Some focus on out-of-focus
During a one-on-one workshop recently we were paying a lot of attention to the subject of depth-of-field. My fellow photographer liked the way her young son looked in a photograph when the background was soft . . . a description often given to backgrounds that are way out of focus. She wanted to know how she could get that affect in her photos. So I set out to explain some of the things that impact depth-of-field. Sounds simple enough for those of us who have already become familiar with many photography techniques and terms. Not so easy to explain to someone relatively new to photography and what you can and cannot do with your new digital SLR (DSLR).
I Started With Aperture:
Aperture is just one thing to consider. What’s aperture? It’s the size of the opening in your lens that let’s light get to the film or digital sensor. (By the way, for the purposes of this discussion, I’ll stick with digital world and refer to the sensor rather than film. For our purposes here, sensor and film are the same thing.)
So, let’s get to it. Most lenses include at least a half-dozen of these opening sizes, or apertures, that we call f-stops. As an example, you might find that your lens has a half-dozen f-stops. It’s common to find at least these f-stops: f2.8, f4, f5.6, f8, f11, f16 and f22.
I like to think of the f-stop numbers as fractions that describe the sizes of the openings. So, I think of them as f-1/2.8, f-1/4, f-1/5.6, f-1/8, f-1/11. f-1/16 and f-1/22. This way, it’s easier for me to remember which f-stop is larger or smaller; which f-stop will let in more light or less light. A fraction that is 1/4 is larger than a fraction that is 1/16. Therefore, a f-stop of f4 (f-1/4) let’s in more light than a f-stop of f16 (f-1/16).
A simple analogy might be to think of the aperture like the diameter of your garden hose and light like it’s the water flowing through your hose. The bigger the diameter of your hose, the more water that flows through it.
Shutter speed in one more thing to consider. Most people readily understand the concept of shutter speed. They find it easy to understand that a shutter speed of 1 second is longer than a shutter speed of 125th of a second just as a shutter speed of 125th of a second is slower than a shutter speed of 250th of a second. It follows that the faster the shutter speed, the less light your lens will let in.
To pick up the garden hose analogy again. Shutter speed determines how long the water flows through the hose.
Aperture and Shutter Speed as a Team:
So, now we can think of the aperture (hose size)and shutter speed (the nozzle on the end of the hose) as a team. They have to work together to be effective. Simply put, it’s the combination of f-stop and shutter speed that determines how much light reaches the sensor.(The longer you keep the nozzle on your hose open, the more water flows).
The aperture/shutter speed combo (such as f2.8 at 250th of a second) is typically referred to as a “Stop.” On most DSLR’s today you can change your settings in one-stop, 1/3 stop, or 1/2 stop increments. For the sake of this illustration we’ll stick to one-stop (or full-stop) increments.
Let’s see how it all works. We’ll begin with a setting of f2.8 at 1/250th of a second. This is but one of many combinations that will determine a specific amount of light reaching the sensor.
If your light meters tells you this is the setting for a correct exposure, that simply means the” right amount of light” is reaching the sensor. As we’ll see in a moment this might also be a great setting if your objective was a soft or out of focus background. More on that later.
Now, let’s see what happens if we want to change our settings, yet not change the amount of light reaching the sensor. We’ll go with a slower shutter speed, say 1/125th of a second and that, naturally, lets more light in because it keeps the shutter open longer. If that’s the only change we make, our image will be over-exposed by to much light reaching the sensor. So how do we see to it that just the right amount light reaches the sensor, the same amount of light as with the f2.8 at 1/250th of a second? It’s really simple, if you think about.
We changed the shutter speed from 1/250th of a second to 1/125th of a second, so we have doubled the amount of time the shutter is open, thus doubling the amount of light reaching the sensor. So, now we use our f-stops. By changing f-stops from f2.8 (our largest in this example) to f4, we reduce the size of our opening by half.
Now that’s interesting isn’t it. We decreased the shutter speed by half (from 1/250th sec. to 1/125th sec.), which effectively doubles how long light pours into the lens and now we’ve decreased the size of the opening that’s letting the light in by half. So, half as much light is streaming into our lens but we’re giving it twice a much time to make the trip.
As straight forward as that may seem, it’s key to capturing images with soft backgrounds. Simply put, the first example, f2.8 at 1/250th of a second gives us the least depth of field with the options we have with this hypothetical lens. Focus on something in the foreground (like the lady in my workshop’s son) and if the background is far enough away it’ll be out of focus. How far behind her son that background is, will help determine how out of focus it will be.
Another important consideration:
Your lens makes a difference, too. We need to keep in mind that the longer the lens, the shallower the depth-of-field. For example, if you have a 200 mm lens on your camera, with a f-stop of f2.8 you’ll have a very shallow depth of field. However, even with a tiny f-stop (such as f22) with a 200 mm lens you won’t be able to keep a distant background and the foreground both is sharp focus.
The reverse is true if you have an 18 mm or a 24 mm mm lens on your camera. It will provide you with significant depth-of-field, even at f2.8.
With a 200 mm lens, frame your subject with someone or some thing in the foreground, set your f-stop to f2.8 and you’ll quickly see the background go soft. That’s what I did to get the soft background behind this image of a butterfly. Conversely, when I wanted more depth so that the two deer in background and the tree trunks would both be acceptably sharp, I use my zoom lens at 120 mm with an aperture of f22. To illustrate the process better I shot this image with the pine cones in the foreground with my lens set t0 18 mm at an f-stop of f22.
If you have been wrestling with f-stops and depth-of-field, I hope you find this helpful. Please feel free to use the comment section below to ask any questions.