There are few terms in cinematography and photography more misunderstood than depth of focus. The term is often used interchangeably with depth of field, but the two ideas actually have two completely separate meanings. Depth of focus is critical to understand if you want to have a crisp image and a firm grasp on your camera operating skills. So what is depth of focus? And how is it different from depth of field?
Define Depth of Focus
Depth of focus definition
Understanding this bit of terminology requires understanding some of the nitty-gritty technical aspects of your camera. It may seem like a complicated idea, but it’s key to making sure that your image is in focus.
DEFINE DEPTH OF FOCUS
What is Depth of Focus?
Depth of focus refers to the space between the lens and the image sensor where the image being filmed will appear in focus to the human eye. If an image sensor is too close or too far from the lens, it is outside of the depth of focus and the photograph or footage will look blurry.
This is the camera depth of focus formula:
In this depth of focus formula, t is the depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the focal length of the lens.
Depth of Focus Components
- Image sensor
The image sensor in your camera is a plate that takes in light waves and translates them into the image you’re capturing. The image sensor is located behind a camera’s lens, the optical device which focuses the light from the scene onto the sensor. It’s the distance between these two camera components with which depth of focus is concerned.
Depth of focus and depth of field
So far, it might seem like depth of focus is pretty much the same as depth of field. What is the difference between depth of field and depth of focus? Isn’t depth of field also about the range of acceptable focus in an image?
The key difference between depth of focus and depth of field is that the former refers to the range of focus behind the camera lens. Depth of field refers to the range of focus in front of the camera lens. Here's a diagram to help put these concepts into perspective.
There’s a reason most amateur photographers and cinematographers know what depth of field is but might not know about depth of focus. Depth of field is often utilized to serve an artistic purpose — a shallow depth of field directs our attention to a very specific area in the frame; a deep depth of field allows for multiple subjects to be in focus at various distances from the lens.
Depth of focus, meanwhile, is rarely altered in service of a creative choice. If your image sensor is outside the acceptable range of focus, your whole image will be blurry, not just certain objects within it.
This isn’t often a desired effect, and if it is, you can usually have your frame be entirely out of focus by adjusting your lens’ focus ring or depth of field rather than moving your image sensor.
Why This Matters
Why is depth of focus important?
For those aforementioned reasons, depth of focus is the more obscure cousin of depth of field, and most casual photographers and cinematographers can get by without ever understanding what it is. But for people who are serious about photography or cinematography and are dealing with high-end cameras, it’s a key concept to know.
A camera operator will most often use the depth of focus formula when they are inserting or removing gels or UV filters from their camera. Gels and filters are often placed between the lens and a camera’s image sensor, and so adding or subtracting them changes the distance between the plate and the lens. Most consumer cameras have built in filters, or filters that are placed on the outside of the lens, so these calculations usually only come into play for higher-end or older cameras.
Depth of focus is a term that is often forgotten until it becomes a problem; cinematographers might drive themselves crazy cleaning filters and adjusting depth of field settings, not realizing that their image sensor is outside the range of focus. So while this concept might not be front of mind when you walk onto set, it’s a good piece of knowledge to keep in your back pocket.