Depth-of-field is one of the many things which I keep seeing highly confusing explanations of everywhere. People talk about lens focal length, light-science and tons of other things which are already difficult to understand when you get the basic stuff, let alone when you're struggling with the easy things.
This isn't especially the fault of filmmaking or the people involved in it, of course. A lot of it has to do with the fact that there are complex forces at work and lots of scientific stuff and the two things don't make for casual conversation. It is possible to simplify it, but the problem is that a lot of the important facts and details get lost along the way...
...which is annoying.
Here, then, I present you with a very simple and relatively quick explanation of what depth-of-field actually is, how it works and other fundamental facts. There's a hell of a lot more to it than this, so don't even think about emailing me and telling me off for not including dozens of important facts. As far as I am concerned, this is what you absolutely need to know.
What the hell is depth-of-field then? Be quick, now. I haven't got all day
No problem. Depth-of-field refers to what you see that is in focus when you look at the image produced by a video camera or DSLR in movie mode (if you already do a bit of photography, you're in luck). An image can have anything ranging from a very deep depth-of-field – as in a hundred metres or more – to literally only 1mm deep.
All cameras with manual functions allow the user to dictate the depth-of-field.
No camera is capable of having absolutely everything in perfect focus, because this is technically impossible. Instead, when things look sharp all over, what you are actually seeing is the illusion that everything you're seeing is in focus. In truth, only a small-to-large area will be in absolute focus and the rest will always be slightly softer. But our eyes cannot perceive the difference, or if they can, our minds are too busy processing dialogue and character movement to do so simultaneously.
4K cameras and even higher resolution cameras are capable of producing a near-sharp-all-over image, which is sometimes described as flat.
Deep depth-of-field is created by using a small aperture and, depending on how much light is available, a higher ISO. In possibly one of the most confusing twists of terminology related ridiculousness ever, this means that the F-stop is large or high – such as F22. Not very helpful when you're just learning, but there it is. We have no choice but to deal with it.
As mentioned in an earlier paragraph, shallow depth-of-field refers to only a very small area being in sharp focus. The rest will appear much softer. This can all be adjusted to a varying degree.
This technique is nothing less than an absolute corker if you want to draw your viewer's attention to a very small area only, such as someone's eyes, finger-tips typing on a keyboard, or even the very tip of a single human hair.
Sometimes described as the cinematic look, shallow depth-of-field is created using a wide or large aperture. This means that you're allowing the camera to absorb lots of light, and also means that your ISO will (probably) also be quite low. The aperture might be 1.5, and the ISO might be 100. Far from ideal for focusing on a moving subject, but perfection for bringing your viewer's attention to a very small area indeed.
Depth-of-field and artistic choices
I'm fed-up with people saying that shallow depth-of-field is cinematic and deep depth-of-field is more of a video camera look. This is almost total nonsense, and this nonsense can be easily demonstrated by the fact that many early films favoured deep depth-of-field (like Citizen Kane). The two things are merely generalisations.
What matters is you, the filmmaker, or you, the client, and what you would like to achieve with your particular movie or video. Ultimately, it's about doing what's right for a particular production, and knowing exactly why you're doing it. Forget about what's cinema and what's video.
All this is why sports games are often filmed with a deep depth-of-field – allowing us to see all the players moving all at once – and movies are often filmed with an emphasis on one particular person's face, or an object, allowing emotion to be communicated in just the right, subtle way.