What is depth of field? Let’s start by determining what it is not. Depth of field is not a tangible thing. Depth of field changes depending on the way one takes a photo or shoots a video. Understanding depth of field is essential in cinematography, where the focus is manipulated to achieve the desired look.
It is more accurate to think of depth of field as a condition resulting from a set of conditions relating to a photograph and its visual arrangement. It describes an area of acceptable focus around a subject placed at a specific distance from the lens. This phenomenon occurs within the lens diaphragm—comprised of tiny thin blades that open and close to create an aperture.
The three factors
Three factors weigh heavily on how depth of field occurs in an image: aperture or iris size (f-stops), Lens focal length (mm) and camera to subject distance—sometimes referred to as focal distance. For the sake of this conversation, we will concentrate primarily on the iris and its effects on depth of field.
Because depth of field is relative to specific f-stops, it can be a consideration when calculating the proper exposure of an image. Depth of field also plays an aesthetic role in how an image looks. Do you prefer dreamy, out-of-focus backgrounds in a portrait, or do you like to see details in the background that reveal a subject’s location? Your preferences become tied to aperture choices affecting over-all exposure. Therefore, the resulting f-stop of an exposure decision allows depth of field to be a compositional choice as well.
Plane of focus
Depth of field relates to something called the plane of focus. Imagine a photo as a 3-dimensional projection with the camera at one end of the projection, and the subject(s) arranged within the “field” of the projection. It’s like a big “slice” of our image field, that gets thicker as we open, or close down our aperture (iris). This “slice” also moves toward, or away from our camera based on our lens size, and subject distance. Here’s an example:
Given this example, as we resolve the subject—the object we want to be in sharp focus—we create a “zone” around that subject where the image is in apparent focus. The zone consists of the area in front of and behind the point of focus, where-in our subject resides. This zone is directly related to the f-stop we select on our lens. We do this by adjusting the lens iris, or aperture (like f2.8 or f16) that controls the diameter of the opening inside the lens. It works a little like the pupil in our eye.
Aperture
By manipulating the size of the aperture, we create a narrow zone around our subjects using a wide-open (large) f-stop. While narrower (small) apertures produce the opposite effect. Did you ever squint your eyes to make distant objects appear sharper? This is the principle behind the lens aperture that controls depth of field.
Aperture and f-stops
Now, here is where it can be a little confusing. When we refer to an aperture as “large,” we signify that adjustment with a low number like f1.4. When we refer to an aperture as “small,” we signify that with a higher number like f16. These numbers relate to a series of aperture “blades” that form the “diaphragm” inside our lens. As these blades open and close, they form a hole where light can pass through the lens and strike our image sensor—or film plane when using photographic emulsion.
F-stop formula
It may help to think of these f-stops as fractions of the maximum possible aperture in which a lens may “stop open.” If f1.0 is a wide-open lens aperture, then f16 is something like 1/16 the resulting size of the original aperture opening. Since f-stops are measured by dividing the effective diameter of the iris—as seen through the rear lens element—by the focal length of the lens, it makes sense that we would think of these measurements as fractions. Therefore, the formula for calculating an f-stop looks like this: F-stop = focal length/aperture diameter.
F-stops and exposure
Every photo/video lens has a diaphragm inside. The size adjusts by way of a menu system inside the camera, a series of buttons on the camera, or an f-stop ring on the outside of the lens. The consequences of each selection have an effect on exposure. Therefore, carefully consider choices for the aesthetic effect, versus the effect on the exposure that results.
Have you ever noticed how your pupils react inside to low light? They get bigger, to allow more light to pass to the retina. However, outside our pupils get smaller with abundant light. This is how to control exposure using the lens aperture. For more on that
see our article on the exposure triangle. If you are a beginner, this advanced concept may require additional study and research.
Put into practice
Now let’s take a look at how the zone of focus around our subjects can change, based on the aperture (a.k.a. f-stop, diaphragm) we select. Here are some detailed illustrations from Michael Bemowski’s internet application, “Depth of Field Simulator.” You can access Michael’s app here.
With the camera at one end of this example and the subject positioned at 5-feet from the photographer, there is a narrow zone of acceptable focus around both sides of the subject. Here, we are using an f-stop(aperture) of f1.4. This selection must be made
manually on a cinema lens.
The above image shows how narrow the depth of field is around our subject placed at 5-feet away from the camera. Notice how little is in focus in front of, and behind our subject? It is the shaded area that will change as we adjust our aperture size.
Here is another example where the f-stop changes to an f2.8. Do you see an increase in the zone of focus? Let’s try another setting.
You should notice a gradual increase in the area around the subject. This is the depth of field. It is a zone that extends from just under four feet from the camera and extends out to over seven feet from the camera–with the subject set at five feet, and an aperture of f5.6 on a 50mm lens. Let’s look at one final example. Then, we want to point out some unique aspects of this depth of the field phenomenon.
This last example shows the most demonstrative change in the zone of focus, or “depth of field,” around our subject, using the same 50mm lens set to f11. At this point, something interesting is happening. Notice the rate of change in front of the subject versus behind the subject, as the aperture gets smaller? This change exists proportionately at a rate of nearly 1/3 in front, and 2/3 behind the subject.
You might notice that the zone behind the subject is reaching toward the background. As a result, while little changes between the photographer and the subject, the background is becoming increasingly more “in focus.” This can be a desirable effect if one is taking pictures while sightseeing, or anywhere that the environment shares an equal interest with the subject placed within it. However, if it is your desire to create a focal point, where the subject is the primary detail you wish for the audience to pay attention to, then the depth of field in the first example may be preferable. With a 50mm lens set to f1.4, the background,
and all of the foregrounds becomes softly out-of-focus–leaving the audience cued towards our model.
As the variables adjust, using Depth of Field Simulator, you see the effects in real-time. By changing the lens focal length, camera-to-subject distance, and f-stop respectively, incremental changes can be seen taking place involving the background. Its apparent distance, and degree of sharpness changes in direct proportion to the size of the aperture! This makes the choice of aperture both an aesthetic consideration, as well as an exposure imperative.
Subject distance and focal length
We have seen how aperture affects “dof” directly. Now let’s discuss subject distance, and lens focal length. Using the depth-of-field simulator, change the camera to subject distance at the bottom of the application desktop. Notice how, as the distance increases between us and our subject, the zone of focus around that subject increases? At the same time, as our camera to subject distance increases, and focus increases, the background becomes more and more in focus. This is a proportional relationship. The further the subject is from the camera, the more the background comes into clarity. Therefore, if you want soft out-of-focus details behind your subject—move closer!
Focal length, or the strength of your lens described in millimeters, can also affect depth of field. Wide-angle lenses–those with a broad field of view, and low millimeter designations—appear to demonstrate more depth of field. While lenses with higher magnification and higher millimeter designations appear to demonstrate less depth of field around a subject. This can be very handy when one wishes to direct the attention of the audience to specific items within the image frame. In this way, depth of field can be a powerful storytelling tool.
Image sensors
With respect to the size of a camera’s image sensor, there are no discernible differences among brands on the matter of effects (if any) imposed on depth of field. Because image sensors of different sizes require different image circle projections, it is possible to experience similar depth of field characteristics if one selects the lenses that offer the appropriate coverage for a given sensor. In other words, match the sensor to your lens system.
A camera with a full-size sensor will require lenses that cover a 24X36mm projection area. A camera with a micro 4/3 sensor will conversely require lenses that need only cover an area of 13X17mm in total image size. If one matches the image circle of the lens to the area of the sensor, the depth of field results is comparable.
The caveat is that the focal lengths necessary for micro 4/3 will be wider than those required by the full-size sensor. Thus, to get shallow depth of field, the m4/3 user will be required to get closer, or further from their subjects as the aesthetics demand.
Useful terms
aperture – the diaphragm inside a lens, comprised of tiny thin blades, that create a measurable hole for light to pass through during exposure.
angle of view – the observable point of view of a lens, described by its size in millimeters, and defined by the image framing.
background/foreground – areas behind, or in front of a subject, that contain details relevant to the image’s overall composition and framing – affected by depth of field.
composition – the artful arrangement of elements within a photograph, or video frame.
depth of field – an area of acceptable focus extending in front, and behind our subject.
diaphragm – the adjustable opening inside a lens a.k.a. “iris.”
exposure – the extent to which an image appears too bright, or too dark.
exposure triangle – a principle that defines proper image tone/balance by discussing manipulation of the lens iris, camera shutter, and the sensitivity of the recording media. Visit the link embedded in the text above for a deeper explanation.
f-stop – a numerical value that describes the size (diameter) of the lens aperture.
focus – the point at which a subject appears sharp in an image.
focal distance – the distance from the lens to the subject being photographed.
focal length – The distance from the surface of the main objective of a lens, and the point of
convergence of light rays that are transmitted through that lens.
image field – all the areas in the foreground, middle-ground, and background of an image that reside within the visible frame. The image field is a factor determined by the lens size, or angle of view.
iris – see diaphragm.
plane-of-focus – the “slice” of an image that exists at the precise point of focus. This slice runs parallel to the camera’s image, or sensor plane–extending infinitely left or right–and moves forward, or recedes from the lens based on lens size, f-stop, and subject distance to camera.
resolve (to) – to achieve focus through the manipulation of a photographic lens.
*Main image “DoF–Shallow Depth of Field,” by P. Namek. Used under GNU Free Documentation
License – Creative Commons. Uploaded to Wikipedia Sept. 2005.
