Aperture & Depth of Field

In the last several posts we talked about how to change the brightness or darkness of an image using three different settings: aperture, shutter speed and sensitivity (a.k.a. ISO). We established that you can brighten or darken a photo equally well by changing any of these three settings. Now the question is, how do you decide which setting to change?

If aperture, shutter speed and ISO only affected exposure, then it wouldn't matter which ones you changed. However, all three of these settings significantly affect your images in many other ways beside exposure. Let's start by talking about aperture and its affect on depth of field.

Simply put, "depth of field" refers to how much of a photograph is in focus. Ever seen a portrait where only the person's face is in focus and the background is just a pleasing, out of focus blur? This is "shallow" depth of field. Ever seen a landscape where the blades of grass a couple feet from the camera and the mountains miles in the distance are all equally in focus? That is very "deep" depth of field.

Got the idea? Ok, now let's revisit the definition above, since its a little bit of an oversimplification. Depth of field does not literally mean what percent of the image area is in focus (as in half the image is in focus, the whole image is in focus, etc.). Depth of field really refers to how quickly in-focus areas of an image transition to out-of-focus areas in relation to distance from the camera.

There's a lot on info in that sentence, so let's use an example. Let's say you focus on an object 3 feet from the camera. Let's also say that in the resulting photo, everything that was between 2.5 and 6 feet from the camera was acceptably sharp, or in other words, in focus. That area, from 2.5 - 6 feet is your "depth of field". With me?

Ok, now let's bring in aperture. Aperture is perhaps the most important setting for determining depth of field. A wide aperture (low f-number) such as f/2, f/2.8, or f/4 gives you shallow depth of field. A narrow aperture (high f-number) such as f/11, f/16, or f/22 gives you deep depth of field. Let's look at some examples:

I shot the above photo with an aperture of f/2. I focused on the second pear from the left, and as you can see that is almost the only thing in sharp focus. The background and foreground are totally blurred, and all the other pears are fairly blurry, especially those farthest from the camera.

The next image was shot at f/4, two full stops difference from f/2. Notice that the depth of field has increased. A deeper area in front of and behind the second pear (where I focused) is now acceptably sharp. The foreground is still pretty blurred, as is the background.

Note: Since I changed the aperture two full stops, I also changed the shutter speed two full stops to keep the exposure level the same. I did the same thing for all the test shots in this post so we can see what we're doing here.

This image was shot at f/8, another two full stops difference. Notice how all the pears and some of the foreground are in focus and the background is starting to be recognizable.

The final image was shot at f/16, a change of another two full stops. Now this is deep depth of field. Foreground to background we are pretty much sharp throughout the whole image.

Bottom line, you can control depth-of-field using aperture, and this is an important consideration when changing your exposure settings. Use a low f-stop such as f/2.8 or f/4 for shallow depth of field to isolate a subject and blur a boring background, or use a high f-stop such as f/16 or f/22 to get front to back sharpness if you want to preserve every detail.