Depth of field Explained

In an image the Depth of Field is the area from front to back, that is in sharp focus. The main control over depth of field can be obtained by using the aperture setting. Other factors that have less of an affect are zoom, distance from the subject and the size of the cameras’ image sensor.

Sensors in a compact camera tend to be small, so the image is squeezed in to a smaller space creating smaller pixels, which has a tendency to make the picture look sharper.

To create a shallow depth of field in a compact camera it is necessary to use extremes. A good use of a shallow depth of field is to try and separate the subject from the background, so if yo

u are taking a photo of the family on a day out, don’t line them up in front of a busy background, bring them away from it and if possible get the camera closer to them than they are to anything behind. If the camera has aperture priority (AV) use that and with the aperture set to the lowest value, usually f2.8, make sure to fill the screen/viewfinder and shoot your picture. Using this technique should blur the background while keeping the subject in sharp focus, and giving the shot a professional look about it.

Bridge Cameras and DSLR’s offer greater control over depth of field, this is mainly down to larger sensors and greater zoom capabilities.

The shot on the left has everything you need for creating a shallow depth of field. This a great example of how the use of a long lens creates a shallow area of sharp focus. The picture was shot at 300mm, which magnifies the image making all the differences in focus easier to see. If this image was taken at 50mm, more of the surroundings will have been included and the subject and surrounding detail made smaller in the frame. This would have the affect of sharpening the image, making it harder for the eye to pick out the different areas of focus.

Learning to control depth of field can greatly improve the look of the pictures you take. separating the subject from the surroundings and background helps to focus attention on what you want the viewer to see.

On the next page you will see how the aperture setting can be used to control the depth of field in your photos.

Controlling Depth of Field

In the images below the difference in the sharp focus depth is very pronounce. The images were both taken using the same camera, with a Sigma 50mm prime lens so the images couldn’t be affected by zoom, and from the same position. So why are they so different.

The first of the two images was shot using an aperture size of f16, which is a mid range setting using this particular lens. Looking closely at the picture it is possible to see that the focus starts to run out on the last bottle. To bring the last bottle in to sharp focus a setting of f22 would do the trick.

In the second picture the area of sharp focus is drastically reduced by setting the f number to f2.8. The first bottle is in sharp focus but the depth of field is so shallow that even the second bottle is out of focus, and from there the amount of blur gets very rapidly worse.

Now that you understand how to control depth of field, when should this new skill be used?

For a shot outdoors of a landscape it is best to use a higher f number like f16/f22 to increase the depth of field, so more of the distant details stay in focus. When focusing the camera for this type of shot, focus on something in the foreground as focus is harder to perceive as the distant details get smaller, and therefore the depth of field will appear greater.

A shallow depth of field is useful when taking shots of people, it can be used to blur out busy or unsightly backgrounds. For full body shots use a slightly higher setting of f8 or so, some trial and error will be necessary to get the perfect setting for the camera you are using. With the Canon 400D a setting of f8 is good for portraits as it gives a depth of field that slightly blurs the background but keeps the whole head and face in focus. As in the second picture above, the depth of field can be very shallow, which means a lower setting could cause an image focused on the eyes and nose to leave the ears blurry.

Whether the f number is high or low, shoot with the camera mode set to aperture priority (AV which stands for Aperture Value) which will leave the camera to take care of the other settings.

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