Some confusion about the circle

[Doug Kerr]

I thought it was about time to give my periodic plea for more care in our use of terminology in connection with the infamous circle of confusion.

To set the stage, note that there are three things we from time to time refer to in this area:

1. Circle of confusion. This is the "blur figure" that is produced on the focal plane from a point on the object when focus is imperfect. It is not a number or quantity, but a two-dimensional physical phenomenon - a "thing".

2. Diameter of (a) circle of confusion. This is the diameter that some real (or hypothetical) circle of confusion, created in some imaging situation, has. We do not choose it - it results from the parameters of the imaging situation.

3. Circle of confusion diameter limit (my term). This is a criterion we choose as a parameter of the calculation of depth of field. It is the largest diameter we allow the circle of confusion to have and still consider blurring to be acceptable (that is, focus to be acceptable).

It is of course common to refer to the quantity in (3) above as "the circle of confusion" (often abbreviated "COC").

The problem with doing this is that it is then almost impossible to speak of item (1) or (2), as we often need to do, without -- confusion.

Thus I recommend using the term "circle of confusion diameter limit" (which we may abbreviate "COCDL") for the parameter in item (3) above, reserving the term "circle of confusion" for item (1) and "circle of confusion diameter" for item (2).