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| p.19 #17 · Official: Zeiss Otus 55mm f/1.4 APO-Distagon |
I re-read the Bokeh write-up by Dr. Nasse, and he definitely didn't mention anything about it. However, it seems like every web site talking about how to interpret the MTF does mention about the convergence (good bokeh) and divergence (bad bokeh) effect on bokeh. Quoting directly from Nikon:
"Using an MTF chart to determine the Bokeh effect of the lens
Another factor that can be read from the MTF graph is the 'bokeh'. Bokeh is a term used to describe the quality of the out of focus areas a lens produces. The bokeh effect varies between lenses and the effect is influenced...Show more →
It is clear that a bunch of things can create bokeh that some people are not going to like. The shape of the aperture can be a problem. For example, I really like the bokeh of the Carl Zeiss Jena 135 f/3.5 except if you stop down and then the highlights from the aperture are pentagons. That bugged me when I had the lens, but other people it doesn't.
Similarly, under corrected spherical aberrations will produce very diffuse background bokeh at the expense of really busy foreground bokeh (over corrected creates the opposite or maybe I had that backwards but you get the point--over or under correcting SAs makes one side of the focal plane very diffuse at the expense of the other).
High CA can also be a bokeh killer. If it is in the focal plane, then I believe it would indicate a busy bokeh. If it is outside the focal plane, then you will get colour fringing in the bokeh. I am less sure what astigmatism and come do to bokeh, but they probably are not good either. So what does this mean for the MTFs and bokeh? Perhaps someone can chime in who knows more, but my understanding is that it is not simply the divergence in the sagittal and tangential lines in the MTF that can indicate CA in the focal plane, but it is a pattern when comparing wide open MTFs to stopped down MTFs in which the divergence between the sag and tang lines gets bigger as you stop down that suggests a potential problem with CA. One way that this pattern can emerge is that a lot of uncorrected SAs mask the divergence between sag and tang lines wide open, but when these SAs clear up as the lens is stopped down the divergence between the sag and tang lines becomes evident. Such a pattern, (at least as I understand it) can be caused by both high SAs and high focal plane CAs.
If this reasoning is correct, then when we consider the 55 Otus we know that it has very low in focal plane CA and since we know that the MTFs are unlikely to give us any other hints about its bokeh. Basically, the Otus passes the one problem the MTFs might show, but there are lots of other things that can mess up the bokeh and there are certainly some APO lenses out there with low focal plane CA that have bokeh that I don't like (e.g., the Leica M 75mm APO). So this doesn't mean that the Otus will have great bokeh. In the same way that knowing that a lens has a nice round aperture when stopping down doesn't mean that it will have nice bokeh.
In contrast, if we consider the new Nikon Noct the close sag and tang lines wide open in no way indicates it will have good bokeh. In fact, if the lens has high SAs (which it probably does) the MTFs wide open would hide any divergence between the sag and tang lines and in no way would indicate low focal plane CA or good bokeh. Even if the New Noct has low in focal plane CA, it still could have terrible bokeh--a lot of other things could make it bad.
So we will just have to see from examples how good the bokeh is for each of these lenses. I think certain things from specifications (like a small number of aperture blades) and from MTFs can indicate that the bokeh might be bad and perhaps even exactly how it might be bad, but I don't think there is any way to know it is going to be good until you see it used in a lot of situations.
The good news is that so far both lenses look like they produce some good bokeh (at least according to my taste) in at least some situations. I hope it is most situations, but I suspect like most lenses there will be some situations in which the bokeh won't look so good. Knowing exactly when each lenses works well (i.e., makes nice bokeh) and when it doesn't will take a fair bit of time to uncover.