Focusing Basics

Depth of Field, Bokeh and Subject Isolation

In the world of photography, the amount of spatial distance that appears to be in focus to our eyes is called the depth of field. Depending on the genre and style of photography, a photographer may choose to have a deep depth of field (majority of the scene is in focus) or a shallow depth of field (small portion of the scene is in focus). Shallow depths of field are useful for isolating subjects from their surroundings, while deep depths of field are useful for displaying as much of the scene as possible.

An important component relating to depth of field is bokeh, which is the visual quality of the out of focus areas of an image. A photographer’s preference for the shape, size and quality of bokeh is subjective, but high quality bokeh is generally characterized by a smooth, circular and butter-like appearance. High quality bokeh can be achieved by creating greater subject isolation, which is the degree to which the out of focus regions are maximized relative to the main subjects.

To better explain the concept of subject isolation, position your finger a few inches away from your eyes and focus on the fingertip. Notice how narrow your visual depth of field is? Now try focusing on something several metres away and take note of the greater depth of field that your vision has rendered. This concept is exactly how camera lenses work; the closer the subject is to the camera (either by physical distance or by using a lens with a longer focal length), the greater the subject isolation will be in relation to the background. 


The Three Focusing Factors

There are three factors which control the depth of field and the amount of subject isolation: aperture, camera-subject distance, and the focal length of the lens. The way that these three factors influence image composition and depth of field will be further explained in this article.


APERTURE

The first way to control depth of field and the amount of subject isolation is by changing the aperture settings in your camera. The lens that you are shooting with will dictate what the usable aperture range will be (e.g. f/2.8 – f/22).

Aperture is the size of the hole in the lens that allows light into the camera. Aperture is represented as f/N, where N is the f-number (e.g. f/1.4, f/11, f/22).

Large apertures are represented by small f-stop numbers (e.g. f/1.4) and are characterized by a larger opening in the lens. Small apertures are represented by large f-stop numbers (e.g. f/22) and are characterized by a small opening in the lens.

The larger the aperture of a lens (small f-number), the shallower the depth of field that can be achieved. Likewise, the smaller the aperture of a lens (large f-number), the larger the depth of field. The easiest way of thinking about this is the larger the f-number (e.g. f/11, f/16), the more of the scene that will be in focus.

Around the larger limits of achievable apertures (e.g. f/1.2, f/1.4, and f/1.8), the qualities of bokeh become much more noticeable. While the difference between f/1.2 and f/1.8 may not seem like much based purely on numbers, the bokeh of f/1.2 will be significantly more appealing to the eye. Because of this, f/1.2 lenses are much more expensive than f/1.4 and f/1.8 lenses.

When trying to capture the entirety of a scene, there is usually a “sweet spot” for which aperture to use in order to ensure the highest level of sharpness with a deep depth of field. When shooting landscapes, the ideal procedure is to use f/11 and focus on the most important subject of the scene—this will ensure a sharp composition without any unintended blurring of the foreground or background. A creative trick in landscape photography is to create foreground bokeh by placing natural components like branches, flowers or grass in the foreground while focusing on a more prominent subject (e.g. waterfall, mountain, human) in the distance.


CAMERA-SUBJECT DISTANCE

The second way to control depth of field and the amount of subject isolation is by adjusting the distance between the camera and the subject.

The general rule is that when aperture and focal length are fixed, the depth of field will become shallower as the camera gets closer to the subject. If a photographer is using a popular portrait focal length like 85mm, it would be best to move closer to the subject in order to achieve greater subject isolation (shallower depth of field) and better bokeh. If the photographer were to back up further and shoot the same subject with the same 85mm lens, they would achieve less subject isolation (deeper depth of field) and less bokeh.

When shooting with a prime lens (fixed focal length), the best way to achieve greater subject isolation and higher quality bokeh is to move closer to the subject while shooting at the largest available aperture. By focusing in closer on the subject, a deeper out of focus region in the background is created. The more out of focus the region, the stronger the resulting bokeh.

However, it may not always be possible to move closer to the subject. Consider the following scenarios:

- A photographer is shooting a soccer match from the sidelines with a 300mm f/2.8 lens, and he cannot move any closer to the players (subjects) that he is focusing on. Some of his images are too far away from the subjects, resulting in deeper depth of field and less isolation of the players. As a result, a large portion of the audience in the background is in focus—serving as a distraction within the image composition. The solution: use a longer focal length to narrow the field of field (e.g. a 400mm, 500mm or 600mm lens), thereby focusing in closer on the subject and creating more isolation against the background.

- A photographer is shooting full body portraits for a client with a 50mm f/1.8 prime lens. The client is unhappy with the quality of the bokeh in the images—there is not enough separation between the subject and the background, leaving unattractive bokeh elements that are hexagonal and “busy” in appearance. The photographer cannot move closer to the subject as it would result in cutting off a part of the client’s legs in the composition. The solution: use a 50mm lens with a larger aperture, such as f/1.4 or f/1.2. This would enable the photographer to maintain the same composition as they were shooting before, but it would create further subject isolation and improve the quality of the bokeh.

If a photographer wishes to achieve a deeper depth of field within a scene (e.g. for cityscapes or landscapes), the photographer is better off using a smaller aperture rather than backing up further from the subject.


FOCAL LENGTH OF THE LENS

The third way to control depth of field and the amount of subject isolation is by using different focal lengths.

To gain an understanding of how different focal lengths are used, let’s look at the common lens preferences within the different genres of photography:

- Landscape photographers often prefer to use wide angle lenses (e.g. 16-35mm) because of their ability to obtain deep depths of field at any given aperture setting and camera-subject distance. 

- Portrait photographers often prefer to use mid-range lenses (e.g. 35-135mm) because of their ability to create shallow depths of field (more subject isolation) at mid-range camera-subject distances.

- Wildlife photographers often prefer to use telephoto lenses (e.g. 100-400+mm) because of their ability to create shallow depths of field at longer camera-subject distances.

Adjusting the focal length either by zooming or by switching between shorter or longer lenses adheres to the same concept of moving the camera closer to or further away from the subject. However, sometimes the photographer is unable to get closer to or further from the subject, so adjusting the focal length is the only way to create more subject isolation.

In order to understand how focal length relates to depth of field, consider a scenario where we have a subject standing at a set distance in our scene. When the field of view is narrower (e.g. we are using a longer focal length like 135mm), the subject will appear closer in the composition—allowing for greater isolation and a shallower depth of field. When the field of view is wider (e.g. we are using a shorter focal length like 35mm), the subject will appear further away—causing less isolation and a deeper depth of field.

It is worth mentioning that switching from a 35mm full frame sensor to a crop sensor (e.g. APS-C) or vice versa, either by changing camera bodies or by enabling an in-camera crop setting, will yield the same effect as what would be achieved by changing the focal length of the lens. This is because a crop sensor camera "crops out" the edges, effectively rendering a longer focal length in exchange for lower resolution.     


SUMMARY

- There are three methods to control depth of field: aperture, camera-subject distance, and focal length of the lens.

- Bokeh is of the highest achievable quality when the subject isolation is maximized.


The following rules hold true if no changes are made to the two other methods:

Aperture:

- Larger aperture/low f-number = shallow depth of field

- Smaller aperture/high f-number = deep depth of field

Camera-subject distance:

- Smaller distance between camera and subject = shallow depth of field

- Larger distance between camera and subject = deep depth of field

Focal length of the lens:

- Shorter focal length/wider field of view = deep depth of field

- Longer focal length/narrower field of view = shallow depth of field


Terminology:

- Bokeh: the visual quality of the out-of-focus areas of a photographic image

- Depth of field: the amount of spatial distance within an image that appears to be in focus

- Subject isolation: the separation of an in-focus subject from an out of focus foreground or background

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