Basic Photography Article - #2 Aperture and Artistic Uses of Depth of Field Controls
Depth of Field
In my last article, I talked about the exposure triangle, and mentioned that in addition to its effect on exposure, the aperture (or f/stop) is one of the determinants of depth of field. Controlling depth of field allows you to determine what is in focus, and helps direct the viewer's eye to what you feel is important. Learning how and why to control depth of field will really help you make more powerful images that say what you intend.
So what is depth of field anyway? Depth of field is the distance between the nearest and farthest objects in a scene that appear to be in sharp focus. Technically only one plane is exactly in focus, but the optical principles of lenses allow for a greater area to be in acceptable focus. Within this area of acceptable focus, 1/3 is in front of the point of focus, and 2/3 is behind the point of focus. So when we talk about "Shallow depth of field" we mean only a limited area is in focus. "Deep depth of field" means a greater area is in focus, in some cases from close to the camera all the way out to infinity. And why should we even care about depth of field? One important reason is that the eye is drawn to objects that are in sharp focus, and tends to skip over things that are soft and not in focus. So right away you can see what a powerful tool this can be in composing a picture. If you want your viewer to pay attention to something, make sure it is in focus. If other elements in the picture are not important, putting them out of focus helps keep the attention on the main subject.
The things that affect depth of field are aperture, focal length of the lens, distance between the camera and subject, and to a degree even sensor size. Let's look at each one below.
Aperture is the first control most people thing about for controlling depth of field. Wide apertures (low f/stop number, such as f/2 or f/2.8) produce a shallow depth of field, and are often used in portraiture to blur the background, and keep the focus (bad pun intended)on the subject. Smaller apertures (high f/stop number, such as f/16 or f/22) produce a deep depth of field, and are often used in landscape photography to allow everything from near to far to be in focus, and thus be important within the picture. See the chart below.
Depth of field
Focusing distance also affects depth of field. At very close focusing distances, depth of field is very shallow. This can be used isolate a subject, but in macro photography where focusing distance may be only a few inches, it can be difficult to get all of our subject in focus. You will often see extreme close-ups shot at f/32 or f/45 in an attempt to get the whole subject in focus. See the chart below.
Lens Focal length
Lens Focal length also affects depth of field. The longer the focal length, the shallower the depth of field. The wider the lens, the greater the depth of field. That's why longer lenses (and large apertures) are often used for portraiture to blur the background and make the subject stand out. This out of focus blur has the fancy name "Bokeh" and is important to portrait shooters. Some lenses have soft creamy out of focus highlights while others have harsher highlights and more chromatic aberration (color fringing errors). Generally the more expensive the lens, the nicer the bokeh. On the other hand landscape photographers often use wide angle lenses (and small apertures) for depth of field that goes from close to infinity. See the chart below. Notice that at the same focusing distance and aperture as focal length increases, depth of field decreases.
Sensor size also affects depth of field. Without going into a bunch of complicated math, the smaller the sensor, the greater the depth of field at a given aperture and magnification. A point and shoot with a tiny sensor will have great depth of field at even f/5.6. DSLRs with APS-C sensors will have more depth of field than a full frame DSLR. Now that you know this, there's not much you can do to change it, other than change cameras. Portrait and fashion shooters prefer full frame DSLRs or the still larger medium format cameras to create that out of focus bokeh. But the smaller sensors have an advantage where getting good depth of field in low light or at high shutter speeds is important.
This image was shot at f/2.8. Notice the very shallow depth of field due to both the large aperture and the close focusing distance.
This image was shot at f/22. Notice the much greater depth of field even though the focusing distance is the same.
This image was shot at f/2.8. Notice the very shallow depth of field due to both the large aperture and the close focusing distance. Only the very front plane of the face is in focus.
The shot below shows more depth of field at the same focusing distance, but f/11. Still the whole figure is not in focus.
Here a wide aperture (f/2.8) and a telephoto lens create a shallow depth of field to isolate the subject.
Here a large aperture (f/2.8) and very close focusing distance create a shallow depth of field that isolates one daffodil from the ones just inches behind it.
Shooting at f/4 with a moderate telephoto allows the background headstones to blur, but still remain recognizable.
f/16 allows everything in this image to be in focus, even with a moderate telephoto lens.
For a given focal length and aperture there is a focus distance that provides the widest possible depth of field. This is called the “Hyperfocal Distance”. It is basically the closest focus point to the lens where the far limit is infinity, giving us total depth of field from this point to infinity. Moving beyond the hyperfocal distance continues to increase the near limit but the far limit is always infinity, so the overall depth of field reduces. Remember that within this area of sharp focus, roughly 1/3 is in front of the point of focus, and 2/3 behind. Hyperfocal distance is often used by landscape photographers to get the deepest possible depth of field so both near and distant objects such as the horizon are sharp. See the example below. At a focus distance of 3m the far limit is 16.8m. As we move towards 4m both the near limit and the far limit get further away until we reach a point at 3.65m where the far limit becomes infinity. This is our hyperfocal distance.
There are 5 possibilities for finding the hyperfocal distance:
1) Use the depth of field preview button (some models call it the stop down button) if your camera has one. This closes down the aperture to the set value and provides you with a visual check. When you use the depth of field preview, your viewfinder will become increasing darker with smaller apertures, making it hard to focus. Focus before using the preview.
2) Use the depth of field scale if your camera has one. In the bad old days of film cameras, single focal length lenses and manual focus, lenses had aperture and distance scales that could be used to determine hyperfocal distance. I can't think of any current DSLR lenses that have these scales.
3) Use a depth of field calculator chart which shows the hyper focal distance for any focal length and aperture combination. See the chart below. There are charts for full frame cameras, and cameras with smaller sensors and multiplication factors such as 1.6 for Canons or 1.5 for Nikons.
4) Use a DOF app for your phone or tablet. There are lots of good ones, and many are even free.
5) Focus on something roughly 1/3 of the distance into the picture and use a small aperture such as f/16 or f/22. This is my most commonly used method, and I also use the depth of field preview button as a quick double check.
In these two pictures I focused roughly 1/3 of the distance into the picture and used a small aperture to keep everything in focus.
Your Practice Assignments
1) Read your camera manual and be sure you can set aperture priority mode, and how to operate the depth of field preview. All of the following exercises will work best on a tripod, both so that your distance to subject stays constant, and because as your aperture is set smaller, shutter speeds will become longer, often to the point where you will not be able to hand hold steadily enough. These sound really boring, but they will really increase your understanding of creative controls.
2) Line up 6 cans or bottles about 6 inches apart, in a line going away from you. Set your camera in aperture priority mode. Start at your lens' widest aperture (f/4 or f/5.6) and if you are using a zoom lens use the shortest focal length. Place your camera 1 foot away from the closest can, focus on the closest can, press the depth of field preview to see the effect and shoot a picture. Repeat focusing on the second can, then the third etc.
3) Repeat the above exercise in the middle of your zoom range and then again at the longest focal length.
4) Repeat exercises 2 and 3 but back up to 3 ft away.
5) Photograph a person with a medium telephoto at the largest aperture of your lens, then with each picture stop down one stop until you are at the smallest aperture. While you are doing this, focus on the eyes, and if the person is not facing you squarely, focus on the nearest eye. 6) Find a scene with a close object, a middle ground and a distant horizon. Use a short focal length and focus about 1/3 into the picture. Shoot a shot at the largest aperture of your lens, then with each picture stop down one stop until you are at the smallest aperture. Notice when both near and distant objects come into focus.
Next article we'll take a look at shutter speed and how it can be used to creatively show or freeze motion.
Also after my article last month, I found this handy chart showing the effects of aperture, shutter speed and ISO. You can download it here. Print it out and laminate it and keep it in your camera bag as a quick reminder.