You've seen that look before in portraits. The people in the photo are tack sharp, but the background is out of focus? No matter what you try, you can't get that look. How did the photographer do it?
Depth of Field & Bokeh
(I'm getting into some technical information first. If you just want to find out how to do it and don't care how it works, you may want to skip this part.)
The photographer achieved that look by using depth of field. What is depth of field? The textbook definition would be something like "the adjacent to the focal plane that appears in focus when viewed at the distance from the focal plane to the camera sensor."
Whoa! That's a mouthful! It can be explained better with an example.
In the image above, the subject of the photograph is standing 12 feet from the camera sensor. We have adjusted the camera focus so that anything 12 feet from the camera sensor is in crisp focus. The focal plane is at 12 feet. However, due to the way that optics work, the entire area between 10 feet and 14 feet is also in focus, although not quite as crisp at 12 feet. This area, from 10 feet to 14 feet, is our depth of field. The background object, since it is outside the depth of field, will be blurry in the photo.
So, the "secret" to keeping the subject in focus while blurring the background is to make sure that the subject is within the depth of field and that there is background that is outside the depth of field.
Frequently, when you read about depth of field, you may see the term "bokeh." (Some people pronounce it bo-kay, some bo-keh, and some bo-kuh. I fall in the bo-kuh camp.). Think of bokeh as the quality of the background blur. "Perfect" bokeh, in my opinion, looks like buttery smooth balls of light. You get some of the best bokeh when the background has lots of contrast, such as lights on a Christmas tree or sunlight filtering through a forest canopy.
Factors that Affect Depth of Field
If you ask any photographer what affects depth of field, I can almost guaranty that they will answer, "Aperture." There are more, but aperture is the factor that is the easiest to control.
Why does aperture affect the depth of field? To avoid going too much into optics and light, think about a dodge ball game.
Instead of a regular dodge ball game, this game is played inside in a gym, and there is a wall going down the middle of the gym. One team on each side of the wall.
There's a gap in the wall, and the size of this gap can vary. Imagine the gap is small. In this situation, the balls that manage to get through the gap will be thrown along paths that are parallel to one another.
As we widen the gap, however, balls thrown in many more directions can make it through the gap.
In this thought experiment, the gap in the wall is the aperture in the camera. The balls are beams of light. When the aperture is narrow, the light that makes it through tends to be "more direct" than the light that makes it through the wider gap. (See image above). This indirect light tends to produce a blurry image while the direct light helps form a tack-sharp image. Thus, when you have a smaller aperture, you have more depth of field than when you have a large aperture
The next factor controlling depth of field is the distance between the camera and the photograph's subject. In a nutshell, the closer you are to the subject, the more shallow the depth of field.
If you can't control your camera's aperture, then this will probably be the factor most in your control. If you're unable to get the background out of focus, try moving closer to the subject of your photograph.
In the two photos above, the only thing that changed between the photos
The greater the distance between the subject and objects in the background, the more likely the background will be outside the depth of field.
This makes sense if you think about it. Look back at the first illustration in this article. If the background object was six inches away from the subject, it would be within the depth of field, and it (or at least part of it) would be in focus. By increasing the distance between subject and background, we can place the background outside the depth of field.
When you hear someone talk about a 50 mm lens or a 200 mm lens, they are referring to the focal length of the lens. The focal length is another one of the ways you can control your depth of field.
Look at the two photos above. On the left, you have a photo taken with a focal length of 50 mm. On the right, the photo is taken at 105 mm. While the house and trees in the background are outside the depth of field on both photos, the depth of field is more shallow on the photo on the right, resulting in more blur of the background.
If you are a regular reader of this blog, you may remember small camera sensors often have more noise than larger sensors.
Small sensors have something else going against them as well. It is easier to get shallow depth of field with a larger sensor than it is a smaller sensor.
Think about photos you've taken with a smartphone or a pocket camera. Very rarely, if ever will you ever see a photo with shallow depth of field that was produced by one of those cameras.
"Wait a minute, Tim!" you say. "My iPhone has portrait mode, and it has shallow depth of field."
Wrong. Not even Apple can change the laws of physics.
iPhones (and any other smartphones that have similar functions) do not have special small sensors that somehow achieve a shallow depth of field. The iPhone's portrait mode works by using a wide-angle lens and a telephoto lens and creating something called a "depth map." This depth map determines how much blur should be digitally applied to the image. Here's all the details on portrait mode if you're curious.
You can even find apps that will apply blur to create similar effects in smartphones without two lenses.
How to Get Shallow Depth of Field
To summarize, if you want to achieve shallow depth of field in your photos, here's what you need to do:
- If you can choose between cameras, choose the one with the largest sensor
- Use a larger aperture
- Stand as far away from the subject as possible
- Have the subject as far from the background as possible
- Use the longest focal length possible
Sometimes, though, let's face it, you can't control one or more of those factors. You simply may not be able to get a shallow depth of field with the equipment and conditions you have.
For those with smartphones, and you don't have a "portrait mode" on your phone, you could try out one of the many apps designed to blur the background and imitate a shallow depth of field.
Or you could hire a photographer. :)