Checking Depth-of-field

Left Picture: Canon EOS 20D, Tokina 20-35mm ATX Pro, 1/60 at f-8.

Depth-of-field is the distance between the nearest and farthest points in a scene that appears in sharp focus. Except in close-up photography, depth-of-field is about one third in front of the subject and two thirds behind it. There are three factors in determining the depth-of-field: aperture, lens focal length, and subject to camera distance. Selecting the right depth-of-field requires you to understand the relationship between these three factors. Aperture is the most critical factor. The smaller the aperture you select, the greater the depth-of-field will be.

 Since depth-of-field increases more behind the point of focus, it is best to focus on the nearest point for greater sharpness. With SLRs equipped with depth-of-field preview, you can check the zone of sharp focus. Lens diaphragm is closed down to the selected aperture, enabling you to see the actual depth-of-field. The viewfinder gets dark when you use depth-of-field preview as you close the lens down which can make it difficult to view the scene. Allow a few seconds for your eye to adjust to the dark viewfinder before determining the right f-stop.

Another way of checking depth-of-field is by using depth-of-field scale on your lens. Not all lenses include this feature. Almost all fixed focal length lenses have depth-of-field scale, but many zooms lack this feature. Depth-of-field scale is marked between aperture range and distance scale on the lenses or just below the distance scale depending on the manufacturer. There are two sets of numbers on each side of the depth-of-field scale. Each corresponding to the f-stop available on the lens. Half of the numbers are on the left and the other half on the right side of the focusing index mark which is usually a gray or red line. To use the scale, first focus on the subject or the main point in the scene. Now read the distances that fall opposite each of the marking on the f-stop you have selected. On the image below, the lens is focused at 70 feet ( 20 meters) with aperture at f-16. By looking at the two f-16 marks on either side of the depth-of-field scale, you can see that depth-of-field would extend from 30 feet ( 10 meters) to infinity. At f-8, range of sharp focus is roughly between 70 feet ( 20 meters) both in front and behind the focused point.

As you can see, this scale is a valuable tool for selecting the right aperture quickly and easily. First, focus on the nearest point you want in sharp focus. In the example above, the nearest point is 70 feet or 20 meters ( note the red mark just below the 20 meter mark). Now, focus on the farthest point you want to appear in sharp focus and take note on that distance. In the example above, it is infinity. To find an aperture that covers this distance, rotate the focusing collar until the nearest and farthest points match up with a pair of aperture marks. In this case, f-16 would be sufficient. Don't be fooled by infinity mark as it does not always mean what it should! On telephoto lenses, infinity focus can be passed the infinity mark depending on the temperature conditions. A very interesting concept.

Don't be tempted to to use the smallest aperture as it can cause blurred images due to slower shutter speed. Diffraction is another problem at extremely small apertures causing loss of overall sharpness.