Depth of focus:
The vertical distance in the observed image through which features within the image are simultaneously in focus.
Like  depth of field, the focus thickness depends on geometric and wave-optical properties. (Murphy)
The NA of an objective on the image side equals the object-side, marked NA divided by the magnification of the objective. It is for this reason that with a high-NA, high magnification microscope objective, the depth of field is very shallow, but the depth of focus can be quite deep and reach several millimeters. (Inoué)
Following is the formula for calculating depth of focus:
 enlarge | Field of view
Explanation
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Depth of field
Explanation
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Depth of focus
Explanation
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