A decade ago, the elusive “cinematic look” was all video producers were talking about and trying to achieve. At the time, the best options available were the 24-frame-per-second framerate, 16x9 aspect ratio, and eventually (if you could afford it), high-definition resolution. However, there was still something missing. That something was the selective focus afforded by a shallow depth of field, which was difficult to produce on most of our video equipment at the time. To unlock this aspect of cinematography, the video producer needed to either use the very end of their zoom range or add some unwieldy gadgets to their setup (anyone remember the Letus adapter?).
Depth of field refers to the range of space in front of the camera lens that is in focus.
What is depth of field?
Depth of field refers to the range of space in front of the camera lens that is in focus. Think of critical focus as a perpendicular plane in front of the camera lens. If I set my focus for a certain subject or object, any other subject or object that is that same distance away from the lens will also be in focus. Depth of field refers to how much space in front of and behind that plane is also in focus. A shallow, or narrow, depth of field means that very little else is in focus. A large depth of field, or deep focus, refers to a broad range that is in acceptable focus.
Today, with the blurring (pun intended) of lines between video production and digital cinema, controlling depth of field is a much more attainable and affordable endeavor. The tricks are the same as when we were using video cameras ten years ago. However, today’s technology gives us much more control…if we know how to control it. There are four variables that affect depth of field: sensor size, the distance between the camera and subject, lens focal length, and aperture. As we talk about these variables individually, remember that the differences are based on all other things being equal. We’ll talk later about changing multiple variables at once.
Today, we can choose to purchase a camera with a full-frame sensor, an APS sensor, or at least a Micro Four Thirds sensor. These larger sensors give us more control over depth of field.
Sensor size
The variable over which we have the least control is the sensor size. This may be a factor in your purchasing decision, but once you have your camera, you are stuck. In general, a larger sensor will offer a shallower depth of field. A smaller sensor will offer deep focus. This is why the camera on your phone typically takes photos where everything is in focus. Now, you might say, “But my phone has a portrait mode that blurs out the background.” News flash: it is faking a shallow depth of field. The previously mentioned Letus adapter was the old way of faking it. In those days we were dealing with 1/3-inch, ½-inch, or at best, 2/3-inch sensors. But today, we can choose to purchase a camera with a full-frame sensor, an APS sensor, or at least a Micro Four Thirds sensor. These larger sensors give us more control over depth of field.
Distance between lens and subject
The next variable is fairly straightforward. Due to the physics of lenses, bringing the subject closer to the lens—thereby necessitating a closer focus plane—will result in a shallower depth of field. Moving the subject farther from the lens will result in deeper focus. Of course, making this change will also change your shot. Perhaps it will necessitate changing the focal length of your lens from wide to telephoto or vice versa, which may be a deal-breaker.
Focal length
That leads to the next variable of lens focal length, which is the variable that makes the biggest difference, in my opinion. A shorter, or wider lens will offer deeper focus than a longer, or more telephoto lens. This was my go-to trick in the old days. If I could move far enough away to use the telephoto end of my zoom lens, I could create a shallower depth of field. There is a trade-off in that a telephoto lens also compresses depth in the shot, making the background seem closer to the subject.
… all other things being equal, a lower f-stop—like f/1.2 or f/2.8—will result in a shallower depth of field. A higher f-stop—like f/16—will result in deeper focus.
Aperture
That leaves aperture, which can also make a significant difference in depth of field and is the variable over which we have the greatest control. It is also the variable that is often misunderstood. Aperture, exposure, iris, f-stop, and T-stop are all describing the same thing: how much light is making it through the lens to the camera’s sensor. Aperture can be a little confusing because a smaller f-stop or T-stop number indicates a more open iris or wider aperture. A higher number means a more closed iris. For the sake of our discussion, I will talk in terms of f-stops. Again, all other things being equal, a lower f-stop—like f/1.2 or f/2.8—will result in a shallower depth of field. A higher f-stop—like f/16—will result in deeper focus. Depth of field is not our only or even primary concern when it comes to aperture. We are mainly concerned with proper exposure, so this is a balancing act.
Of course, we can change the amount of light in the scene in order to adjust the aperture: less light means we can open the iris, lowering the f-stop. We can change the shutter speed on the camera, although this technique should be used sparingly as it will change the motion blur captured by the camera. The general rule of thumb is that the shutter speed denominator should be double the frame rate—1/60 for 30 fps, 1/48 or 1/50 for 24 fps, etc. We can also add neutral density to the lens: the photographic equivalent of putting on sunglasses. Finally, we can change the gain or ISO of the camera, although again, this should be done with caution. Cameras have a native ISO that offers the greatest dynamic range and least amount of noise, so deviating from this can have negative consequences. I typically approach adjusting the aperture in this order: adjust the lighting in the scene, use neutral density if necessary, adjust ISO if necessary, and then adjust shutter speed if necessary.
So, we always want the lowest possible f-stop and the shallowest depth of field possible, right? Ah, no. If you have ever shot an interview close-up with a 200mm lens at f/2.8, you know this is not true. As soon as the interview subject leans forward or backward an inch, they are out of focus. This is where knowing your goal and utilizing the various variables together comes into play. Sometimes selective focus is necessary, and so creating a shallow depth of field is desirable. But other times, you want to be able to see and make out what is happening in the background of a scene, so deep focus is warranted. In the case of an interview close-up, I try to set my f-stop around f/4 because it gives the subject some room to move without going out of focus, and many lenses are their sharpest around f/4.
Knowing how each of the variables works will help you to construct a shot with the correct depth of field for your artistic vision and needs without sacrificing the quality of your image.
