The Ultimate Guide to Learning Photography: Aperture & Depth of Field

— Photo: Georg Krewenka, Student

A Beginner’s Guide to Depth of Field: How to Blur (or Not Blur) the Background of a Photograph

Most new photographers reach a point when they’ve mastered snapping an action photo without blur — yet at the same time, wonder how to add in a different kind of blur. Background blur. Blurring out the background of a photograph while keeping the subject sharp gives an image depth and eliminates distractions.

That background blur? It’s called depth of field (DOF). A photograph’s depth of field indicates whether the entire scene is sharply focused, or if everything but a small section is blurred into oblivion.

Aperture and depth of field are directly related, but there are a number of other factors that play a role in that blurry background effect too. Here’s what new photographers need to know about aperture and depth of field.

What is depth of field?

— Photo: Aaron Van Swearingen, Student

Understanding depth of field (DOF) is a bit simpler once you understand a bit of how a camera — and a camera lens — works. A lens redirects light to the camera’s sensor. The different pieces of glass inside the lens adjust the light rays so that they come together at the image sensor. When the points of light meet at the sensor, the image is in focus. The points of light that meet before — or after — the image sensor are out of focus.

Focus is dependent on distance. Those different glass pieces inside the lens adjust so that the light bouncing off your subject meet at the sensor, creating an in-focus image. A camera lens with a longer focal length will result in a narrow angle of view, with the focal point of the image being farther in the distance. Any lens with a shallow focal length could be considered a wide angle lens. A wide angle lens captures a wider field of view, but fails to capture much detail.

But what about the other objects in the image? Since those items are at a different distance from the camera than the subject, the light rays won’t meet together at the sensor, which means those objects are out of focus.

But how far away do those other objects need to be in order to be out of focus? Will an object a foot away be in focus? What about an inch? A millimeter?

Depth of field determines the range of distance where objects will remain sharp. For example, with certain camera settings (which we’ll talk about next), objects a foot or even several feet away from the subject will still remain acceptably sharp. But, if you take that camera setting to the other extreme, objects an inch away from the subject will appear blurred.

A shallow depth of field means that range of distance where objects appear to be sharp is very small. For example, a shallow depth of field could mean objects that are farther than an inch from the subject are blurred. In images with that narrow range, the background appears very out of focus.

A wide depth of field, on the other hand, means that range is wide. With a wide depth of field, even objects several feet from the subject would still be sharp. Of course, one inch and several feet are just examples — the exact range varies, but short ranges are called narrow and long ranges are referred to as a wide depth of field.

Depth of field also goes both ways — if an object that is six inches behind the subject is in focus, an object six inches in front of the subject will also be in focus. If you are taking an image of three objects at three different distances — like a group photo with three rows, for example — focusing on the middle object will help you use that entire range of focus. Depth of field influences the foreground, or the front of the image, just as much as the background.

Aperture and depth of field (DOF)

— Photo: Mar Washburn, Student

Aperture allows the photographer to control just how large (or small) that range of distance is. As one of three different exposure settings, aperture influences how much light comes into an image by changing the size of the opening in the lens. A larger opening in the lens creates a brighter image and vice versa.

But as the lens’s aperture or opening gets wider, the distance that produces a sharp object gets shorter. A very wide aperture, like f/1.8, produces an image with a very narrow depth of field where even objects that are an inch away are a bit fuzzy. A portrait taken by focusing on the subject’s eyes at f/1.8 will have the subject’s nose — and maybe even the tips of their eyelashes — out of focus.

As the lens aperture shrinks, the range of distances that will produce a sharp image gets wider. With a smaller aperture, the objects further from the subject will come into focus. An aperture of f/22 will let in very little light — but it will also keep most of the scene in focus. Objects a few feet from the subject will appear just as sharp.

F/1.8 and f/22 are extreme examples — there are plenty of apertures in between as well. If you are back taking that portrait and you want the eyes and the nose to be sharp, you can narrow down that aperture a bit to get the entire face in focus and still get a decent amount of background blur.

What else influences depth of field?

Aperture is one of the biggest players when it comes to depth of field — but while aperture may be the MVP, it’s not the only player on the court.

Focus distance also plays a role in an image’s depth of field. The closer you are to a subject, the narrower your depth of field becomes. Macro photographers often use very narrow apertures because they are so close to the subject. An f-stop of f/2.8 used on a close-up of a bumble bee may leave his eyes sharp but his stinger would be lost in the blur. That’s because the closer you are to the subject, the narrower your depth of field is.

Farther from the subject, that depth of field narrows back down — if you are using the same focal length. Using a zoom lens has the same effect as getting in close to the subject: it narrows down the depth of field, leaving the background blurred.

A common misperception is that it’s the zoom lens that actually creates that background blur — but if you took a photo from close up, then used a zoom lens to create that same composition from far away, the images would have an almost identical depth of field. Zoom lenses do not add to the background blur, but they do create the same level of blur as if you were shooting up close to the subject. So while an f-stop of f/1.8 is impossible to find on a 300mm zoom lens, an f/4 is often sufficient because the lens’ zoom qualities will already create a narrow depth of field effect — just know that it’s because of distance, not the lens itself.

Since objects within that depth of field range are sharp, the farther an object is from the subject, the blurrier it will look. If you take a picture of a bird in your yard but the nearest trees are 50 yards away, those trees will blur simply because they are so far from the subject. While a wider aperture would blur those trees more, distance also impacts the level of background blur.

There’s one more factor that affects depth of field, but it’s out of your control unless you update your camera: sensor size. The sensor is the digital equivalent of a piece of film. The larger the sensor is, the narrower the depth of field is or the blurrier the background is. That’s why it’s easier to get a photo with a blurred background from a DSLR and much tougher to do with an iPhone. While the iPhone has a fixed f/2.8 lens which is pretty wide, it has a small sensor which makes it harder to achieve that background blur. In the same way, a Micro Four Thirds camera will have less background blur than an APS-C sized sensor and even less than a full frame sensor.

Application & Troubleshooting

Understanding aperture and depth of field allows photographers to get creative with blur — like keeping the details of a stunning landscape intact with a narrow aperture or blurring out the distractions in a portrait with a wide aperture.

But, knowing how aperture and distance play a role in depth of field is a tremendous tool for troubleshooting. The tricky group photo is a perfect example. Say you have three rows of people (which means they are at different distances from the camera), and the front row is in focus but the back row is blurry.

First, you can make sure that you’re autofocus point is placed on the middle row. If you focus on the front row, you are wasting half of your depth of field because it’s in front of all your subjects.

If you are focusing on that middle row and still getting background blur, use a narrower aperture. For group photos, an f/8 or higher is often necessary.

Now pretend you have a much smaller group — just two people. If you don’t want to use a narrow aperture, perhaps because you are shooting with limited lighting like inside a church, you can place those two people the same distance away from the camera to get them both in focus with a narrow aperture. Since millimeters come into play with apertures as wide as f/1.8, it’s not a good idea to go super wide or one of them could be just out of focus, but you don’t need that same f/8 that you used when you had rows of people at varying distances.

Narrow apertures also make it easier to get a sharp focus. If you notice your action shots are a little soft, try narrowing down that aperture. By putting more of the image in focus, you’ll have a better chance of nailing that focus, even when you have very little time to focus the shot.

Understand how distance plays a role in depth of field allows photographers to work within the limitations of their own gear. Say you are shooting a portrait against a brick wall, but you only have your kit lens and you can’t get any wider than f/3.6. If you aren’t happy with how sharp that brick wall looks, simply move the subject farther away from the wall.

Since the distance between the camera and the subject also plays a role, you could instead move closer to the subject or use a zoom lens. Of course, moving closer will change your composition, which is why having those wide apertures available is a nice option to have.

Aperture and buying lenses

The lenses in your camera bag may not be able to get as wide as f/1.8 or as narrow as f/22. Most kit lenses have a maximum aperture of f/3.6 — you can’t shoot any wider than that because the physical parts necessary to get that wide aperture are simply not there.

Upgrading to a lens that can achieve f/1.8 will allow for more background blur as well as better low light performance, especially over a kit lens. But, the wider a lens gets, the bigger the price tag becomes.

Prime lenses, or lenses that have a fixed focal length without zoom capabilities, are often a good alternative because they have a wide aperture range without an overwhelming price tag. Prime lenses aren’t as convenient as zooms, but for applications without quick moving subjects such as portrait work, they offer a lot of aperture flexibility for the price.

As the lens’ zoom increases, it also becomes much harder to find wide apertures — it becomes physically impossible to stop down the aperture that much. Remember that zoom lenses imitate the effect of getting in close to your subject, so not having access for an f/1.8 on a telephoto lens isn’t such a big deal.

Aperture: Things to Remember

Aperture can be a confusing concept to remember for beginners. If there’s one thing beginners need to remember about aperture, it’s this:

Wide aperture = narrow depth of field = out of focus background
Narrow aperture = wide depth of field = sharp background

Here’s a memory trick: The size of the aperture is the opposite of the range of distance that will be in focus. Wide apertures have a very small range of distance that’s in focus, while small apertures have a large range of distance in focus.

Just like learning shutter speed or ISO, heading out with your camera and trying out the different aperture settings will help all those details about aperture and depth of field to become second nature. Experiment with using different distances to adjust the level of background blur too.

Another way to help remember aperture and depth of field? Bookmark, pin or share this guide for easy reference later.