What causes this weird bokeh

Started Mar 8, 2018 | Discussions
J A C S
J A C S Forum Pro • Posts: 19,559
What causes this weird bokeh

See this thread:

https://www.dpreview.com/forums/thread/4261378

It happens with long lenses, like 400mm+, and a filter can make it worse. But it happens often without a filter, too. I am talking about double or triple, etc., bokeh slightly behind the focus plane.

So the question is - why long (and relatively slow) lenses? My guess would be - increased role of the diffraction which creates a few rings, and relatively deep near focus area (because of the FL), which may catch more objects than a shorter lens.

TheBlackGrouse Veteran Member • Posts: 3,586
Re: What causes this weird bokeh
1

J A C S wrote:

See this thread:

https://www.dpreview.com/forums/thread/4261378

It happens with long lenses, like 400mm+, and a filter can make it worse. But it happens often without a filter, too. I am talking about double or triple, etc., bokeh slightly behind the focus plane.

So the question is - why long (and relatively slow) lenses? My guess would be - increased role of the diffraction which creates a few rings, and relatively deep near focus area (because of the FL), which may catch more objects than a shorter lens.

Part of image with Black Roe Deer and weird bokeh (Canon 7D with 70-300 L)

In a thread I started years ago my idea was that IS may contribute to this weird bokeh too. Let's say that not every one agreed

But some did, like gbdz:

'(...) having the OOF branches striped like that could very well be caused by the IS element trying to keep the optical axis so abouts stable. It does so by changing the axial angulation, I presume but I do not know. How else could it do it?

If this is the case, it would seem logical that the parts of the picture that are outside of the focal distance behave strangely. I would imagine that the in-camera stabilisation where the sensor dances to keep the the focused area centered, would not make these kinds of artefacts.

Most certainly, you have described a phenomenon that could in fact be traced back to the internal optical IS. It would be interesting to hear what the Canon guys say about this. I am sure they know.'

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Ian Stuart Forsyth
Ian Stuart Forsyth Veteran Member • Posts: 3,935
Re: What causes this weird bokeh
1

spurious resolution ?

I know if I have enough of a repeating pattern in places that are OOF they can give the appearance of the same pattern. I think its just 2 out of focus edges that when placed on top of one and other will give you some funky patterns.

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Tom Axford Veteran Member • Posts: 8,515
Interaction between foreground and background bokeh

TheBlackGrouse wrote:

Part of image with Black Roe Deer and weird bokeh (Canon 7D with 70-300 L)

This looks like an interesting interaction between foreground and background bokeh. You don't give the EXIF, but if this shot was taken at something like 300mm f/5.6, the aperture diameter was 300/5.6 = 54mm (just over 2"). At that aperture, the lens can "see through" small objects less than 2" diameter (or, if you prefer, it "sees around" the object).

Looking at your picture the interesting striped bokeh appear to occur within the bokeh produced by small branches (less than 2" diameter) that are much closer to the camera than the deer. Notice that the stripes are roughly parallel to the direction of the branch.

To understand how this can occur, it is necessary to understand how blur (bokeh) is formed in out-of-focus parts of the scene (both foreground and background).

See this post for more explanation of background blur formation.

The small branches in the foreground effectively change the shape of the aperture, which, in turn, changes the shape and size of the bokeh (do a web search to find plenty of examples).

Karl Persson Regular Member • Posts: 246
Re: What causes this weird bokeh
2

J A C S wrote:

See this thread:

https://www.dpreview.com/forums/thread/4261378

It happens with long lenses, like 400mm+, and a filter can make it worse. But it happens often without a filter, too. I am talking about double or triple, etc., bokeh slightly behind the focus plane.

In the linked image the weird bokeh comes from something in front, not behind. Presumably a twig/branch from brush or tree.

Not at all unusual.

So the question is - why long (and relatively slow) lenses? My guess would be - increased role of the diffraction which creates a few rings, and relatively deep near focus area (because of the FL), which may catch more objects than a shorter lens.

Tom Axford Veteran Member • Posts: 8,515
Re: Interaction between foreground and background bokeh
19

As a follow-up to my previous post, I did some quick experiments to check my explanation.

This shot was taken with a 75mm f/1.8 lens at full aperture (42mm) and manually focussed on a point closer than the background so that the background is thrown out of focus:

Bokeh from the background with no foreground present

The next two shots are exactly the same except that a dark-coloured pencil (about half an inch in diameter) was held in front of the camera lens and several inches away from the lens:

With the pencil held horizontally in the foreground, the background bokeh show approximately horizontal stripes.

With the pencil held vertically in the foreground, the bokeh show approximately vertical stripes.

HumanTarget Senior Member • Posts: 1,738
Re: Interaction between foreground and background bokeh

Nice demonstration!

TheBlackGrouse Veteran Member • Posts: 3,586
Re: Interaction between foreground and background bokeh

Tom Axford wrote:

As a follow-up to my previous post, I did some quick experiments to check my explanation.

This shot was taken with a 75mm f/1.8 lens at full aperture (42mm) and manually focussed on a point closer than the background so that the background is thrown out of focus:

Bokeh from the background with no foreground present

The next two shots are exactly the same except that a dark-coloured pencil (about half an inch in diameter) was held in front of the camera lens and several inches away from the lens:

With the pencil held horizontally in the foreground, the background bokeh show approximately horizontal stripes.

With the pencil held vertically in the foreground, the bokeh show approximately vertical stripes.

Thanks! 

Busy reading about what you said but this gives a good impression. It is too early for me to give an opinion.

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NewToNikonD850 Junior Member • Posts: 30
Re: Interaction between foreground and background bokeh

Hi there,

Someone pointed out your thread to me and I thought I'd share a few images from my Nikon 70-200mm E FL ED VR that show this same splitting OOF effect.

I can sympathize with you. It's a very unpleasant look, and it's difficult to predict when it will rear its ugly head.

Someone recently suggested that the "reality" was that I was being a "bokeh perfectionist" in lamenting over this effect. What a laugh. I don't need OOF perfection, but I have had others lenses and camera bodies before and this ($3,500) lens mangles out-of-focus areas in ways that can't be ignored or rectified in post.

Btw, the photos below were taken without a filter.

In your search for answers, I hope you don't come across too many know-it-alls who aren't as interested in being helpful as they are in establishing themselves as the arbiters of all things photography.

Note the split antlers of the reindeer in the background, as well as the "split" fur and mangled grass. How do you fix that in post?

Tristimulus Veteran Member • Posts: 8,550
Re: What causes this weird bokeh

Suspect aspherical lens elements also play a role in busy bokeh.

Just defocus stars and lenses with aspherical lens elements and will at certain focus positions show a darker center, or even a black hole, in the defocused image of the star.

Not so when using lenses containing spherical optical elements only (unless having severe zonal errors, but then the lenses are strictly speaking no longer spherical).

But this is old news in this forum...

Bernard Delley Senior Member • Posts: 1,770
Re: Interaction between foreground and background bokeh

Very interesting start for problem analysis !

The funny shaped extra aperture formed by close foreground can do surprising things. So lesson one is to avoid that. Can easily happen with shooting through fences in the zoo. Also watch out  for branches crossing the view in front of the lens.

My question is, when this is avoided, can such strange bokeh still happen. The remaining suspects are the vibration reduction system (can also be turned off) and lens aberrations. The aberrations cannot be turned off, but can be controlled somewhat by the aperture. At least the lens aberrations at a given aperture of a particular lens are always there, means the problem is reproducible. Reproducibility is greatly helpful for evaluating and testing.

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(unknown member) Senior Member • Posts: 2,180
Re: Interaction between foreground and background bokeh
1

FWIW, it looks like that is to do with the IS element being allowed angular motion, rather than just a decenter in its motion, or the used IS mode recenters the IS unit at the beginning of the shot and there is a small timing error.

You can try to remove it by disabling IS (at 1/1250s, this is fine but maybe not for the compositional aid of the stability) or changing IS mode.

CutTheShot
CutTheShot Regular Member • Posts: 221
Re: Interaction between foreground and background bokeh
2

nisen bokeh...

caused by over correction of spherical aberration. this is a lens design issue.

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J A C S
OP J A C S Forum Pro • Posts: 19,559
Re: Interaction between foreground and background bokeh

BobdoIe wrote:

nisen bokeh...

caused by over correction of spherical aberration. this is a lens design issue.

It looks different than the usual "double line bokeh" to me. I think I see multiple lines.

NewToNikonD850 Junior Member • Posts: 30
Re: Interaction between foreground and background bokeh

Nisen bokeh! It even has a name!

Thank you for providing the answer to my query about the bokeh rendered by my 70-200mm f2.8 E FL ED VR!

J A C S
OP J A C S Forum Pro • Posts: 19,559
Re: Interaction between foreground and background bokeh

NewToNikonD850 wrote:

Nisen bokeh! It even has a name!

Thank you for providing the answer to my query about the bokeh rendered by my 70-200mm f2.8 E FL ED VR!

Giving it a name is not an answer.

NewToNikonD850 Junior Member • Posts: 30
Re: Interaction between foreground and background bokeh
1

My comment was for Bobdole, who did more in his response than give this ugly bokeh a name, J A C S.

Thanks so much for your constructive input, though

J A C S
OP J A C S Forum Pro • Posts: 19,559
Re: Interaction between foreground and background bokeh

NewToNikonD850 wrote:

My comment was for Bobdole, who did more in his response than give this ugly bokeh a name, J A C S.

Thanks so much for your constructive input, though

Well, speak for yourself.

I am not convinced that his explanation is correct. I am well familiar with artifacts produced by overcorrected lenses and this does not look like what I have seen. There is what looks like a phase cancelation effect, for starters.

CutTheShot
CutTheShot Regular Member • Posts: 221
Re: Interaction between foreground and background bokeh

J A C S wrote:

NewToNikonD850 wrote:

My comment was for Bobdole, who did more in his response than give this ugly bokeh a name, J A C S.

Thanks so much for your constructive input, though

Well, speak for yourself.

I am not convinced that his explanation is correct. I am well familiar with artifacts produced by overcorrected lenses and this does not look like what I have seen. There is what looks like a phase cancelation effect, for starters.

i was responding to the images posted of nisen bokeh.
i don't like to quote images, but i suppose it would have helped here. you do agree that the pictures posted with twigs and a bird in the tree show nisen bokeh, right?

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c h u n k
c h u n k Senior Member • Posts: 1,693
Re: Interaction between foreground and background bokeh

Im just lurking this forum and Im def NOT gonna try to enter an optical science thread in any meaningful way, but, fwiw, when I shoot my telephoto through my very old, crappy windows, the bokeh is very similar to some pics shown here. Similar affect to results I got with the very first ND filters I ever bought. Suffice it to say, some of my first purchases were a waste of money.

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