Bokeh Issues on the Pen F 38 mm f/1.8

Laserguy

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I have been using my little Pen F 38 mm a lot over the past month or so, mostly to take pictures of my newborn son. (Sorry, I can't post any of them; my wife has a very strict "no pictures of the kid on the unprotected internet" policy.) For this application, it's usually great. However, I have been dissatisfied with the bokeh from the lens. In addition to having a severe case of ni-sen, it tends to look sort of "swirly." The picture below is a missed shot, but it illustrates this "swirliness" very well.

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I didn't quite figure out what was going on until I looked at the corner of another image, and saw this:

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An out-of-focus point source not only produces a pretty nasty-looking ring, the ring itself isn't circular! It looks as if the bokeh is terribly astigmatic, with a much smaller extent in the tangential plane than in the saggital one. It's pretty strange-looking, and I haven't seen anything like it on my more modern lenses. It's a bit of a bummer for a bright, portrait-length lens like this, since it puts some pretty severe limits on the sorts of backgrounds that will produce an acceptable image. Perhaps it could be used to artistic ends...

Does anyone have experience with the Pen F 40 mm or another similar-length Pen F lens? Do they have this bokeh issue as well?
 

Jonas B

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Swirl:
What you see is normal for any lens with a small (or "normally" sized front element) and is an effect related to the Cat's Eye effect. As I understand it this is a result from "optical vignetting" (sometimes called "artificial vignetting") and it is present with most lenses, to different degrees.

Non-circular ring:
It's the same thing, but here it's simply called the Cat's Eye effect. Here you can see it with a big 85/1.4 lens on a FF camera:
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and to get back to our µ4/3 world we have these Panasonic G20/1.7 samples:
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Both the lenses above may be considered modern, no?

Bright rings:
That is unfortunate. The reason can be discussed. Some say it is about a lens that is overcorrected against spherical aberrations, some say it is a lens where the manufaturer concentrated on resolution and contrast rather than pleasing rendering. I think people are talking about the same thing there.

What to do? To me it is about learning the gear. That sounds boring but after having spent years, and dollars, looking for the perfect lenses I now believe we have to find equipment we like to work with, learn it well and then the good results will come. In this case you have to avoid specular highlights in the background, or learn at what distances the phenomen is the least disturbing. And now you know you should stop down one or two stops before shooting the flower bushes.

And then there are all those lenses waiting for you to explore them...

regards,

Jonas
 

Laserguy

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Those shots are interesting; I had never noticed the effect on any other lens. (Now, of course, it will be all I see...) The astigmatism does seem to be a lot less pronounced in the two modern lenses than it is in my old 38 mm Pen F. Part of that is because the two modern lenses you used have pretty smooth bokeh while the 38 mm has those nasty bright rings, but the aspect ratio of the out-of-focus disks are clearly different with the different lenses.

Like you said, it's a matter of just working to the strengths of the equipment. I know now that the 38 is good at smaller apertures or against smooth backgrounds, but that it is not good at all with wide apertures against very busy backgrounds.
 

Brian S

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It's caused by astigmatism in the lens, manifests itself as football shaped objects with the elongation increasing as you get off-center.
 

Jonas B

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It's caused by astigmatism in the lens, manifests itself as football shaped objects with the elongation increasing as you get off-center.
No. I know the terminology can be a mine field and perhaps this has been described as astigmatism somewhere. To me astigmatism is associated with the "spoken wheel" and differences in tangential and sagittal resolution.

Sometimes I have sen astigmatism used as a term for the typical gull wings seen at the corners/borders in an image taken at long distance of street lamps (in focus), or spot light sources. It can all be a bit confusing but I don't think astigmatism can be used to describe the bokeh problem laserguy described.

regards,

/Jonas
 

Jonas B

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Laserguy,

The circles becoming ovals can be seen also in top notch lenses, like the OM90/2 Macro, both on FF cameras and 4/3 cameras.

For the most of the time the problem is never a problem. Sometimes it shows up and most people don't care. Here is an image from a series of images taken at an open air exhibition:


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Above: Photographer Mitsy Morel trying to protect her images against the rain, "Planket", Gothenburg, Sweden 2009


If anyone is interested in more images taken with this lens you can find the above one and some others here.

regards,

/Jonas
 

Brian S

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Nikkor-S 5cm F2 in F-Mount, wide-open.

View attachment 145164

Most optics books that I've seen describe the footballs as being caused by astigmatism.

The Summarit is well known for swirlies, caused by astigmatism.
 

Bokeh Diem

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Swirlies happening here with the Panasonic 45-200. Toilet bowl effect enhanced with PP... I figured if you've got it, flush it.

Background is about 1500mm beyond subject.

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Jonas B

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Nikkor-S 5cm F2 in F-Mount, wide-open.

View attachment 145167
As I said, it is about terminology. To me the two images above are only loosely connected and the first one is a schematic showing different focal planes making for the differences in tangential and sagittal resolution. There you have the spoken wheel. The second one shows the Cat's Eye effect caused by vignetting.

They are loosely connected as the OOF highlights take the form of the aperture blades. When optical vignetting affects the shape of the focal blades or, when wide open, the inside tube shape of the lens, you get different effective aperture openings for tangential and sagittal objects therefore making it look like astigmatism.

In my opinion the oval opening depicted should be seen as the Cat's Eye effect caused by vignetting: if you imagine a lens with a very big front lens element helping to minimize this vignetting the effect will lessen. That has however nothing to do with that lens' ability to render tangential and sagittal objects sharp at the same plane or not.

Most optics books that I've seen describe the footballs as being caused by astigmatism.

The Summarit is well known for swirlies, caused by astigmatism.
Or maybe the Summarit is well known for its swirly bokeh caused by optical vignetting...
I don't know about "most books". I stick to the terminology, or at least I try to - sometimes I make mistakes, as used by Paul van Walree. His page on astigmatism is here. (Check out figure 7close to the bottom.) Then click away to his page on vignetting here.

Paul is active at another forum I'm taking part in and I trust him. At his site he also presents his sources and references.

Brian, I can't stop you from calling the Cat's Eye effect for astigmatism but I don't think it is a good idea to do so. The terminology is changing and we can for example see longitudinal CA slowly being exchanged into defocus CA. I don't say anyone is exactly right or wrong. In optics nothing is isolated from anything else and I think that is the reason for different words for the same, or nearly the same, phenomenas.

regards,

Jonas
 

Brian S

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I believe that a combination of astigmatism and over-correction for spherical aberration causes the effect shown. Unless you have the prescription of the lens and run it through a ray-tracing program, the world may never know. I believe that the stretch of the "footballs" increases with distance from the center, which can be explained by astigmatism. The bright contours of the footballs could be caused by over-correction for spherical aberration. The "footballs" have an unbroken outline. The vignetting that I've observed in several of my lenses tends to be at the edges. The Canon 50/0.95 comes to mind on the latter, and the contours of the highlights are asymmetric.

Canon 50/0.95, wide-open.

View attachment 151854


But again, it's all speculation without having the optical prescription and a ray trace.
 

Jonas B

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To put it simply. Some lenses have nice bokeh, others have rather ugly bokeh. Not necessarily old vs. new designs just a trait of a particular lens design. I love the photograph your bokeh thread on GetDPI.
Nah, that is to put it too simply. ;)

GetDPI... have we identified a lurker? Or do you gave another nick over there?

regards,

Jonas
 

Brian S

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The OP will understand this comment...

I asked Bernie about it. That's where I got the answer.
 

LisaO

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I'm losta there, opposite of here where I'm first name last initial, there I'm first initial last initial. I post there sometimes but not too often.

Nah, that is to put it too simply. ;)

GetDPI... have we identified a lurker? Or do you gave another nick over there?

regards,

Jonas
 

Jonas B

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To me this was an interesting thread with some explanations, speculations and conclusions (some both right and wrong, that's not bad) and with an interesting old Pen lens as starting point. But, less than 140 views and few attenders.

It sure is fine being a happy snapper but it isn't wrong understanding why images look the way they do, and in the end become a better photographer. I'm sorry if I made people feel uneasy and caused less interest for the thread than it deserves (imo). Or is it that very few are interested in stuff like this?

Jonas
 

Jonas B

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I stumbled upon a Zeiss white paper the other day. I haven't read it earlier but my guess is that it has been around for some time. There is a paragraph on oval "circles" discussed earlier in this thread at page 31, and also a lot of other things of interest.

I know this thread is old but it should never be too late to get things right, right?

regards,

Jonas
 
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