Re: [Leica] Test versus trial?, part 2

[Stephen Gandy <Stephen@CameraQuest.com>]

Erwin,

it's amazing you  spend so much time and effort trying to dismiss Mike
Johnston's bokeh testing, only to finally ironically  agree with him in your
last statement.

yes, more fact finding is needed. That is why Mike advised photog interested in
the subject to test their own lenses and come up with their own conclusions.
What you, or I, or Mike, think about bokeh of OTHER  photog's lenses is
unimportant.

What the individual photog thinks of the bokeh of  THEIR OWN  lenses in their
own camera bag is important.  If others don't see exactly what another photog
sees,  so what?

Stephen Gandy

imx wrote:

> Bokeh is generally defined in a very loose way, which in itself hinders a
> good discussion of the phenomenon.
> Often bokeh is presented as a recent discovery which is not true. The intra-
> and extra focal out-of-focus blur is a wellknown topic in every optical
> handbook. New is only the idea that these blur effects have a subjective
> dimension and are supposed to be instrumental for the qualitative
> appreciation of a picture.
> Blur has a real and a subjective aspect.  The analysis of bo-keh is in
> itself simple: we know from optical theory how  the focused rays will be
> distributed in the unsharpness areas in front of an behind the plane of best
> focus. We can also calculate the energy distibution within the blur circles,
> located on the several unsharpness planes at any required distnce form the
> plane of focus. This is Canon's approach and by studying the changes in
> energy distribution patterns, they can predict what effect will result. The
> optical cause of these different energy distributions within the blur circle
> is the level of correction of spherical aberration. So if I know the level
> of SA in a lens, I can safely predict the way the image will be reproduced
> in the unsharpnes plane.
> An optical manual from 1933 (no typo!) already gives a lucid account of this
> phenomenon.
> If we would do a series of experiments with our lenses to see how they
> reproduce the out of focus blur circles and how these have impact on the way
> the out of focus images are shaping (deforming) the image structure and
> details.
> As it seems the case that the characteristics of the image (shape, structure
> of details, contrast) in the out-of-focus plane are decisive for the type of
> blurring in the  rendition of out-of-focus foreground and background  and
> the  perception of the eye to this type of blurring, we need to establish
> some clear guidelines what structures we have to look at.
> Some experts note that bokeh is confined to out-of-focus highlights, which
> are influenced by optical design and number of diaphragm blades. But not one
> experiment (and I checked more than a hundred sites on internet to become
> familiar with what is the general opinion) has conclusively decided which,
> if any, of these supposed causes is the real one.
> Others hold that bokeh is only relevant for macro- and portrait photography.
> Bokeh is variously described as "hard edged blurs", "abrupt transitions from
> light to dark", "poor definition in out of focus objects", " doubling of out
> of focus lines","soft edged blurs", "smooth even transitions in areas of
> contrast". Then we have all kinds of descriptions of the blur spot itself:
> donut shaped, butterfly shaped etc. Not one description refers to focal
> length, aperture, distance and relative position of fore and background
> location. Now "(fore)background"  is not an isolated plane, but a portion of
> reality with depth. So where exactly are you loking at when discussing
> bokeh?
> As the origin of bokeh is the blur circle and its physical shape, dimension
> and energy distribution, any study of bokeh would have to control the usual
> blur circle effects: focal length, distance to plane of focus, distances
> from PoF and print magnification. Enlarge a negative 5 times or twenty times
> and the size of the blur circle changes and so the relative diatance from
> unsharpness blyr to PoF also changes. Diffeernt portions of the out of focus
> image are affected.
> To get ahead in this barrage of definitions and options, we need to
> establish a common situation. You can not compare a picture taken with a
> 50mm at 5.6 of a girl at 5 meters in a sunny environment and enlarged 8
> times with the background at 15 meters, with a picture of a 90mm lens at 4
> of a flower at 1 meters in contre jour to get nice specular highlights in
> the background, where the background is at 3 meters and the enlargement is
> 15 times. And so on.
> Without a description of an experimental situation, that can be accurately
> reproduced in several testsessions, without the definition of the camera
> position and distance, aperture and the definition of the specific o-o-f
> planes where the bokeh effect should be studied, without specified
> enlargement factors, without a specification of the blur effects to look for
> and so on, we are simply getting nowhere in the direction of studying the
> phenomenon.
> I do assume, wading through hundreds of pages of lens descriptions by people
> who discuss bokeh, that  concepts like "openness", "surrealness", "airbrush
> creamy smooth" and so on are all attempts to describe the impression that
> the observer gets from looking at a picture.
> Such an approach makes for a fine discussion that may be satisfactory for
> the participants, but is so open-ended and vague that I wonder what the goal
> is here.
> If you pick up a art book like " how to look at photographs" or 'The
> photograph", the content and meaning of a photograph is discussed in terms
> of visual language and the whole weight of visual art critique.
> If you pick up an optical book, you are treated with physics, and the
> fundamentals of visual perception.
> Both approaches cover the whole spectrum of how to analyse and discuss a
> photograph. And it is very enlightening to study both fields.
> The bokeh discussion on the contrary is quite confusing and has no added
> value (at least not in the present state of vagueness and lack of any
> experimental base). In the Japanese culture,  the bokeh discussion may be of
> great value as it fits in in their tradition of visual awareness and
> sensitivity.
> If we wish to improve the quality of the bokeh discussion, we should move
> beyond the trial stage and do some substantial fact finding based on
> repeatible experiments.
> Erwin