Understanding Depth of Field (DoF)
The “Depth of Field” (DoF) determines how focused your subject is when taking a photograph. The more technical definition of it is:
The distance between the closest and farthest objects in a photo that appears acceptably sharp.
The distance here is measured from the camera to the subject, where the subject can be an individual, place, thing or object. When we say camera, we actually mean the sensor which is what captures the image from the lens. The camera lens is what focuses the image to create the exposure that the sensor captures.
When you focus your camera, either manually using the focus ring or the autofocus feature, you are actually determining the depth of field. You look at where the subject is sharpest from the viewfinder during image composition. Once you have determined the sharpest focus, you create the exposure. The DoF can either have a narrow or large focus.
Let’s take a look at this illustration to show an example of the two types of DoF.
Narrow Depth of Field — Only a small part of the image is in sharp focus.
Large Depth of Field — More of the image is in sharp focus.
When you take a narrow or small DoF, you only focus on the subject and blur out the background. This is also called a “bokeh effect”. The background is insignificant to the composition, while your subject is the main focus. This is best used in portraitures and product shots. A good head shot often uses a narrow DoF in which you have a shallow focus. Often, a photographer sets the focus point of the eyes during a portrait head shot. Otherwise the focus can be set to the subject in the case of a product shot, like an earring for example.
To accomplish a narrow DoF with a good bokeh, the aperture needs to be opened more. That means more light through the lens hitting the sensor. Portrait photographers who take headshots will use an aperture of f/2.8 or even lower. The larger your aperture, the more light you are allowing to the sensor, therefore you have a shallower focus or narrower DoF. A good focal length really depends more on the “Angle of View” (AoV) because it is all about focusing on the subject.
The longer the focal length, the narrower the angle of view and the higher the magnification.
Longer focal length lenses are ideal for greater magnification. When you can get greater magnification on your subject you create a shallower focus. This is not the only way to get a narrow DoF. You can also use what are called “Prime” lenses that have a fixed focal length to take flattering portraits with a narrow DoF. A prime 35 mm or 50 mm is often used by headshot photographers to take up close and personal images. If you are using a 300 mm telephoto lens, you need to be a certain distance away from your subject to focus on.
Another way narrow DoF is used would be in macro photography. This is because the shorter your distance is to the subject, the smaller the DoF. Macro images are extremely close up shots, and often of very small subjects like insects and tiny objects. The technique used magnifies the image many times over to get details. Great macro shots like a bee on top of a flower are best shot with lenses that have a minimum focusing distance. Lenses with a focal length of 90 mm and 105 mm are the most popular in use.
A large DoF focuses more on an entire frame and has a wider angle. It has a much sharper focus on a large part of the image, rather than just a small part of it. This is an ideal technique for shooting landscapes and wide angle shots. Picturesque sceneries often use a large DoF because of the way it allows a sharper focus on most of the image. When you have a large DoF, you put more of your image if not the entire image into focus. Many photos are actually taken with a large DoF since many digital cameras don’t have extra zoom or magnification features.
The shorter the focal length, the wider the angle of view and the lower the magnification.
Landscape photographers shoot images with a large DoF. The ideal type of lens for creating these images have a short focal length, < 35 mm. A 15 mm lens would be great for taking a wider angle shot of a city’s skyline. When you shoot a scene like a skyline, you want to have most of the scene in sharp focus. In terms of aperture, the more DoF, so less light hitting the sensor. This means a smaller aperture will need to be set to create the image. This corresponds to a higher f-stop e.g. f/14.
Aperture and focal length are just two factors that affect the DoF. A camera’s sensor size would be the third factor.
Smaller sensors have larger depths of field, larger sensors have smaller depths of field.
This is why when you take a photo with your smartphone camera, you don’t usually get a shallow depth of field. This is because smartphone cameras have smaller sensors than a DSLR or mirrorless digital camera. Smartphones that use AI get shallow depth of field using software techniques rather than their optical hardware. A camera with a full frame sensor (35 mm) is great for taking photos that require a small DoF.
To get the most DoF, it is important to understand what hyperfocal distance means. It is defined as:
The focusing distance that gives your photos the greatest depth of field. That is the closest focusing distance that allows objects at infinity to be acceptably sharp.
In order to achieve this, the photographer needs to focus on a point between the foreground and the background of an image. If you focus on the foreground, then the background will be blurry and vice versa.
The variable “circle of confusion” means the part of an image that is acceptably sharp. It is measured in mm (millimeters) and it is the point of light on your camera’s sensor that would appear due to being out of focus. A larger value would mean more blur in an image. This is just a brief explanation of hyperfocal distance since that in itself is a topic worthy of its own article.
In the end, perspective will determine the best composition and DoF. How close the photographer is to the subject and how they frame the composition will produce different results. There is a difference in how an image’s DoF will turn out based on the distance to the subject, aperture, focal length and sensor size. There is really no wrong or right rule on DoF, but there is a correct technique for narrow and large DoF. What a photographer decides to do with it is how they create their image.