Re: [Leica] Diffraction limited; bo-ke

[Dante A Stella <dante@umich.edu>]

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.

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Dante Stella
http://www-personal.umich.edu/~dante