Depth of Field

By Steve Thomas
The Powerful Photography Coach

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Depth of field

Depth of field is the range of sharp focus of a picture.  You can use depth of field to select how much of a picture is in focus.  With a short depth of field you can consciously choose a part of a picture to be in focus and thereby show that it is the most important part of the picture. If the background of a picture is distracting, and you can’t change it for some reason, making the background blurry can help to minimize the distraction.

Short depth of field	Longer depth of field

There are two things that affect the depth of field – the aperture and the focal length of a lens.  By far the easiest way to change depth of field is to use your camera’s aperture setting as shown above.

The “electronic” or “digital” zoom that many cameras have doesn’t work the same way as zooming in with a lens and doesn’t change depth of field at all – it merely crops the image so only a portion of the image is saved. We’ll talk about the disadvantage of using digital zoom later on.  I always turn off digital zoom on my camera and do any desired cropping using photo editing software on the computer.

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You can also change the depth of field of a picture by changing the aperture of your camera’s lens. With some cameras – mostly of the “point-and-shoot” variety – you may not be able to do this. The depth of field of the two flower pictures was changed by changing the aperture. The shutter speed needs to be changed along with the aperture to maintain the correct exposure. With older film cameras all of these adjustments were controlled manually. As electronic control of cameras became available a few decades ago, much of the routine work of getting proper exposure in average situations was built into the camera so the photographer could concentrate on the artistic aspects of photography and let the camera take care of the exposure details. The camera doesn’t know what the photographer’s intentions are, so manual override of shutter speed and aperture was needed to give the photographer freedom to compose pictures that aren’t average.

Aperture

Aperature is the opening in the lens that allows varying amounts of light to reach the film or sensor. The smaller the opening the greater the depth of field and the more of the picture will be in focus. If you are taking pictures in bright light, the aperture will generally be smaller to get the right amount of light to the film or sensor. As the light level drops, the aperture has to be bigger to let in enough light for proper exposure.

Focal length

Focal length is a measurement of how much distance it takes to cause the light entering the lens to converge at the focal point at the film or sensor. For a wide angle lens this distance is short which causes the depth of field to be deeper than for a telephoto lens.

Picture taken with wide angle lens	Picture taken with telephoto lens

These two pictures were taken with the camera on a tripod so the position didn’t change. The picture on the left was cropped from a picture taken with a wide angle lens so it showed the same image as the one taken with the telephoto lens. Except for some image degradation, that probably won’t be visible over the Internet, the images are the same. If I want to get a smaller depth of field to blur the background, I would need to move closer to the subject and use the wide-angle lens or select a larger aperture such as f/2.8 instead of f/16 or both.

The following two pictures were also taken with wide angle and telephoto lenses. In this case, however I changed position between the two shots so the subject was framed approximately the same. Examine the two pictures and you can see some dramatic differences. For the telephoto shot, the depth of field is far shorter so objects in the background are blurry. Also, the subject looks flatter in the telephoto picture. If you want to see larger versions of the images, just click on the image or the title and a new window with just the image will open for you. You’ve probably seen pictures of people’s faces taken very close so their nose looks too large for their face. You can now make a conscious choice how you use distance and your zoom lens to achieve the result you want.

Picture taken with a wide angle lens close to the subject	Picture taken with telephoto lens further away from the subject

Most, if not all, automatic cameras have a method they use internally to choose the shutter speed and aperture. Since camera movement causes the overall picture to be blurry, the camera will first try to get a reasonably fast shutter speed and once the light level is high enough, it will then make the aperture smaller to get proper exposure. Usually the choices made by the camera result in a good picture, but not necessarily the best picture. Knowing the power of the camera’s settings to improve your pictures gives you the freedom to create better pictures.

The Disadvantage of Digital Zoom

Digital zoom works by expanding a portion of the image captured by the digital image sensor to fill the frame. The image sensor has a certain number of pixels on its sensing surface. When you use digital zoom, the image is created from a portion of the center of the sensor which reduces the effective resolution of the picture. If you think you are going to crop the picture to get the image you want, do the cropping using your computer picture editing software, not in the camera. Once you have thrown away part of the resolution of the picture, you can’t get it back and your picture won’t be as sharp as it could be. 

Go Out and Play

Now, go out and play with your camera to practice changing the depth of field. Try to use everything you have learned so far in these lessons to take pictures that will make your proud.

If you have pictures you would like me to see, you can e-mail to me at pictures@goodphotographyinfo.com. If you would like me to make comments and suggestions, please include the words “comments please” in the subject line of the e-mail. Right now there is no charge for this service. That may change in the future

Lesson 1 Subject Placement
Lesson 2 Leading Lines
Lesson 3 Light and Shadow
Lesson 4 Point of View
Lesson 5 Available Light
Lesson 6 Outdoor Lighting
Lesson 7 Close-up Photography

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(c) Steve Thomas 2008-2010 All rights reserved. This is copywrited content and may not be reproduced in any form without the express permission of the author.