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Daily photo tips: Understanding depth of field

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Posted 23-09-09 at 11:22 AM by Ian

No.5: Depth of field; creatively controlling what's in focus.

Depth of field, (also known as dof) in a technical sense, is the distance in front of and behind the subject that you have focused on. It's most noticeable when the background of an image is blurred, but sometimes you can see objects nearer than the subject that are out of focus, too.

Basically, you can control the range of distance from the camera that is in focus by adjusting the lens aperture. The smaller the aperture (larger the f-number), the greater the range that is in focus, or the greater the depth of field, and vice-versa.

These roses (above) were photographed at f/2.8, which is a wide, bright, aperture. Only part of the rose focused on at the right is in focus.

Here (above) we have the same roses photographed at a smaller aperture (f/7.1). The second rose is still not in sharp focus, but it's more defined. The nearer rose on the right is now completely in focus. This is because there is more depth pf field thanks to the smaller aperture used.

Also, note the background to both images. The defocused, blurred, area in the background of an image is often referred to as 'bokeh'. The smoother it is, the better. The bokeh is better in the picture with the larger aperture at the top because there is less depth of field.

Depth of field and focal length

Creative use of limited depth of field is easier to achieve with a longer focal length lens. Conversely, wide angle lenses give you much greater depth of field, even at wide apertures.

Focus closer to limit depth of field

A great tip for deliberately limiting depth of field for creative effect is to get closer to your subject. An extreme example of this is macro close-up photography, where photographers actually struggle to achieve enough depth of field to ensure the whole subject, like an insect, is in sharp focus.

Depth of field and camera sensor size

Compact cameras usually have much smaller sensors than, say, DSLR cameras. The smaller the sensor is, the smaller the lens needs to be. The smaller the lens becomes, the smaller the aperture becomes, physically, and so the depth of field gets greater.

Above, this picture was taken using a DSLR camera. The background is out of focus thanks to limited depth of field.

Here is the same scene (above), this time taken using a compact camera. I made sure the cameras were set using a similar aperture. The compact camera exhibits much greater depth of field and this has resulted in a more definition in the background. The problem is that the background is competing for the eye's attention. The DSLR result is more pleasing as the background is more blurred.

Of course, there will be times when you want more depth of field to keep everything in focus, in which case a compact camera or a smaller aperture will be the solution!

But there is a limit to how far you can reduce the aperture. This is because the smaller the aperture becomes, physically, the more prone your picture becomes to the softening effect of diffraction. This is why compact cameras usually have a more limited aperture range than larger cameras.

Tomorrow: Image noise - what is it?

Incidentally, please don't hesitate to post a question about this Daily photo tip if you have one!
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