Extremely Toxic - Mitakon 0.95 35mm MIC

Started Aug 18, 2012 | Discussions thread
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ProfHankD Veteran Member • Posts: 3,736
Big vs. Pretty Bokeh... Aperture vs. Apodization

El Matadurr wrote:

the bokeh on that is extremely subject-specific. In some of the shots it looked fantastically creamy, but in others (especially ones with highlights) all it did was distract.

Actually, the bokeh of most very fast lenses, especially retrofocal wides, are rather harsh. Basically, larger apertures make larger OOF PSFs (out of focus point spread functions), but design constraints to get good IQ commonly result in bright-edged OOF PSFs that are misshaped off-axis by internal vignetting.

The traditional trick to improve bokeh is to undercorrect SA (spherical aberration). However, that actually trades a smoother PSF after the focus point for a more heavily outlined one before the focus point. Overcorrecting SA does the opposite, and the sample shots from the Mitakon show some signs of this: closer things look better.

The way to get really smooth bokeh is to add an apodization element -- shaping the OOF PSF like the Minolta/Sony 135mm STF does. It is fairly easy to make an add-on apodization element, for example as a front-mounted piece of film (filter) with an appropriately-sized Gaussian blur-diffused clear center leading to black edges. (Actually, the STF lens uses spherical shading, which they build in a very clever way.) Of course, the dark edges cause some light loss; the STF lens is f/2.8 but T4.5. Of course, if you make the clear area too large, the filter will vignette rather than apodize. Incidentally, the use of an apodizing element also slightly improves sharpness of in-focus portions of the image.

In summary, better bokeh don't come from faster lenses, but from apodization. Some lens formulas, especially certain double Gaussians, naturally have modest apodization effects (e.g., the old Takumar 50mm f/1.4), but nothing works as well as apodizing by design.

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