Not so fast.
I want that shallow depth of field look, OK?” Well, not so fast, hombre. While most people intuitively know the look of a shallow or narrow depth-of-field, creating it is not necessarily a slam-dunk in the age of super sensitive, camera-imaging chips.
It is a visual ethic that both creator and viewer gravitate to naturally. Not only aesthetically pleasing to the eye, controlling what is in and not in focus goes to the very heart of controlling the narrative. Other than camera framing, there is probably no better cinematic technique at focusing the viewer on precisely what the director considers the most important element of a shot. A shallow depth-of-field is the best at establishing the primacy of subject over environment. With apologies to the photographically proficient among us, here are some very general thoughts to consider before you start harping on your DP because they can’t “make that background softer.”
First, what is depth-of-field (DOF)? In photography or videography, it is at it’s most basic, the effective focus range or distance between the nearest and farthest objects in a shot that will appear to be in focus. A lens will focus on only one distance at a time in your shooting space. There is then a natural, gradual decrease of sharpness in front and behind this point. Manipulating the distance or range (or in our case minimizing) of what is considered “in-focus” is the trick and there are many technical and optical variables that come into play. For the sake of a concise discussion, lens focal length, depth of the cinematic environment, lens aperture or f-stop, camera ISO and format size are the most germane here.
Generally speaking, for any constant format size or imaging device, the longer the lens, or the further it moves from a wide-angle field-of-view to a telephoto view, the shallower the apparent depth-of-field will be. So, everything generally appears to be in focus when you employ a wide-angle lens. This is why it is extremely difficult to generate a shallow DOF in let’s say, the tight confines of doctor’s small examining room or a tight office cubical where the background might be only inches away from the subject. DP’s are invariably forced to use relatively wide lenses to capture the subject matter in these tight spaces. Therefore, you will often see the DP trying to find a more distant shooting position, maybe outside a doorway to be able to employ a longer focal length lens. It is just one of the evitable production struggles between logistics and aesthetics. The corollary problem here is just depth. Of course, it’s hard to throw a background out of focus when it is only two feet behind the subject’s position.
In another generalization, the size of the recording surface or pickup chip in a camera has an inverse relationship to the apparent DOF. This partly explains why with the first generations of video cameras everything looked in focus. This early technology utilized 1/3” or ½” sensing surfaces as opposed to current sensors that have the larger dimensions of a still 35mm camera.
By far, the biggest challenge to obtaining a shallow DOF is f-stop or the aperture of the lens. As a rule, the wider the f-stop is set, meaning the more light permitted to pass thru the lens into the imager, the more shallow the DOF. It is an inverse relationship. As confusing as it may sound, the higher the f-stop number on the lens, the smaller the lens aperture and less light passing thru the lens. An f-2.8 lens setting is infinitely wider or more open than an f-16. All things being equal, a lens set at f-2.8 will have a much shallower DOF than one set at f-16.
However, the newer generation of cameras presents real difficulty in achieving this shallow DOF aesthetic. Some of more advanced camera chips are incredibly sensitive, making it much more important to sufficiently control the amount of light reaching the sensor. The three obvious solutions are to reduce the camera’s ISO, or ASA in old parlance (which still may not be sufficient when shooting outside on a bright sunny day) or reduce the f-stop or aperture (which is self-defeating for obtaining a good, shallow DOF) or simply using less light. Putting neutral density filters in front of the camera lens is often the only solution. However, as any gaffer or DP will confirm, if you are doing interior lighting with very low intensity so that you can correspondingly lower the f-stop, there are a whole other set of aesthetic control problems introduced.
Moral of the story; the next time you ask for the shallow depth-of-field aesthetic, realize that there are a whole lot of variables that must be manipulated to achieve this desired look.