Originally posted by UnknownVT Thanks, I do not mean to prolong this -

If you'll allow me to go back to basic fundamentals for a moment -

CoC is more about human physiology than it is about technology (sensors or pixel count)....

That's it - nothing to do with pixel size, count or film grain resolution limit etc.

Yes, that's the starting point for one way to describe the geometric optics relating to depth of field, but it doesn't complete the story.

Once the CoC term is put into the equations, one must state the qualification that there is a lower limit to it that defines the domain of validity for the relationship. Without such qualification the relationships imply there is no enlargement limit for a camera system.

Your own language says there is a lower limit to CoC that occurs when there is insufficient sensor pixel density.

But I suppose this argument is more about purity of terminology origins vs utility of geometric optics & descriptive mathematics applied thereto.

In fairness, it should be pointed out that the limits to depth of field are most likely better described by near field equations & "Depth of Focus" concepts. It turns out the geometric optics description of the focal point of an image being the intersection of two straight lines is incorrect. The actual image of an in-focus point is roughly an ellipsoid elongated in the direction of the optic axis.

Here's a figure from

Olympus showing the shape of an "in-focus" point; The left image is for standard optics; the optic axis is vertical: (PSF is "Point Spread Function" - the image of an infinitesimal point in space)

Notice that the length of the focal disk along the optic axis is a few times its diameter.

This length is purely a function of near field interaction of light waves at a focal plane and has nothing to do with human perception of a print nor pixels on a sensor; it determines the obtainable resolution of a lens based image in the depth direction.

Dave

PS in real life I think one runs into the pixel spacing limit to image resolution (in x, y, and z directions) before the diffraction based limits shown above (but I'm not certain of the details.)