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?
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.
There is only one distance at which an object is in focus.
Depth of field says that, within a certain range, the object is acceptably sharp. So it will not be in perfect focus. That definition of acceptably sharp is also relative to its print size. Most DOF tables are calibrated for an 8" x 10" print but if you are looking at 100% from the output of a full-frame DSLR, you most most likely looking at the equivalent of a much much larger print.
What you are probably missing is that your DoF table assumes a display size of 8x10 viewed at 10 inches by a person with 20/20 vision. If you change the display size (or viewing distance, or visual acuity of the viewer, or any combination of those factors) then you have changed the magnification factor between the sensor size and the display size and must also compensate by changing the acceptable Circle of Confusion used to calculate the Depth of Field for a particular focal length and aperture.
Understanding what DoF is and what it is not is important here.
In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image. There is no magic barrier beyond which everything is equally blurry and inside of which everything is equally in focus!
Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focal plane.
Viewing a 22MP image zoomed in at 100% on a 96ppi 23" HD (1920x1080) monitor is like looking at a portion of a 60"x40" print!
You need to take the circle of confusion to be the size of one pixel, for CMOS sensors this is typically about 4 micrometers. If we call this r, and we denote the focal length by f and the F-number by F, then the so-called hyperfocal distance is given by:
H = f^2/(F r) = 78 meters
Focusing on infinity will keep all objects a distance H or farther away in focus. Suppose now that you focus on an object that is a distance d away and that d is much less than H. Then all objects within a distance d^2/H from that object will be in focus, the blur will be less than one pixel wide. In your case for d = 6 feet, d^2/H = 4.3 cm, so the DoF is 8.6 cm = 0.28 feet. The blur of a point one foot away from the focus point will thus be about 7 pixels wide. This may be acceptable when you are not zooming in, but it will obviously be noticeable when you blow up your picture.
