Troubleshooting Photos –
About Depth Of Field
For any given lens at any focus setting, there is one distance at which the picture is perfectly in focus; every other distance is out of focus. Fortunately, for some distance in front of that perfect point of focus and for a greater distance behind that perfect point, our eyes aren't sharp enough to see the lack of focus, so it looks as if the point of focus has depth. The photo world calls this illusion the depth of field.
Because cameras have depth of field, you can shoot a picture of your sweetie standing in front of Niagara Falls and have both be in (apparent) focus even though the Falls may be around 1,000 yards or meters away. Alternatively, you can take the same picture, setting the camera' depth of field so sweetie is in focus and the Falls are a romantically out-of-focus haze behind.
The first of these photos, deep, has a lot of depth of field and the details of all the plants show clearly whether near or far. The second photo, shallow, has shallow depth of field and this separates the picture into a foreground that is sharply focussed, and background that is less sharp. This turns the cattails in the foreground into a distinct subject and the plants behind into mere background, changing the feel of the picture.
The ability of depth of field either to include everything in the camera's view, or divide that view into 'subject' and 'subject's surroundings' or 'backdrop', makes it a powerful aesthetic or illustrative tool.
Controlling depth of field
The optics of depth of field are quite complicated (and are explained at length in many sites on the Web, if you're interested) but control of depth of field is fairly straightforward – all else being equal, a larger aperture equals a shallower depth of field, a smaller aperture equals a greater depth of field. Measuring from the point of perfect focus, there is approximately 1/3 of the available depth of field in front of the point of perfect focus and 2/3 behind that point.
Before going too far, it's important to understand that some cameras can control depth of field, while many of the more simple point-&-shoot cameras cannot (or at least, not without trickery – see below). First, many point-&-shoot cameras have fairly deep depth of field (for technical reasons that we won't get into) so a shot like shallow, above, is hard to achieve – they will normally yield a shot similar to deep, above (particularly in bright sunlight – this will close the aperture down and yield a deep depth of field in any automatic camera unless you can over-ride it). To take control of depth of field, you need either direct or indirect control over your camera's shutter speed and aperture:
- On a simple point-&-shoot camera, the camera's natural depth of field is likely to be deep. However you can trick it into a shallower depth of field by using a scene setting that includes a faster-than-normal shutter speed (look in its manual for Sports, Kids, Action or some phrase that suggest capturing quick movement, which requires a faster shutter speed). This will yield a more open aperture and a shallower depth of field. If you need additional depth of field, look for a setting like Fireworks or Long Shutter Opening as a means of achieving a smaller aperture. To use a longer shutter opening without motion blurring, you may need to brace the camera against something or use a tripod.
- On a semi-automatic camera, control depth of field with an aperture-priority or shutter-priority setting. An aperture-priority setting allows you to achieve deep depth of field by setting the aperture to f16 or so (try to avoid taking the aperture down to its smallest size, as image quality will be slightly better at one f-stop above the smallest setting than at the very smallest). If the camera has a shutter-priority setting, a fast shutter speed will open the aperture, reducing depth of field, while a slow shutter speed may require a tripod if you are to avoid camera shake while taking advantage of the greater depth of field.
- On a manual camera (or the typical DSLR, which allows you to decide precisely how automatic or manual you want to be), either the aperture-priority, shutter-priority or manual settings allow you to control depth of field to a fine degree.
- Closing down the aperture restricts the light entering the camera; the compensation is to increase the exposure time. If you have steady hands, you can hand-hold a camera with a normal (not telephoto) lens down to about 1/30th of a second. Below that, you must steady the camera with a tripod or other means.
Depth of field is proportionate to the distance between the camera and the scene it is capturing. In other words, if you're close to the subject, your depth of field may be measured in fractions of an inch or in millimeters. If the point of focus is far away, depth of field may be measured in miles or kilometers.
In this photo, the camera is close, about 15" from a brooch that is about 1" in depth (the complete photo can be seen here). At this distance, the depth of field is very shallow. In a 200% enlargement, the leather background is in focus but the stone is not.
