Clive- I've seen what Drew's referring to. Years ago, a friend was having trouble getting sharp prints. She thought it was her magnifier, so I brought my Peak over, and we focused wide open. Then, closing down 2 stops, the print still wasn't sharp. Checking the Peak, at the chosen aperture, it wasn't sharp, and needed refocusing. It was a fairly cheap lens. She started focusing at the chosen aperture, without issue, even with her cheap magnifier. I haven't seen it happen with better lenses, though.
Something I did years ago when my father was having a slide show was to cover up half of the lens with my hand thinking it would block out half of the image. Instead you get the whole image at a reduced brightness.
Steve, you're right when the image is in good focus. Out of focus, different result. It's easy enough to explain the general principle.
Imagine a plane through the center of the lens (as in a cutaway showing the lens construction). Imagine pointing the lens directly toward a distant, small subject, such as a star (imagine something larger if that helps). We know that this subject illuminates the entire front of the lens, BUT... consider only the light on one side of our plane - that is, on one half of the lens. As this light converges to form an image, all light is coming from one side of the imaginary plane. As the light passes the ideal focus position, it crosses to the other side of the imaginary plane.
Now repeat this process from the OPPOSITE side of the lens. This light wants to also pass through the focus point and diverge on its opposite side of the imaginary plane. Thus, a double image, but only when out of focus.
I don't consider this as very useful for focusing an enlarger, but a lot of experienced photogs would probably bite on it as a barroom bet.
There are all kinds of hypothetical optical issues one could discuss. The main point, however, is that most enlarging lenses are not ideal wide open. You might need full apterture for general composition and approximate focus, but silver grain or dye clouds might not focus crisply at full aperture. So it's
a good idea to refocus a stop down (not necessarily at working aperture, which might be too dim to
focus well - just depends). But never assume a focus device is itself correctly aligned. You need to test.
And nothing will make senses if your enlarger is not perfectly aligned on every plane, or if your negs
are not truly flat in a glass carrier. I don't want to open a can of worms in the latter respect, but at a
minimum, you should have a glass carrier on hand with a focus target for the sake of general calibration. Some ten buck Home Cheapo level isn't the ticket.
If I am using a reasonable quality enlarger lens I expect it to stay at that point of focus when stopped down and therefore fail to see the point of refocusing again at one stop down.
Modern enlarging lenses are optimized one or two stops down, and if any focus shift occurs it is likely
to be only one the first stop. But I don't know about old enlarging lenses or cheapo ones. And in my
own collection of enlarging and process lenses, only one lens exhibits any focus shift at all, at it's the
least expensive one of all, but still very usable a stop down. Some of the very best lenses are very
sharp wide open, but might have excess illumination falloff compared to stopped down. One just needs
to test the variable, which is quite simple to do in terms of basic characteristics.
As you stop down you increase depth of focus, so how can you judge this is better or worse than when you focused at full aperture?
I have always focused wide open with the paper I am using under the peak device corrected for my eyes.
I want to see grain, sharp grain. I then close down two stops and work, I have never seen the reason for stopping down.
I have always used APO lenses and my film is always in glass, we check alignment periodically.
Seems to work for me over the years.