears to "snap" in and out a lot quicker than at the short end which gives an impression of a smaller DoF. By having a "smaller" area of background, this too can give the impression of a smaller DoF. Also an unsharp object (such as a plant) which is far in the background with a wide lens is "bigger" due to the compression of a telephoto and, therefore, more noticeably present and unsharp.
Try a few tests and you'll see what we mean.