In a depth of field app, I entered my camera's sensor size, focal length of 50mm, aperture of f8, and focus distance of 6 feet. The app tells me that DoF is 2.1 feet, from 5.1 to 7.2 feet.
I then focused on my subject at a distance of 6 feet, and then moved the subject back so the distance was now 6.9 feet. But, to my surprise, it was no longer in focus.
The subject is well within the 5.1 to 7.2 foot range ie, what am I missing here?
Answer
Depth of field occurs on a plane. At 6 foot is where complete, "perfect" focus is achieved, and anything before or after that is not in complete focus and is, in fact, slightly out of focus. So, the range from 5.1 to 7.2 feet is the range of "acceptable focus." And, as you're seeing, it's somewhat subjective based upon image size. At 6.9 foot, your subject is at the edge of that "acceptable" range, and in your case is not acceptably focused.
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