Re: [Leica] Diffraction limited; bo-ke

["Dan Post" <dpost@triad.rr.com>]

Guys-
Maybe I am reading this wrong, but I think that the idea of diffraction
limiting is somewhat different than the idea of bokeh- similar to discussing
apples and oranges, again!
The way I read it, diffraction limiting is the effect of using a too small
aperture, for instance. The lens has an optimum aperture as designed and
built. At the widest aperture, the effects of spherical aberration, and coma
will make a point in perfect focus form an imperfect circle of light on the
target/film. As one stops down, this effect diminishes- to a point. At the
diffraction limit, the images ceases to become less sharp, and the effect of
light siffracting around the edge of the diaphragm, causes flase increas,
and lowers contrast. It is similar to pouring water out of a glass, and
having it flow oround the edge and down the side of the glass. Light, as it
passes an edge is bent, diffracted, and will then be scattered in the
penumbras or shade of the aperture so that it strikes in an area not
illuminated, and therefore reduces contrast. I think that this can be
measured, and so a lens can be tested and found out to have diffraction
limiting of the image at a certain point.
Bokeh on the other hand, seems to be a subjective evaluation of the
characteristic of the out of focus elements in a scene.
I tend to think of it in terms of how the aperture is designed and where it
is placed. I see this on the motion picture screen a lot. In some films, if
you notice the rendition of a very out of focus renditions of somewhat
specular highlights it is very noticeable.
For example, in the movie, The Patriot, I noticed that when they had a
closeup, that is there was a light reflection- like from a leaf or the
rendition of a lamp in the distant background, it was indeed a blob of
light, but the edges distinctly showed what looked like a twelve pointed
star! Even though the out of focus spot of light was rendered as a circle of
confusion, the edges were definitely defined as a star shape. In other
films, most in dfact, the spot of out of focus light is rendered like a
bubble, with the edges more or less ill defined and usually with fairly
smooth edges.
The image in focus, however, may be indistinguishable from one taken with
another lens; The in focus image, however, would beging to suffer for
diffractive limitation when the in focus image started to loose contrast,and
detail, the out of focus portion notwithstanding. I can see diffraction
limiting affecting the out of focus portions of the image very little, if at
all- in any discernable way.
The 35 Summicron is fiarly good wide open, better than 95% of the lenses out
there, and Leica designs its lenses so that there is minimal abberations
wide open- one of the reasons they are so expensive. They are designe to
perform well wide open and so you may find the in focus portion of a shot
taken at f2 is as sharp as one at f5,6. Diffraction limiting, it seems, is
inherent in any lens with an aperture- essentiall all of them, since to let
the light through, there logically has to be a hole!
How the diffraction around that hole handled- whether it is wide open or
very small, determines the diffraction limiting of the image. I have seen
some lenses that at f5,6 are quite low in contrast, and I have had to assume
that the placement of the aperture, and the internal contruction of the lens
had a lot to do with it.
Dan ( Still trying to get that 'perfect' negative!) Post

- ----- Original Message -----
From: "Dante A Stella" <dante@umich.edu>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Saturday, July 22, 2000 5:37 PM
Subject: Re: [Leica] Diffraction limited; bo-ke


>
> Mr Puts:
>
> Maybe I am misreading something you wrote on your web site, but if
diffraction
> is not limiting the performance, why pray tell should you shoot at larger
> apertures?  What is the limiting  factor that agitates for this?  I am
using a
> 35/2 4th generation Summicron and I believe you indicated that the top
> performance for lenses of this type was at f/5.6.  Do the aberrations
begin to
> increase at some point past that?
>
> The easiest shorthand that I have heard on bokeh is that good bokeh means
a
> smooth transition from sharp to unsharp suggesting greater depth of field
than
> really exists.  Bad bokeh has a sharp and sudden transition that doesn't
> reproduce the way we see (real) scenes.  We don't see wiry tree branches
in
> real life, but we do in a lot of photos taken with more modern lenses.
>
> Dante Stella
>
> Erwin Puts wrote:
>
> > 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.
>
> --
>   ------------------------------------------------------------------------
> Dante Stella
> http://www-personal.umich.edu/~dante
>