Re: Bokeh:a mythical construct

[Erwin Puts <imxputs@knoware.nl>]

>Date: Thu, 20 Nov 1997 22:39:06 +0100
>To: leica-users@mejac.palo-alto.ca.us
>From: Erwin Puts <imxputs@knoware.nl>
>Subject: Re: Bokeh:a mythical construct
>Cc:
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>Gee, I now wished I had kept my big mouth shut.
>Sorry for not answering within a day. Sometimes I have to work for a
>couple of days in circumstances where my mind is not tuned to Leica stuff.
>To start: I am very much aware of the fact that any lens can only give a
>sharp rendering of an object in a flat plane. So any  3D object  has one
>sharp plane and every other extension of that object in front or behind
>that plane is unsharp in various degrees. The 'thickness' of the sharpness
>plane is governed by the familiar circle-of-confusion, which is also the
>basis of the various DOF tables. All these tables are based on the fact
>that the permissible circle of confusion is 0.03mm. This is nowadys
>hopelessly inadequate as anyone who enlarges his negatives or projects his
>transparencoes can readily see for him/herself.
>So the phenomenon of unsharpness as such is a fact of optics.
>The way the unsharpness is rendered by any lens is a result of two kinds
>of computations. First of all the computations that ensure the sharpness
>of the in-focus-plane are relevant for the unsharpness regions. The better
>the optical correction of the sharpness plane, so will be the rendition of
>the unsharpness areas. BUT: if the corrections of the sharpness plane are
>extremely well done, then the visual or perceivable diffence (that is the
>gradient from sharp to unsharp) will be quite steep. This happens with all
>modern Leica lenses of the recent (3) generation. Older lenses have a
>smoother gradient, not because of any special characteristics  of the
>unsharpness rendition, but simply because the in-focus-sharpness was not
>as well corrected as it is today.
>Second: any lens designer can concentrate on maximizing the
>characteristics of the sharpness (more general: the rendition) of the
>in-focus-plane and ,speaking relatively, neglect the correction of optical
>defects in the out-of-focus areas.
>That is what most japanese lensdesigners do as they optimize for high MTF
>values in the plane of focus. Therefore the noted diffences between for
>instance Leica and Nikon/Canon.
>Nothing new or mythical here. The effect of the placement of the diaphragm
>and the number of blades is greatly exaggerated. The basic optical
>characteristics are laid down by the optics not the mechanics. Only in
>highly unsymmetrical lenses the diaphragm placement has a certain role to
>play. The number of blades has an insignificant effect.
>So 'bokeh' as a construct has nothing new to reveal. But as soon as we
>start stating that the 35 Summicon (nonASPH) has better bokeh than the
>actual Summicron 35 ASPH we make the wrong turn  from a factual view.
>Certainly if we imply that Leica lenses are specifically designed to show
>good bokeh (whatever that is) we are sliding into mysticism.
>Just as we can rationally discuss the correction of optical aberrations in
>the sharpness area, we can do the same for the unsharpness area. To make a
>bold statement: if Canon (which I give a higher value optically than
>Nikon) were to decide to design a lens according to Leica standards, the
>rendition of the out-of-focus-areas would be identical.
>
>>    Erwin, are you saying that you believe that all lenses of
>>    the same focal length render out-of-focus areas identically
>>    when opened to the same aperture?
>>
>
>No I do not believe that all lenses of same focal length focused at an
>object at the same distance and stopped down to the same diaphragm render
>the o-o-f areas identically. But that is as stated a function of the
>corrections of the sharpness areaand the careful attention to the optical
>aberrations in the unsharpness areas. Optical aberrations are not
>different for the sharpness and the unsharpness areas. The same laws and
>mathematical equations work here and there.
>
>As for testing. As always you can only compare in a meaningful way any
>differences when you take comparative pictures with the same film and the
>same object at the same distance with different lenses set at the same
>aperture. Pictures taken in these settings with the Summicron 35
>(non-ASPH) and Summicron ASPH revealed hardly any  visual difference in
>the rendition of out of focus areas. The superior bokeh of the nonASPH
>version I failed to note.
>Erwin
>
>
>