from article - On Focus in Photography

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How does CoC affect focus considerations?

There is only ONE plane of exact focus. Anything outside of this plane is rendered soft. The further away from focus plane, the softer the detail. Why so?

A point photographed at plane of focus will be represented as point on the film, to be exact light rays will re-converge, after passing through the lens, at the film plane, requirement for sharp rendering.

Now let’s look at a point that is away from the plane of focus.

If it is behind it, its image forming light rays will re-converge before the film plane, so will have to travel a bit more before they reach the film. This will result in a circle rather then a point. It’s size will depend on how much before the film plane rays convergence occurred.

If our point is before it, the rays will re-converge behind the film plane. So they will first travel through the film, before convergence takes place. This will also result in a circle rather then a point on the film. And again, the size of this circle will depend on the distance between film and the rays-convergence point behind it.

In either of the latter two cases, the rendering of the circle will result in a soft looking point as recorded on the film. It is the size of this circle we’re concerned about. Our calculations will use earlier assumptions, mostly based on critical lab measurements, as a starting point.
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