Depth of Field Scale Part II: © Wendy Folse Jun 25, 2001 Now let's practice reading the depth of field scale:
On the scale above, find the two markings for f8 on the depth of field scale. The lens is focused at 3.5m as indicated by the diamond marking. Now look at the line above the depth of field scale. One line is marked in meters and one line is marked in feet. This is the distance scales. The points between the two f8 markings on the aperture ring that correspond on the distance scale give you the depth of field range. The reading on the distance scale indicates that we have a depth of field between 5m and 2.5m. Everything in that range will be in focus. Anything closer than 2.5m will be out of focus and everything beyond 5m will be out of focus. The depth of field is a total area of about 2.5m. (5 ‐ 2.5=2.5) Why is this important? It is important because we know that we have a margin of error of about 2.5m inside of which everything will be in acceptable focus. As long as what we want to be sharply in focus is within that boundary, then we are assured that it will be reasonably sharp. If it moves outside that boundary it will be less sharp or out of focus. The depth of the field of focus is what we are talking about. Remember that the larger the f‐stop number the smaller the opening in the diaphragm or aperture. The smaller the aperture, the larger the depth of the field. The larger the aperture, the smaller the depth of field. Suppose we have our subject at the same distance, 3.5m, and we want to blur the background slightly. What f‐stop should we use? Look at the first diagram and find the two f16 markings. What is the range? Using f16 will give us a range of 15m to 2m. Everything in front of 2m will begin to lose focus and everything beyond 15m will lose focus. Is that enough for the effect we want to achieve?
The copyright of the article Depth of Field Scale Part II: in Photography is owned by Wendy Folse. Permission to republish Depth of Field Scale Part II: in print or online must be granted by the author in writing.
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How about f11? What is the depth of field for this aperture? The range is approximately 2.25m to about 7m. For those people not familiar with the metric scale, let’s look at the scale again and read the depth of field in feet.
In the above diagram for a 50mm lens, with a focus point of about 11ft, F11 has a depth of field range between 7.5ft and 24ft. That is a margin of error of 16.5 feet so to speak. If you wanted to do the math to get a more precise estimate, divide 16.5 by three. Why three? Because remember that you have one‐ third in front and two‐thirds behind the focus point. We get three equal parts of 5.5ft each. So we have 5.5ft in front of the focus point and 11ft behind it, since the focus point is 11ft away, 11+11=22ft. The calculations are about 5.5ft to about 22ft. That is pretty much what the scale already told us. We don't need to make the actual calculations because the manufacturer has done it for us. How did I know the subject was 11ft away. Easy, the scale told me. When you look through the viewfinder, focus on the intended subject then read the distance scale. The point when the image comes into sharp focus corresponds to the center marking or diamond on the distance scale. Viola! Remember at this stage you are composing your picture and deciding how you want the image to appear. Knowing the distance from camera to the focus point is important when you want to use depth of field creatively. It is also important if you want to know if your flash range is sufficient. If you don't have a distance scale on the lens then you have to estimate the distance or measure it conventionally. At this range the background will only be slightly out of focus because it is still a broad range. If we really want to throw the background out of focus we need to use a larger aperture. Remember we originally started with an aperture of f8. What is the range in feet? Find the two f8 markings on the aperture scale diagram. Read the distance scales to get the depth of field range. F8 gives us sharp focus from 8ft to 22ft. We are focused in at about 11ft. Everything 5 feet in front of the focus point will be sharp and everything 9 feet behind the focus point will be sharp. If our subject is a person standing in a field of wildflowers, this setting would give a very nice effect. What if we want that dreamy watercolor look of our subject in a blurry field of color with only a hint at the flowers surrounding them. Using f4 our range would be 10ft to 12ft. That is a depth of field of 2ft.
The copyright of the article Depth of Field Scale Part II: in Photography is owned by Wendy Folse. Permission to republish Depth of Field Scale Part II: in print or online must be granted by the author in writing.
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Anything 8" in front of the subject will be out of focus and everything more than 16" behind the subject will be out of focus. Still not quite enough for the effect we want because the wildflowers at the subject's feet and behind the subject will still be too sharp. Let's try f2. There are no markings for f2. Now what? That is where the diamond comes in and where depth of field gets interesting. The diamond is now marking the critical spot at which the lens is focused. At this point, the ranges of numbers have converged into only one point. Only the points around 11ft will be sharp and everything else will start to blur. There is a very limited margin for error. You have reached the maximum aperture for this lens. Will it be enough? Is it too much? That depends on the size of the subject. Your range is now less than 12 inches. Divide12 by three and you get four. So you have 4 inches in front and 8 inches behind the critical point. For our original subject and theme, a person standing in a field, this range is perfect for the effect that we are trying to achieve. But what if there were two people, say a couple. How would you have to place the couple in order to get the same effect but have both people equally sharp? How could you place more emphasis on one person and less on the other? You've seen those dreamy wedding photos of the bride and groom, where the bride's face is in focus and the groom standing right beside her is out of focus. Selective focusing and depth of field is how they do it. Once you have decided on the aperture you want to use for a certain effect, you have to then compensate for the change by setting the appropriate speed on the camera. If you metered the scene and found that f8 at 1/60sec was the proper exposure, refigure the ratio for the new aperture setting. F8 at 1/60sec = f5.6 at 1/125 sec = f4 at 1/250= f2.8 at 1/500sec= f2 at 1/1000 Try it for yourself. Go out and shoot a roll of film with only one subject. Work with the depth of field and experiment with different effects.
The copyright of the article Depth of Field Scale Part II: in Photography is owned by Wendy Folse. Permission to republish Depth of Field Scale Part II: in print or online must be granted by the author in writing.
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