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Controlling Depth of Field
Have you ever looked at a picture that had a very narrow field of focus where everything was blurred beyond what you were looking at, and wondered how they did it? Was it a special camera? Was it done in Photoshop? Why is it some pictures are sharp through the entire frame, while others have blurred aspects? I hope to answer these questions and more with this installment of Tips and Tricks.
The term used to describe how blurry or sharp an image is portrayed is Depth of Field. Those that have been reading Photo Trek for any length of time have heard this phrase before, but may not know exactly what it means. One of the tools that photographers use is depth of field. There are times, such as portraits, when you want to draw the attention of the viewer to a specific element in the photo. This can be done by making the image appear out of focus, except for that one element which is represented in sharp focus. Other times, such as landscapes, the photographer wishes to represent the entire picture in sharp focus. Both of these tasks can be accomplished with the same camera.
How is this possible? Depth of field is a result of controlling the aperture of the camera. The aperture is the actual opening inside the lens that allows the light to pass through. It is often referred to as the f-stop. The concept of f-stops takes a little while to get used to as the numbers seem backwards. When the lens is completely open (letting the maximum amount of light in) the aperture is said to be f/1. As you close the opening in the lens the number gets higher, such as f/32. The thing to keep in mind is that the shutter speed works in unison with the aperture. As you stop down the aperture (making the opening smaller), you must slow the shutter speed an equal number of stops. This will allow the same amount of light to enter the camera and keep you exposure the same. Conversely, if you open the aperture up, then you must increase the shutter speed to maintain the correct exposure.
Now that the basics are covered, on to how aperture affects depth of field. The wider the aperture, the more narrow your depth of field will be. That is to say that only a small portion of the picture will be in clear focus. The rest will be progressively blurred with distance. This is good if you want to highlight a certain element in the photograph and draw attention to it.
Blooming Stages Photo Copyright © 2006 G. Kiser
In the above picture, I wanted the white flower to stand out against the background. With everything in focus, there would be just too much confusion in this image. I used a wide aperture setting (f/2.8, if I'm not mistaken) which decreased my depth of field. I focused on the center stem of the flower using my manual focus ring. The same could have been done using a spot auto focus setting on the camera. The resulting image showed a clearly focused subject with the background blurred just enough to reduce its visual impact.
On the other end of the spectrum, there is maximizing your depth of field. This is accomplished by stopping down the aperture to a higher number. This allows the photographer to capture more of the image in apparent focus. I say "apparent" because there is only going to be one plane of sharp focus in the picture--ever. using a narrow aperture gives the illusion of crisp focus throughout the image.
Peaceful Photo Copyright © 2006 G. Kiser
In the above picture, it was important to me to have both the foreground and the waterfall appear in focus. This called for a maximum depth of field. As it turns out with waterfall photography, you most often want to shoot with a narrow aperture. This particular shot was done at f/8 and the focus point was the first "step" in the stream. Why there you ask? Remember, I said that there will be only one plane that is in absolute focus. To gain maximum depth of field without having to calculate a hyperfocal distance, you should focus on a point about 1/3 of theway into the scene. The camera will focus a short distance in front of that point and about twice that behind that point. If you pick your spot right, the entire image will appear in focus, even though we know that is not technically possible.
It is not always as black and white as high number=maximum depth of field, low number=shallow depth of field. Another aspect that must be kept in mind is your focusing distance. The further out your subject is, the larger the depth of field will appear at any given aperture. for instance, in the following picture, I was probably a mile or more from the subject of this photograph and a good distance above.
Doughton Park Photo Copyright © 2006 G. Kiser
Due to being so far away from all that I was photographing I was able to use a wider aperture while still gaining full depth of field. I used an aperture of f/5.6 for this shot. This was all that was needed to provide sharpness front to back. Simply put, the further the subject is from the camera, the more depth of field will be produced for any given focal length.
Another consideration is your focal length. As a general rule, the wider the focal length, the wider the depth of field. For instance, if you are shooting a broad landscape using a 28mm focal length then you can expect to have greater depth of field than if you were to zoom in on a portion of it, say using a 400mm focal length at the same aperture setting.
If you use a long focal length (200mm) and a wide aperture (f/2), then your depth of field will be extremely narrow. This is why it is important to use a narrow aperture when you are zoomed in on a subject at close range. Conversely, if you are using a wide focallength (50mm) and a narrow aperture (f/16), then your depth of field will be fairly wide.
When putting all of this to use in the field, you might have to make some concessions because of conditions. There are times that you will have to sacrifice depth of field to allow for a faster shutter speed because of wind. To keep your maximum depth of field, you would stop your lens down which would necessitate a slower shutter speed. This will cause the motion to be captured in the picture. This is not always wanted, so you can open up the aperture allowing for a faster shutter speed to freeze the motion. However, you have lost some of your depth of field. There are always tradeoffs to be made when creating a picture. You just have to decide which concessions you are willing to make.
To make use of depth of field in your camera, you will need to have the ability to shoot in either aperture priority or full manal modes. Some cameras have a program shift mode where you can control the aperture and shutter speeds together while maintaining a correct exposure. The control here is limited though. Many compact cameras have these settings, as do all SLR's (Single Lens Reflex). Refer to your owner's manual to learn how to enter these modes.
Stay tuned for my next installment of tips and tricks, scheduled for release on the 1st of November. Remember, if you have a question you would like to me answer, just ask. You can either leave a comment here, or you can email me at NCPhotoTrekker@aol.com . Happy shooting!
Written by Greg A. Kiser
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7/23/08 8:39 PM
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