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swirling bokeh

richard314159 PRO 3:05pm, 1 July 2010
Hi I was reading about the Petzval lenses and their so-called "swirling bokeh". (Best example I found is here: my.net-link.net/~jsmigiel/images/technical/collodion/Robi... by Smigiel. There are other examples on Flickr, including Russian lenses mounted on DLSRs)

So before I bid £££s on ebay and perhaps get the wrong lens or wrong focal length for my camera / image sensor / film size, I wondered how easy it would be to reproduce the effect with HM lenses. Indeed, what causes the effect?

I presume that the effect is due to very curved image plane and out of focus bokeh away from the certre of the image that isn't circular as we'd expect because of the image curvature. If that's all it is, it should be easy to reproduce. Maybe using two acrhromatic doublets like a standard symmetric arrangement, but put one of the doublets the "wrong way round" (thus adding the effects of the curvature rather than cancelling them out as would be for a RR).

Perhaps focusing is impotant for the effect. The centre must be *just* in focus, so that the areas away from the centre are well off focus. Choice of focal length, image circle size for the sensor or film area is important too.

Anyone done this already (i.e. made such a lens)? Any hints? If not are there others that want to try?
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johnnyoptic PRO Posted 8 years ago. Edited by johnnyoptic (admin) 8 years ago
It is my understanding that swirly bokeh results from astigmatism in the lens, not from field curvature. (Field curvature can cause the outer part of the image to be blurred to produce an effect like the lensbaby, but that is different from swirly bokeh.)

Regarding what commercial lenses produce this effect, I recommend looking in flickr's swirly bokeh group.

I've managed to get this effect in one of my semi-random lens hacks in which I pieced together recycled components, including the lens from an old 8 mm movie camera. Unfortunately, the movie camera lens is a complicated "black box" which makes optical analysis of the lens impossible.

Here's an example I shot with this lens.
infinite

I'm continuing to look for a reproducible way to get this effect from a homemade lens. It would be great to find a fairly simple lens design with low spherical aberration and field curvature but high astigmatism. I suppose experimenting with a fast Petzval would make sense.
richard314159 PRO Posted 8 years ago. Edited by richard314159 (member) 8 years ago
Right - I have done some more work and reading and thinking and a few little experiments prior to trying something out properly at the weekend. I think I have a workable set up to do experiments on - it does give me swirly patterns but I couldn't tell in the light this evening if it's the right sort of swirl. (I am not convinced all the "swirly" effects in images we see are all all the same.) I take your point about astigmatism (third-order astigmatism, as it is called on the wiki) though I am not yet convinced that that is all there is to it, or at least not for the petzval or your example.

Meanwhile, here's a guess. Make a lens in a long tube with lots of air spacing between the elements and use it fully open. Then you might get the effect reproducibly.

More later...
janeskodavid Posted 8 years ago. Edited by janeskodavid (member) 8 years ago
I had never heard of this effect the background effect looks similar to some parts of this:
 by janeskodavid

this is a single element HM Lens, the image is light reflecting off wet stones in a fountain. without the effect the pic would be just points of light. i got similar effects shooting x-mas lights.

I thought J-Optic turned the camera on the grass pic above.
Thanks 4 the info
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johnnyoptic PRO 8 years ago
David, You've got some very interesting and very complicated off-axis aberrations in that image. And yes, mid way from center to edge does look like the swirly bokeh effect.

In general, I find that in simple lenses, off-axis aberrations tend to increase with faster, wider angle lenses (which tend to be positioned close to the focal plane).

Richard, I'm looking forward to seeing the results of your experiments.
richard314159 PRO Posted 8 years ago. Edited by richard314159 (member) 8 years ago
Nothing much to report yet. I played with lenses, and some of my HM lenses indeed have very significant astigmatism, which I now know how to measure (to a certain extent). But still didn't get petzval-style swirling bokeh. Swirly effects are easy to obtain - any imperfect axially symmetric lens will do this, but the point is how to get the petzval effect (and indeed what is it precisely). Oh and practically, the tricky thing is how to arrange that the image circle is about the same size of the negative.

Some quick ideas:

1. With two more-or-less-equal lenses in a long tube, astigmatism should be introduced more readily because one lens "sees" more of the bad part of the other.

2. The long-tube lens fully open would have a strange shaped effective aperture off-axis - like a gibbous moon. That I am sure adds to the petzval effect.

3. I strongly suspect the effect has something to do with the relative position of the sagittal and tangential image planes, but i don't know how to control this. I could get nice swirly effects by over-focusing (the lens closer to the image than the effective focal length) but of course then the centre wasn't sharp as required. If only the image plane was curved away from the lens instead of towards it....
richard314159 PRO 8 years ago
Curved space 1 Curved space 2

These are my best efforts yet. The lens is a doublet, a long space and a meniscus lens. It's quite critical for astigmatism which way round the meniscus and doubet are - try all four possibilities for your set up, some will have much less curvature and/or astigmatism than other combos.

The images above were taken with the same lens but focussed differently. The image curves in the "normal" way. some swirly bokeh is seen, but not I would say of the petzval type.

My best and simplest demo of astigmatism in an otherwise well-behaved lens is this. Take an ordinary SLR lens (I used my 35mm SLR's 50mm f/1.8 standard lens) and put in front a strongish meniscus close-up lens as normal, except you flip the meniscus so the convex surface is facing the object. (Unscrew the retaining ring and put it in backwards.) This shows astigmatism (try focusing crosses drawn on paper but near the edge of the image).
sketchySteven PRO 8 years ago
slick!
richard314159 PRO Posted 8 years ago. Edited by richard314159 (member) 8 years ago
I think I am close to getting it. Not as sharp as a Petzval, but swirls in a similar way. More tests with this lens in a couple of days. I need to find better subject matter than I can find in this flat.

Test (swirling bokeh?)
richard314159 PRO 8 years ago
Grasses (swirling)
traskb 8 years ago
looking good.
richard314159 PRO 8 years ago
Objective achieved.

Trembling
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johnnyoptic PRO 8 years ago
Not only did you get the swirling bokeh, but you also got impressive sharpness along with it. We are eager to see the details of the lens.
richard314159 PRO 8 years ago
All the details are at www.flickr.com/photos/richard314159/4872799252/

I also worked out how to modify the lens so that it works on different formats. I was able to swap the same lens between my 35mm SLR and my 4/3 format DSLR and with only a few small changes to the apertures I could change the image circle and swirl to get the same effect on each. Sharpness is there, but chromatic aberration is there too. When I get the chance I will build one like this for my LF camera from doublets. I think that will improve sharpness and aberrations a lot. The corrections for spherical aberrations will also help enhance the bokeh effect. All this will take a few weeks - I need to get the right glass....

I also think I understand how the swirly bokeh comes about. See the comments on the link above. I think astigmatism is at best a rather minor effect.
Thomas Sommer 8 years ago
Hi,
is this what you call swirling bokeh?
Swirling Bokeh ? by Thomas Sommer


I was not searching for it but today I got this background in my pictures.
It looks as if there is swirling bokeh but on the other hand
the edges of some photos look more like zoomed...
IMGP006091-o by Thomas Sommer
richard314159 PRO Posted 8 years ago. Edited by richard314159 (member) 8 years ago
Hi Thomas .. those are very interesting. They certainly swirl, and they are not quite the same. What I have been looking out for is the way an out-of focus point of light is rendered as a non-circular disc in different positions off-axis. Because your lens is a short focal length, most of the image is sort-of focused anyway, meaning these discs should be small and are rather difficult to see at the resolution available. It seems quite plausible that your lens has swirly bokeh in the middle sections of the image just like mine, for similar reasons. Most of the grasses picture is like this. (I'm intrigued about the differences between the two. Was the grasses one taken more stopped-down than the chickens one? Or is is it cropped and represents a smaller section of the image? I presume focusing distance was about the same.)

There is certainly chromatic aberration going on here too, and a curved image plane. And barrel-distortion (the leftmost grass is not straight.) I see features that may be comas, but I am not sure about this. I think the sharp outward-pointing lines towards the edges are due to astigmatism. If I am right, these are areas where the light-points are sagittally focused but not tangentially, and is an effect I saw on some of my experimental lenses, but not the ones I used for my images above. The other thing I wasn't able to determine from your images is the area where light points at infinity are in tangential focus - if any - and whether this contributes to the swirl we see in the centre.
Thomas Sommer Posted 8 years ago. Edited by Thomas Sommer (member) 8 years ago
Hi Richard, The gras is only very bit cropped - left and below. The chicken is more cropped - also left and below. The head was about in the middle of the picture. The appel in my fotostream is completely uncropped.
I'm not so familiar with all these distortions but what you wrote, sounds quite good expecialy about sagital and tangential focus.
I agree about the barel distortion, it's very visible.
I only played around and wantet to build another soft lens but my
example perhaps may help you on your way to the perfect swirling bokeh.
richard314159 PRO 8 years ago
Thanks for the details. Yes, it was very helpful! I am still interested in the role of astigmatism, and yours are useful examples.
richard314159 PRO 8 years ago
Made a 4x5 version now. I was dodging showers to do a test shot this morning, but it seems to work well. Details on the links to the photo.

Silver Birch
Genius! Anymore details or improvements?
Fav'd and Bookmarked.
motorbiker1985 5 years ago
How would you like this bokeh?

www.flickr.com/photos/motorbiker1985/9730094702/

It is combination of optics from russian lens and czechoslovak overhead projector meotar (polar 60mm, f/1.3)

But compare it with real use of petzval on DSLR

www.flickr.com/photos/motorbiker1985/9481058966/

It is not the same.
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