Depth of Field

Forums and Facebook groups are full of someday want-to-be professional photographers and an extremely common question they ask as beginners is “How do I get that blurry background in my photographs?” The blurry background, or bokeh, is an effect accomplished by knowing how to create a shallow depth of field. It is a compositional tool used in many genres of photography such as portraits, flowers/nature, and products.  A quick word of caution, overuse of any one compositional tool can create a portfolio that is, well… boring. So don’t rely only on “blur” to create memorable images. Check out our blog post on Composition Basics here.

While bokeh creates a pretty effect for many images, some genres require crisper focus throughout and blur is not desirable. Landscape, night photography, and some types of street photography are examples where a wider depth of field is common.  To accomplish this wider depth of field, hyperfocal distance is often used.  (You can read about Hyperfocal distance in next month's blog, but it is recommended you first have a solid understanding of basic depth of field and factors that affect it as discussed in this post).

What is Depth of Field?

Simply put, depth of field is the distance between the nearest and farthest objects in an image that are in acceptably sharp focus. In some photographs, everything seems to be in focus throughout and in some photographs the foreground or background, or both, may have blur. This is achieved by manipulating the factors that affect depth of field.

Depth of field is calculated using the lens’ focal length, distance to the subject, the acceptable “circle of confusion” size, and the aperture setting of the lens. Thus by changing one or more of these factors, different depths of field can be created.  Let’s investigate the effects of each factor on the depth of field, starting with shallow depth of field.

How does one get shallow depth of field?

Shallow depth of field, also called a small or narrow depth of field, means that only a part of the image is in focus. Most often this is accomplished by using a wide aperture (small f/stop number) on the lens and shooting in aperture priority or manual mode. If a lens has the ability to shoot at f/2 at its most open setting, the resulting depth of field will be small or narrow. If that same lens was set at f/22 and its position and the subject/background positions are not changed, the result will be a much deeper or wider depth of field.

In these images above, the only factor that was changed was the aperture, or f/stop. Notice in the first image shot at f/32 there is more of the image, from front to back, in focus. The main flower and the flowers behind it are more defined and the bars on the iron door can be seen. In the second image, the aperture setting was changed to f/5 so much less of the image is in focus. There is more blur behind the main flower in this image and the bars on the iron door are completely out of focus, so much so, they almost disappear into the background. This is one advantage of a blurry background; to get rid of distractions behind the subject.

Shallower depth of field can also be accomplished with longer focal length lenses or by changing relative positions of the camera, the subject, and the background. A photo taken in close range will have a much smaller depth of field. Moving the camera closer to the subject and moving the subject farther from the background will increase blur in the background and make the depth of field narrower.  For example, a portrait photographer will want to put some distance between the person/subject and the background to get the desired blur effect. A flower photographer might use a longer focal length lens to get a smaller depth of field and blur behind the bloom.

The images below demonstrate this effect. In the first image, I was much farther away from the flower arrangement which produces a wider depth of field. Some of the background, like the door behind the flower arrangement, were already somewhat blurred, but most of the flowers in the arrangement are in focus. In the second image, I have moved closer. There is a narrower depth of field. At this very close range, the background is fully blurred.

A camera can only get precise focus at one exact distance from the lens. Everything in front of or behind that distance will be blurred. The blur “spot” will be shaped like the aperture of the lens, thus almost a circle. If these spots, or circles, are small enough they are almost indistinguishable from a point of light and they appear to still be in focus. When this happens we have acceptable sharpness or acceptable “circle of confusion.” The “circle of confusion” size is related to a camera’s sensor size, and is a complicated concept that could be a whole article in and of itself. Any blur one can see in a final photograph is simply the blur spot as it registered on the camera sensor, only enlarged (on your screen or in print). How big this blur can get without being noticeable, is the acceptable "circle of confusion" size.

The take-away here, is that even the most crisp looking landscape image one has ever seen actually has areas that are out of focus or blurred, it’s just the blur is so minimal one’s eyes can’t tell unless the image is magnified or the viewer gets closer. The mechanisms of the camera and the way light bends as it enters the lens makes it impossible to have an image that is totally in focus from edge to edge at every given distance. 

How depth of field and the “circle of confusion” changes from a crop camera to a full frame isn’t something most photographers will need to know. If one plans to make super-sized prints or enlargements, then it might be beneficial to know the sensor size and "circle of confusion" calculation in relation to the anticipated print size. More important for most photographers to understand are the effects of a specific camera body and lens combination on depth of field. Most photographers would not change camera bodies to achieve the depth of field they want when they can change position, lenses, or settings instead.

A full frame camera sensor will create a shallower depth of field when adjustments are made to keep the same field of view (remember a 50mm lens on a crop camera will only “see” the field of view of approximately an 80mm on a full frame – see our blog article on focal lengths here for clarification). Otherwise, this formula shows that depth of field is unaffected by sensor size if aperture, focal length, and distances are kept constant.

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How do you get wider depth of field?

Wide or deep depth of field results in a much larger area (front to back) of the image being in focus. Basically by doing the opposite of any of the above, one can accomplish a wider depth of field.  Using a shorter focal length (for example switching from an 85mm to a 50mm) or setting a larger aperture (from f/5.6 to f/11) will increase the size of the depth of field and make it deeper. The next set of images show the results of changing the focal length.  

I remained stationary, but used the zoom on my lens to change the focal length from 18mm to 400mm (these images were also cropped to fit together). The shorter focal length in the first image creates a wider depth of field. The iron door is clear and the door knob appears mostly focused. In the middle image, the depth of field is narrowing. By the time I zoomed all the way in for the last image, the depth of field has become much more narrow and the only part that is really in focus is the solitary flower.

Changing the distance between camera and subject will also change the depth of field. If the subject is placed closer to the camera the depth of field is smaller. When you want to blur the background the subject should be closer to the camera, but keep some space between the subject and the background. If the subject is placed closer to the background, and the camera is moved farther away, the depth of field will be wider. In the example above, if I had remained in position after the third image and had moved the flower arrangement farther away from me (moving the subject farther from the camera), then the depth of field would have gotten wider again.

How is shallow depth of field used as a compositional tool?

In portrait photography, shallow depth of field creates that soft bokeh or blurry background behind the person (and sometimes in front of the person as well). The blur can give a photo a moody look or be used to create a dreamy, mysterious, or romantic “feel.” Additionally, the blur is sometimes used for an abstract effect.

The selective blur creates dimension and reduces distractions from the background. Since our eyes are drawn to and gravitate towards areas in focus, creating a composition where only some of the image is in focus allows the viewer to be guided to the subject. In portrait photography it is considered an important standard practice that the most crisp focus point be on the eyes.

Blur in the foreground can also remove distractions that are in front of the subject, like fencing around a cage, allowing the focus to be on what is beyond. In this image below, the wire fencing on the bird enclosure "disappears" and I am able to shoot "though it" even though my lens was several feet on the other side and the lens glass far too large to put between the wires.

Shallow depth of field is most often used in the genres of portrait, nature, travel, and to some extent street photography. With street photography, too much blur will take the subject out of context and the story behind the image can be lost.

How is wide depth of field used as a compositional tool?

Wide depth of field is desirable for landscape photography especially. This is where understanding hyperfocal distance becomes important. 

Hyperfocal distance, in a nutshell, is the point in the foreground that is the closest point the camera can focus while still having acceptably sharp image quality throughout the rest of the image to “infinity” (which is the background or horizon in most images) In other words, it is the point of focus that will yield the greatest depth of field.

The hyperfocal distance point does not create an equal amount of focus in front of and behind the focus point. Typically, 1/3rd of the range of focus will be in front of the point and 2/3rds will be behind that point.

For a detailed look at hyperfocal distance and how to make calculations in the field, check out our hyperfocal distance blog next month.

There are hyperfocal distance charts and apps (like DoF Table, Digital DoF, and PhotoPills) that can provide a good starting point for reference, but they are not always accurate and they are one-size-fits-all, not taking into account the actual scene. Some lenses also include markings on the side of the lens barrel that give you these calculations, but the easiest method is using the approximation method of “double the distance.”

How do I use “double the distance?”

A very simplistic way to achieve equal sharpness in the foreground and background is to use a method called “double the distance.” Find the closest object or element in your composition and determine (approximately – exact accuracy is not necessary) how far away it is from the camera sensor (not the end of the lens).  Then double that distance and focus at that point. For example, in the image below, if the closest rock that needs to be in focus is 10 feet away, then make the focus point 20 feet away (where the water ripples are). Use a small aperture (large f/number). Use live view if available on the camera and use it zoomed in. This method does require some practice at estimating distances, but can be quite effective and efficient once one gets the hang of it.

Keep in mind that depth of field increases with smaller apertures, so if the closest object is not in focus at a certain aperture, then one may have to adjust the aperture. For example, if the camera is set up using a 35mm lens and the closest object that needs to be in focus is 8 feet away (focus point 16 feet away) and f/8 is not working, increase the f/stop to f/11 or f/16 to bring the focus closer (increased depth of field). Other adjustments may then be required to shutter speed and ISO, so an understanding of the exposure triangle is also essential to achieving the desired outcome.

Image by strikers on Pixabay.

What is the take-away?

In summary, manipulating the depth of field is a compositional tool used by photographers in many genres. It is typically accomplished by changing the aperture of the lens, the lens focal length, or the positioning of the subject and background along with the positioning of the camera.

A shallow depth of field creates a nice blur effect in front of and behind the subject which results in a softer or more abstract feel, whereas, a wide depth of field has crisper focus throughout the image from front to back.

Knowing the calculations or keeping a focus distance chart is not necessary. An understanding of the exposure triangle, and the effects of focal length and aperture settings on your specific camera are essential. Practicing with your camera and lens in different settings is the best way to see the effects on depth of field in practice.

For improved landscape photography, narrow the aperture and use the “double the distance” method to get a wider depth of field. For portraits and flower blossom, put space between the subject and background and use a wider aperture or a longer focal length, or shoot in close range of the subject.

With an understanding of these basics, one will be ready to move on to a more detailed look at hyperfocal distance, or some practice with other compositional tools.