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This is the fifth of the four-part series on digital photography. You might call it an encore, except for a lack of applause, so instead we’ll just admit that we left an important item out of the first four parts, and now we return to remedy the situation. The subject is depth of field, sometimes called depth of focus. It should be discussed because it is different for digital cameras. First of all, what is depth of field? When you take a picture and after focusing carefully on a subject, you notice that objects which are closer to the camera, and objects that are more distant than the subject are “out of focus.” However, there is a range of distance, in front of and behind the subject where objects will appear to be “in focus.” The range of distance in front of and behind the subject that appears to be in focus is called the depth of field. Some years ago, I was studying portrait photography, and I ruined several rolls of film by focusing incorrectly. I had pictures where the tip of the nose was in focus, and the eyes were out of focus. The pictures, although fine in all other respects, were horrible. I learned from that experience to focus on the eyes. Recently, Richard Avedon exhibited many of his portraits at the New York Metropolitan Museum of Art. In some of his portraits, I noticed that where the head is turned slightly, one eye is in focus and the other is slightly out of focus. I estimated that he was working with a depth of field of about a half-inch. In direct contrast to this, I recently took a picture with my digital camera and noticed that the depth of field was much greater than I would have expected. Objects close to the camera were almost in focus, even though I was focusing on a more distant subject. To check this I ran some experiments, comparing a digital camera to a high quality 35mm film camera, and confirmed my suspicions. Digital cameras do, indeed, yield greater depth of field when compared to film cameras under the same picture-taking conditions; that is, with the same subject and f-stop. I asked myself why. The depth of field that you will achieve with a photograph depends on several things, in particular, the f-stop, the focal length of the lens and how close you are to the subject. The f-stop, or f-number, is defined as the focal length of the lens, divided by the lens diameter, or aperture. You can have any f-number you want, but cameras are usually designed to offer “stops,” a defined selection, such as f-2.0, f-2.8, f-4.0, f-5.6. The f-stop is important because it determines how much light is going to fall on the film or light sensitive element. The smaller the f-stop number, the larger the aperture and more light falls on the film. Because the f-number is based on the ratio of focal length to aperture, the amount of light falling on the film is dependent only on the f-stop, so f-5.6 for a 50mm lens gives you the same exposure as you would have with f-5.6 and a 300mm lens. We set the f-stop based on light conditions, film speed, and time of exposure. To answer my questions, I reviewed the equation governing depth of field. I found that depth of field is proportional to the ratio of f-stop to focal length. Remember that f-stop is the ratio of focal length to aperture, so the ratio of f-stop to focal length is the reciprocal of the aperture, that is, 1/aperture. The equation is telling us is that depth of field is inversely related to aperture. This makes sense, because the old pin-hole cameras we made as kids had, theoretically, infinite depth of field. But we don’t know the aperture when we take a picture, since the camera is not set up that way. We only know the f-stop and the focal length. For our purposes it is the ratio of f-stop to focal length that governs depth of field. If you take a picture at f-5.6 with a 50mm lens on a 35mm film camera, then the depth of field is about 12 inches. By the same measure, if you are using an 8x10 view camera with a 400mm lens at f 5.6, your depth of field is about 1.5 inches. How does this affect digital photography? The CCD light-sensing element in a digital camera is much smaller than in a film camera. The 35mm film frame is 24x36mm. The size of the CCD in my 4-megapixel Olympus E-10 or the 5-megapixel Nikon 5700 is about 6x9mm. Because the film frame is smaller, the equivalent focal length for the digital camera is one fourth of that for the same picture taken with a 35mm camera. Therefore, the aperture for the same f-stop is one-fourth that for the 35mm camera. Another way to put it is that I can use f-2 on the digital camera and achieve almost the same depth of field that I would normally get at f-8 on a 35mm film camera. Caution: these numbers I am quoting are approximate, because there are second-order elements in the equations. However, they accurately represent the direction and magnitude of the relationships. So depth of field is, in fact, determined by the kind of camera that you are using. Today’s digital cameras, with their small CCD chips, give greater depth of field for the same f-stop than a standard 35mm camera. Similarly, a 120 film size camera, such as a Hasselblad or an 8x10 inch view camera, provide a much shallower depth field. Some future, high resolution digital cameras may have light sensing elements (CMOS chips) that are the same size as 35mm film, so their depth of field will be the same as for film cameras. Our objective in this article is not to go into detail about depth of field in general, but only to explore differences that exist between digital and film photography. That being said, there is another important factor to note and remember. Depth of field depends also on the ratio of the subject size to the film size. This ratio is determined by the size of your film or light-sensing element, the focal length of your lens, and the distance between camera and subject. Using a diagonal measure, a portrait has a size of about 36 inches. For a flower, the subject size might have a diagonal of five inches, and for a group photo, perhaps 10 feet. So, using the same equipment, the depth of field for the flower photo is much less than that of the portrait, and for the group photo, much greater. This is why depth of field is important in macro and portrait photography, and less important in group photos. Interestingly, depth of field also becomes important in scenic photography, because a good scenic usually has some object in the foreground which should be in focus in order to emphasize depth to the image. In those situations where depth of field is important, today’s digital cameras provide greater flexibility and ease of use. |
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Richard Ten Dyke is a member of Danbury Area Computer Society who has had a long interest in both photography and computers. He started his photogr aphy career with a Leica IIIC in 1952, and his computer career working with an ERA 1103 in 1956. He currently is retired from IBM and resides in Bedford, New York. You can reach him at tendyke@bedfordny.com. |
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