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bokeh test target

What is good bokeh is somewhat subjective, but in general it's anything that makes out-of-focus objects be as non-distractive as possible. My bokeh tests are here. But those tests neutralize aperture and focal length (because their aim is to compare lenses).

Here I run some tests to see how aperture and focal length (perspective and distance) affect bokeh.

In any case, I must say I'm no expert: I'm just experimenting with my lenses as I learn about these things myself.

Quantity of bokeh: aperture tests
Compare these results with aperture neutralized (changing the focus point so that the size of the highlights is the same on all tests) with the results that leave the focus distance unchanged.
The first set tells me that my Carl Zeiss Jena Sonnar 135mm f/4 has somewhat better bokeh than my Leitz Elmarit-R 135mm f/2.8. The second set shows how the fact that the Leitz is faster is actually more important than the slight difference in bokeh quality.

Neutralizing aperture:

Not neutralizing aperture:

Focal length and bokeh
People often say longer lenses deliver more bokeh. The scenario that best exemplifies this issue is when you can control how far you get from your subject (you can step backwards to use a longer lens) but the subject-to-background distance is fixed.
To see how important is the effect of focal length and perspective on bokeh, let's compare results with perspective neutralized (moving the camera so that the out-of-focus background is always the same size, and changing the focus point so the amount of blurring in the background is always the same) with the results that don't neutralize perspective (the subject is placed at a fixed distance from the background, and the camera is moved so that this in-focus subject is the same size at all focal lengths).
These shots are all taken at f/5.6 with the Canon 18-55 f/3.5-5.6 IS kit lens. The left column tells us bokeh quality doesn't change when we move through the zoom range, but the right column shows how the perspective of a longer focal length blows up background bokeh.
The problem with this is that often you can't step backwards in order to use a longer focal length; if distance and type of shot (e.g. closeup) determine the focal length you have to use, there's no point in considering whether a shorter or longer lens would deliver better bokeh.

Canon 18-55 f/3.5-5.6 IS kit lens set at f/5.6
mmneutralizing perspectivenot neutralizing perspective

Some random conclusions from these and other bokeh tests

* First and foremost: this bokeh thing is a very comprex issue...

* I always thought my Carl Zeiss Jena Pancolar 50mm f/1.8 and my Carl Zeiss Jena Sonnar 180mm f/2.8 had great bokeh; it turns out, they both have pretty bad bokeh, they're just making up for quality with quantity: the Pancolar is my fastest lens at f/1.8, and the Sonnar, at 180mm f/2.8, is both long and fast.

* Vintage glass is a lottery with regards to bokeh too: that Sonnar that came out so bad in my tests is considered one of the mythical "best bokeh ever" lenses (see here).
(the other possibility is that other people also got fooled by the focal length and speed of this lens, but the fact that my other two Sonnars, the 135 and the 300, have much better bokeh, seems to point to my 180 being sub-par; maybe it was just badly assembled: go here to see how that can affect both sharpness and bokeh).

* Another thing that points to vintage glass being a lottery: I expected to come out of this with a conclusion to the tune of "how I learned to stop worrying and just buy Leitz glass"; but the 135mm (which I just bought about a month ago) is actually not stellar: at f/2.8 it's not great, and at f/4 the much cheaper Sonnar 135 is actually better; if my next camera can work with M42 lenses, maybe I'll sell that Leitz.

* Aperture affects both bokeh quantity and bokeh quality: I hardly ever use the Pancolar wide open, because it becomes relatively soft in the corners; as it turns out, that's also helping with bokeh big time (stopped down to f/2.8, it is both sharp and pretty nice in terms of bokeh).

* Cheap, slow, lenses can have very nice bokeh; they just have very little of it.

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