> I own EL-Nikkor 50 f2.8 and 80 f5.6 lenses. Given that enlarging
> lenses are used at around f8 most of the time, I doubt that the
> f2.8 is worth trading 'up' to from the f4. Especially if you bear
> in mind that you should focus through a grain magnifier at the
> exposing aperture-i.e. f8 for critical sharpness. Focussing at
> maximum aperture and stopping down will cause a small loss of
> grain shrpness due to a focal length change in the lens. Try it
> with a grain magnifier at 2.8 and 8.
Not quite ...
The focal length itself does not change (it is determined by the
surface shapes not the aperture). When certain aberrations (eg
spherical aberration) are present in the lens, their effect can be
balanced by a slight defocusing (compared with the Gaussian
image plane) of the lens. When you stop down the lens and reduce
(some of) the aberrations the balance point moves and you need to
refocus to obtain the optimum image quality (best OTF/ pont
spread function) with that lens.
Usually (but not always) the change in depth of field is sufficient to
overcome the need to refocus. So with any enlarger lens it is worth
checking by focusing at max aperture and then stopping down in
order to establish if you need to refocus.
