I am reading the LLC by EP. I found this topic is very interesting and worth discussion.
EP said,
The presence of aberrations decreases the ability to detect a defocus, as the result of aberrations is a loss of contrast, blurring of sharp edges and of fine details, just as the effects of defocus, we described above. A lens that shows less image degradation in the out-of-focus areas, (has good bo-ke) must be, therefore a lens with a higher level of residual aberrations.
New or recently redesigned Leica lenses are more highly corrected than older lenses and therefore have a steeper transition from focus to defocus areas. A lens in case is the Summicron-M 1:2/35mm (3) from 1979, which is credited with very good bo-ke and the current Summicron-M 1:2/35mm ASPH, which is supposed to have a different kind of bo-ke. The aberrations still present in the older version do indeed decrease the effect of the out-of-focus blur.
Which lens should we choose?
1. a highly-aberration-corrected lens which has excellent image quality for the in-focus object but only so-so out-of-focus image, or
2. a lens with "good bo-ke" but only so-so in-focus image.
For those who are interested, you can download
LLC for free from EP web site.
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