[Leica] Diffraction limited; bo-ke

["Erwin Puts" <imxputs@knoware.nl>]

Someone suggested that I might have said that the Summicron 50 and other
lenses are diffraction limited at f/8 and smaller. And therefore you should
use smaller apertures to get the best performance. I did not state anything
close to this. The Summicron is not diffraction limited at any aperture and
so are many Leica lenses. Only a few lenses in the Leica stable are really
diffraction limited, like the R- 4/280. The R-2/180 is for all intents and
purposes diffraction limited at f/5.6 and should not be stopped down further
if best performance is required. Diffraction limited means that the optical
aberrations in a lens are so small that the physical limit of the Airy disc
is approached.  This means also that the lens automatically performs better
at larger apertures as the diffraction effects (loss of contrast, loss of
resolution, loss of encircled energy) increase when stopping down. It is
always best to use the widest aperture that is feasible in a given
situation. (when using modern leica lenses).

The topic of bo-ke has been popping up occasionally on this list. The email
by Mr Johnston (provided by Mr Gandy), stating that bo-ke can vary with a
number of parameters, is like flogging a dead horse. The parameters he cites
are the same that govern the representation (or recording) of any out of
focus plane. This is obvious to anyone who knows the difference between  a
plane of correct focus and a out-of-focus-plane. Any out-of-focus plane has
a higher aberration content than the plane of correct focus. In fact, one
could describe the effect of the sum of all optical aberrations on an image
as a defocus effect. The out-of-focus plane then shows a higher level of
aberrations than the true focus plane. The whole idea of bo-ke (at least as
interpreted by its students) boils down to a description of an out-of-focus
representation of a section of a solid (3-D) object by a lens, that has some
specified aberration content. There is a very close relationship between the
o-o-f representation and the level of aberration correction. Most lenses
have a different represestation of o-o-f objects in front of and in back of
the plane of focus. That has nothing to do with bo-ke, but with simple
geometry of the lens.
While bo-ke is a useful concept, it is not a new concept and any lens
designer is aware of its basics. The study of bo-ke is simply the study of
o-o-f representation as governed the optic properties of a lens and by the
residual aberration content of a lens. No new revelations or need to
introduce new concepts. The  claim that many current descriptions and
explanations of lens performance are inadequate, because lacking in a taking
account of the bo-ke characteristics that may govern or influence the visual
properties of an image, is like the claim of the famous but uncomprehensible
current French philosophers that a new language and new concepts are needed
to describe social reality.
But I admit that the phenomenology of the picture is a rich breeding ground
for semantic gymnastics.

Erwin.