Have ever seen a picture where a single flower was in sharp focus and every other flower in the picture was blurred and wondered how they did that? The answer is that you’ve seen an example of compressing the DEPTH OF FIELD, or “DOF” by increasing the size of the aperture inside the lens. Blurring certain regions of your image while others remain in sharp focus is an elegant way to direct the viewer’s attention toward the part of the picture that YOU want to feature.
Depth of Field is defined as that portion of your scene that’s in focus, measured from FRONT to BACK. If your depth of field is very deep, objects in the foreground will be as sharp as those that are far off. If your DOF is shallow as in the picture above, objects in the background AND in front of your subject will be blurred, while the subject itself will remain in sharp focus.
When you meter for a shot by pushing your shutter release button down halfway, your auto-focus goes to work finding a POINT on which to focus. When it’s finished, it’s really given you a DISTANCE from the camera that’s in focus, and everything that’s that distance from the camera is also in focus. That’s why diagrams often show the focus point as a flat plane standing like a wall in front of the camera. The Depth Of Field describes how much of the space both in front of and behind that plane that’s also in focus. The actual point of focus is really about a third of the way into the DOF, from the camera’s point of view. That means for instance that if you’re photographing people standing three deep and would like everyone to be in focus, you should focus on someone who’s roughly in the middle of the group, rather than on those in the front or back rows.
Your camera’s APERTURE, in addition to being one of the three exposure controls, is also known for the effect it has on your DOF. The word “aperture” simply means the round, or nearly round opening inside the lens THROUGH which light flows on its way to our camera’s sensor. You’ll need to remember that the SMALLER the aperture, the DEEPER the depth of field. Large apertures result in shallower DOFs. Now if that’s all we needed to know, we’d be finished: But it’s not. Our cameras don’t have markings that say “deep field” and “shallow field”. They also don’t say “large aperture” or “small aperture”. What you WILL find on your camera are mysterious numbers called “f-stops”. If you have your camera with you, turn your Mode dial to the A, or Aperture priority setting (If you have a camera that has the letters Av for aperture VALUE, choose that: It’s the same thing). Your camera should now be prompting you to choose from a series of Aperture “F-STOP” values.
Unless you’re a mathematician, don’t expect those numbers to mean much at first. F-stops are a RATIO–specifically between the lens’s focal length to the diameter of the aperture. We introduced the concept of a STOP earlier as meaning a doubling or cutting-in-half of light (depending on whether the light is increasing or decreasing), and that’s exactly what the word means here. Consider the following list of aperture values. Notice that though the numbers seem to be increasing, each number is really a one-stop DECREASE in light from the value before it: f/1, f/1.4, f/2, f/2.8 f/4, f/5.6, f/8, f/11, f/16 and so forth. Don’t worry, you don’t need to memorize the numbers. What you do need to memorize though, is the idea that the smaller the f-stop NUMBER, the larger the APERTURE – the larger the OPENING to let light through. For example: Lenses are often described by the largest opening they’re capable of achieving. An “F/1.4 LENS” has a larger maximum opening than an “F/2.8 LENS”. The f/1.4 is said to be the FASTER of the two lenses, because light travels through it faster because it has a larger opening.
Creating a shallow, or SMALL depth of field calls for a LARGE aperture, which would be a SMALL f-stop number. Some photographers shortcut that by just remembering that the smaller the f-stop NUMBER, the smaller the depth of field. However you handle that is up to you, but you do need to know how to connect the numbers to what’s happening in your image somehow.
Learning the aperture rules is important for blurring backgrounds, but they’re not the whole story. Actually there are THREE variables that contribute to your ability to compress your DOF, and your camera’s aperture setting is only one of them. We point that out because often those using the so-called “kit” lenses that come with entry-level SLRs are disappointed to find that they can’t seem to blur their backgrounds much if at all, no matter how hard they try. The reason for that is that kit lenses are typically very short for WIDE ANGLE shots that will include as much of the scene as possible, measuring from left to right. Depth of Field compression is more effective when using a LONGER LENS. That’s the second factor.
The third is focus distance, or how far you’re standing from your subject. The closer you are, the shallower, or more compressed your DOF will tend to be. I know: That’s a lot. Let’s summarize all three of those in one sentence: “To get the SHALLOWEST DOF, use the LARGEST aperture — with the LONGEST lens — at the SHORTEST distance from your subject possible”. I should warn you here that if you use all three of those at the same time, you’ll probably not be happy with the results. It’s possible to get a depth of field that’s so shallow that if your subject is a person’s face for instance, and if his eyes are in sharp focus, both his nose AND his ears will be blurred, and that’s probably not what you had in mind!
You’ve heard this before here, but let’s say it again: Remember that aperture is primarily an EXPOSURE TOOL: Compressing depths of field is a secondary function. Some photographers forget that, and shoot as if blurring backgrounds is the only important skill to know. In fact, blurring from accidental DOF compression can easily creep into images as a flaw, especially when shooting with longer lenses. Watch your shots of groups of people especially. We mentioned the need to find a person in the middle as your focus point. Don’t forget though, to zoom all the way in afterward on your review screen, checking carefully to be sure that those in the front and back rows aren’t focus-blurred by a shallow DOF, just to be safe.
Aperture Priority mode is probably the most popular of all the PSA and M settings. Blurring backgrounds is a GOOD skill to have in your tool kit, but don’t forget that NOT blurring backgrounds is an essential skill too! It pays to also keep in mind that all this attention to Aperture carries a DANGER, and it’s that you can lose sight of your camera’s choice of shutter speed if you’re not careful. Changes in lighting can easily result in slower shutters, resulting in un-fixable motion blurring. With experience you’ll begin to notice slow shutters by their sound, but always remember to check ALL of your settings in the viewfinder before taking each shot, and to double-check your results by zooming on your images in the review screen afterward. Shooting in Aperture Priority mode is fine, but if you find that you keep ruining shots with motion-blurring, it might be a good idea to switch to Shutter Priority where you’re sure to keep those shutter speeds up – and to save Aperture for special applications instead.
