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Tuesday, May 25, 2010

Isolated Sharpness

Tips and techniques for using shallow depth of field to add impact to your macro shots

This Article Features Photo Zoom

3 The single stalk of golden grass would have been lost in a sea of other stalks if Sheppard hadn’t used such a wide-open ƒ-stop.
Creating Shallow Depth Of Field
Shallow depth of field starts when you choose a wide aperture for your lens. Often, it helps to set your lens at its widest (maximum) aperture, such as ƒ/2.8 or ƒ/4, or at most, stopped down one stop from that maximum. This gives the least amount of depth of field for a given distance and focal length of lens. Avoid the middle ƒ-stops of ƒ/8 or ƒ/11 for this effect (with some lenses, one stop down from maximum may be ƒ/5.6, which is okay, but borders on the middle apertures).

Focal length has an effect on depth of field. A wide-angle lens gives more apparent depth of field than a telephoto. If you combine a telephoto lens with a wide ƒ-stop, you get even more limited depth of field for a stronger effect. Then if you close in on a subject with a telephoto set to a wide ƒ-stop, you get a very narrow depth of field.

With wide-angle lenses, it’s harder to get shallow depth of field because, even at maximum aperture, the lens gives more depth of field than a normal or telephoto lens at a similar ƒ-stop. The shorter the focal length of the lens, the more depth of field, e.g., 20mm has more depth of field than 50mm. This means that if you use a full-35mm-frame format compared to an APS-C or Four Thirds format, you use a longer focal length for any given angle of view. (For example, a 105mm lens on a full-35mm-frame-format camera gives you an equivalent angle of view and framing of 70mm on an APS-C-format camera.) The result is that a full-35mm-frame-format camera consistently gives you less depth of field.

In these diagrams, you can see how the depth of field changes with the aperture setting. With any ƒ-stop, there will be a plane of absolute, sharpest focus. On the other side of that plane is the zone of acceptable sharpness, which we call depth of field. With a bigger aperture (and smaller ƒ-stop number), the zone of acceptable sharpness is narrow. A smaller aperture (larger ƒ-stop number) gives you more depth of field.
That may be a discussion point for a camera club, but it can affect how you photograph. If you shoot with a smaller format, you get increased depth of field with a given angle of view or framing of your subject because you’re using a shorter focal length. That can be great if you want depth of field. On the other hand, it can be a challenge to get really shallow depth of field with the smallest formats, such as Four Thirds, Micro Four Thirds or with a compact camera like the Canon PowerShot G11. With these cameras, you need to get close to your subject, use longer telephoto focal lengths and use the widest ƒ-stop possible.

Background Distance

The distance to the background behind your subject is an important part of using a shallow depth of field, yet it’s something that’s often missed. The distance to the background is a different thing than the distance to your subject. It’s easy to pay attention to the distance to the subject and not notice how close or how far the background is behind the subject. The closer the background, the more it is in focus; conversely, the farther the background, the more out of focus it becomes.

For example, many people photograph flowers from above the plants. Imagine doing that and think about how close the background is behind the flowers. Next, think about getting lower so that you shoot the flowers from at about their level. How close is the background now? Often, when you get lower like this, the background suddenly gets very far away compared to the ground when you shoot down on photographs. That distance affects how sharp the background is, especially when you’re using larger ƒ-stops.

Look at your subject. When you’re up close, often you can move a slight distance up or down, left or right, and change what the background looks like simply because you found an angle that puts the background farther away. Having a more distant background frequently gives you nice, soft tonalities and colors.


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