[Leica] Apo enlarging lenses

[Mike Johnston <michaeljohnston@ameritech.net>]

The primary advantage of an APO enlarging lens is that its optimum
f-stop is wider than that of a non-APO lens. That's about all.

However, this is significant.

All enlarging lenses should be used at the widest stop that minimizes
aberrations, since diffraction is a far, far worse problem in enlarging
than it is in taking lenses. (There is a good article on exactly why by
Ronald B.J. Wisner in a back issue of D&CCT--I can't reference the issue
just now since I don't have it at home).

Ctein and Arthur Kramer argued for years as to whether the Apo-El-Nikkor
105mm (which Arthur owns*) was better than a conventional 6-element
enlarging lens such as the Rodagon, since the Apo-El was
diffraction-limited at f/5.6 (its widest aperture) and the Rodagon at
f/4.5. Ctein felt the extra half-stop made up for the difference.

You should never stop down a 50mm enlarging lens to f/8 or, worse, f/11.
If you don't believe this, simply get a very good grain magnifier and a
negative with crisp grain and WATCH the image of the grain as you stop
down. Stop down a few clicks and watch the grain magically get sharper,
then continue stopping down and watch it get mushy again. With virtually
all lenses there is a quite clear optimum--and it is this aperture that
you should always use for the sharpest prints.

With my cherry-picked Apo-Rodagon-N 50/2.8, the optimum stop is between
f/3.5 and f/4, whereas for the non-Apo versions it is between f/4.5 and
f/5.6. This is enough to reduce diffraction effect, and it makes a
difference in print sharpness.

If the rest of your enlarging technique is no good, as Ed B. suggests,
an Apo lens makes very little difference. Ditto if you are not going to
use the optimum aperture.

- --Mike

* Actually he has two--one on his enlarger and one cast in lucite which
he uses as a paperweight on his desk. Arthur is a man of class, wealth,
and style.