Depth of Field Scale Part II:
© Wendy Folse
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 fstop 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 fstop 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?
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, lets 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.
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