Originally Posted by MattKing
Some lenses do suffer from focus shift, but most don't.
So Michael described a method of checking whether your lens is one of the problem ones.
Perform the suggested test. If you observe focus shift, you will need to build a "focus at working aperture" step into your workflow.
If the test doesn't reveal any shift, you can confidently focus at maximum aperture for all your work.
But how can you judge when you are viewing with an increase in depth of focus?
“The contemplation of things as they are, without error or confusion, without substitution or imposture, is in itself a nobler thing than a whole harvest of invention”