Technically it is called "Eberhard effect", though "Mackie lines" are the same thing.
Originally Posted by tull777
Google will provide nearly two million hits on such topics, for anyone who actually does attempt to find it. Searching on something like 'film agitation sharpen' ("acutance" can replace "sharpen") will provide fewer hits but may result in material that is more readable for most people.
Here's one that I really like, given certain comments that have been made (and deleted by the moderator):
For a fairly good discussion of sharpening aimed at photographers:
"The phenomenon due to the fact that the density of a photographic plate, given a uniform exposure through a metal plate with an opening, varies with the size of the opening, when an organic developer is used."
To avoid further confusion... I did not say that increasing agitation causes sharpening. (I said that decreasing agitation increases sharpening!)
And let me point out that only an advanced darkroom worker (as opposed to an advanced photographer who happens to do a little darkroom work) would be likely to adjust film processing to control sharpening. The gross interaction between different effects when doing anything with chemicals is too complex, and most of the time consistently following the instructions for a given film and developer was hard enough!
With digital, sharpening can be done with no effect on contrast or density, which was not true with film. Yet note that even with digital most photographers want to merely set something that works with a camera setting and then never touch it again, much the same as they did with film!
With digital tools both sharpening and Un-Sharp Mask are easy techniques to work with; but they still are significantly difficult to understand.
Another suggestion (in a deleted article) was that using Un-Sharp Mask is "unprofessional". Note that while sharpening is an advanced darkroom technique, USM is absolutely for only the most advanced and well equipped darkroom! Which is to say that most magazines such as Life, Playboy, and Time were equipped to do it, but even large newspapers were not.