Like a lot of photographers, I have my special little problems I try to solve.
This happens to be a tree in my mom's back yard, which I've been working on trying to get just the right image of for years. Because it's the bark of the tree that appeals to me and that produces some pretty bizarre bits and pieces, I generally work on this puzzle as an abstract. So all the discussion of the last two weeks still applies.
But today's lesson is a bit different. I haven't really written anything about bokeh (pronounced bow-kay), which is the concept of how the out of focus (OOF) areas of an image look. Here I want you to look at the brightest green out of focus highlight. Notice how it has just a hint of octagon to it? That's because this was shot with a lens that uses 8-blades in the aperture, and those blades weren't rounded.
Indeed, the very top of the OOF highlights have a bit of a point to them while the rest of the blade intersections are better masked, something I call bokeh asymmetry. On the other hand, this lens doesn't have another problem that's common with OOF highlights: edge reinforcement.
Lenses with lots of chromatic aberration, especially longitudinal, tend to form a slight ring at the boundary of the OOF highlight. Here, the highlight falls off naturally and has no real hot spots of its own.
Since there's little in the bokeh to distract, I'd tend to call this good bokeh. It's not great, as it does have the non-circular defect, but it's still not that bad.
These days, of course, the temptation would be to just use Photoshop's tools to produce a better blur in the background. Still, the bokeh of the lens will intersect with that: a greatly distorted circle would still be a greatly distorted circle after Photoshop's blur, unless you just obliterated the background completely.
I've heard a few photographers say that because Photoshop now has handy tools for this (and onOne's FocalPoint is another such tool), that you don't need a lens with good bokeh. I'd argue that you still do. Better data in, better data out.