The Naked Truth About Bokeh

Started Jul 22, 2016 | Discussions thread
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Marianne Oelund Veteran Member • Posts: 6,711
The Naked Truth About Bokeh
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Several years ago, I acquired a handy optical device for producing high-quality pinpoint light sources, for study of lens acuity and aberrations.  I have finally gotten around to using it, but not for lens acuity studies.  Rather, I've found it is very suitable for studying the COC, or blur circle, profiles of a lens.  I couple it with a green LED light source, which further helps by narrowing the wavelength bandwidth and eliminating most chromatic aberration effects.

Looking at the blur circle formed by a lens from a pinpoint light source gives the clearest view possible of how different bokeh characteristics are formed.  It strips away the confusion and clutter, or cloak if you will, which masks the essential lens behavior in photographs, leaving it rather naked for all to see.

I will begin with two lenses which are known for pleasing bokeh:  The Nikkor 58/1.4G and 85/1.4G.  The 58G gives an especially "dreamy" look to its images when used wide open, or even at f/2, and has extraordinarily smooth background bokeh.  Looking at the pinpoint-source blur circles in the background direction, we see that the blur circle fades out gradually toward the perimeter and does not have any abrupt cutoff.

The 85/1.4G represents more typical lenses with under-corrected SA; although its blur circle does decrease in intensity toward the boundary, there is a well-defined perimeter which does not produce such a soft bokeh:

Unique 58G wide-aperture background blur circle with diffuse perimeter and bright central peak

The 58G also has a rather bright and small central peak which tends to maintain high resolution at hard image edges, while contrast falls off gradually due to the highly diffused outer area of the blur circle.  This is readily seen in images which have white lines on a black background.  The precise plane of focus at f/1.4 is difficult to decide upon; if you view these scales at different distances, you are likely to pick different optimum-focus locations for the f/1.4 scale:

The closer you look, the further back you are likely to judge the precise focus plane for the 58G at f/1.4

There is quite a bit of information in these figures, and much more could be said, but I will leave the discovery to you.  Hopefully you will find some interesting surprises.  

As I have time, I will add posts for other lenses.  The next candidate is the Sigma 50 Art, which has a reverse characteristic to the above lenses.  You will see that it is a good choice for smooth foreground bokeh.

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Source credit: Prov 2:6
- Marianne

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