Hi

Please help me out with a specific question on, Pentax or any brand, lenses with regards to bokeh.


***Blabbering background

Bokeh? Indeed subjective, immeasurable and somewhat esoteric. Hard to describe and also a comprehensive conception. We have foreground bokeh, background bokeh, bright rings, areas in focus and the rendering of the stuff in focus and out to areas which are out of focus. We have aperture blade shapes visible in specular highlights and we have some liking to see that and those that wish they weren't there. There is sometimes axial CA (or Longitudinal Chromatic Aberrations (LCA) as Klaus at photozone says) playing in the OOF areas. We even have CBF that was easy to understand as soon as it was explained/defined, and more, like the cat's eye effect and other things.


***Question:

My question: What lenses, any brand and in the FL range 50-100mm, do you know of that consistently produces either good or neutral background bokeh according to the definitions *below*? I'm interested in the rendering abilities wide open, and stopped down 1 and 2 stops.

Hint: Don't give me the SMC 50/1.4 or FA31 or A85/1.4 or... According to this somewhat tough definition they don't consistently produces good or neutral background bokeh. They can all render beautiful pictures that are pleasing to the eyes, but this is purely about the rendering of background highlights at different distances.


***Definitions in _this_ case:

Bad bokeh:
http://photos.imageevent.com/jonas_b/dprmonthly/enstakabilder/1_poor_rockwell_bokeh_definition.jpg

Fig. 1. Poor Bokeh. This is a greatly magnified blur circle showing very poor bokeh. A blur circle is how an out-of-focus point of light is rendered. Note how the edge is sharply defined and even emphasized for a point that is supposed to be out-of-focus, and that the center is dim.


Neutral bokeh:
http://photos.imageevent.com/jonas_b/dprmonthly/enstakabilder/2_neutral_rockwell_bokeh_definition.jpg

Fig 2. Neutral Bokeh. This is a a technically perfect and evenly illuminated blur circle. This isn't good either for bokeh, because the edge is still well defined. Out-of-focus objects, either points of light or lines, can effectively create reasonably sharp lines in the image due to the edges of the sharp blur circle. This is the blur circle from most modern lenses designed to be "perfect."


Good bokeh:
http://photos.imageevent.com/jonas_b/dprmonthly/enstakabilder/3_goodl_rockwell_bokeh_definition.jpg

Fig. 3. Good Bokeh. Here is what we want. This is great for bokeh since the edge is undefined. This is also the result of the same spherical aberration, but in the opposite direction, of the poor example seen in Fig. 1. This is where art and engineering start to diverge, since the better looking image is the result of an imperfection. Perfect bokeh demands a Gaussian blur circle distribution, and lenses are designed for the neutral example shown in fig 2 above.

(I have taken the illustrations from the infamous Mr Rockwell's site, from this page:
http://www.kenrockwell.com/tech/bokeh.htm
I haven't asked Rockwell and can only hope he doesn't mind. I tried to call him and ask for his blessing and permittance to grab this part of his site, but his wife told me he is in Nirvana and unreachable for us common mortals for the moment.)

regards, and thank you!

--
Jonas