While depth of field is generally measured in macroscopic units such as meters and feet, depth of focus is typically measured in microscopic units such as fractions of a millimeter or thousandths of an inch.
The choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with smaller formats.
Placement of items behind the lens will alter the optics pathway, shifting the focal plane.
Therefore, often this insertion must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus.
If the depth of focus relates to a single plane in object space, it can be calculated from[1] where t is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length.
Moreover, the simple formula will always err on the conservative side (i.e., depth of focus will always be greater than calculated).
This practice is now deprecated; it is more common to base the circle of confusion on the format size (for example, the diagonal divided by 1000 or 1500).