# Bokeh onion rings



## Eldar (Apr 14, 2014)

The Zeiss Otus 55 f1.4 is a phenomenal lens, but it has been criticized for having onion ring bokeh. I have numerous examples of this, some posted over at this thread:

http://www.canonrumors.com/forum/index.php?topic=18169.msg387033#new

Attached are 4 examples. The first one has AC light bulbs. The second one is candle lights. The third is Christmas tree lights and reflections from the decorations on the tree and the fourth is a DC lightbulb.

As you can see, the first and third have the rings, the other two does not. I do not understand why this happens, but it seems to be related to AC light vs. natural or DC light.

I have tried this with numerous lenses (Otus, Sigma 35/1.4A, 85/1.2L II, 24-70/2.8L II) with the same result. The sharper the lens, the more obvious it becomes. So to me it seems that this phenomena has its cause outside of the lens.

Can someone please explain what this is.


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## neuroanatomist (Apr 14, 2014)

Some good info here: http://toothwalker.org/optics/bokeh.html


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## Eldar (Apr 14, 2014)

neuroanatomist said:


> Some good info here: http://toothwalker.org/optics/bokeh.html


Thanks John, interesting read. I still don´t understand why the DC light and the candle is clean though.


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## danski0224 (Apr 14, 2014)

It must have something to do with the varying intensity of the AC light source.

Do you have any LED light sources? Those, in my experience, also have a frequency to them and may show the onion ring bokeh.

Incandescent AC bulbs on a dimmer may show a different pattern, same for dimmable LED bulbs.

Have you tried a CFL light source?

It is interesting, maybe the lens designers never thought to take out of focus pictures of certain types of light sources.


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## neuroanatomist (Apr 14, 2014)

Were the distances similar?


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## Eldar (Apr 14, 2014)

neuroanatomist said:


> Were the distances similar?


+/- one foot. I have so many examples, so its must have something to do with the type of light source. I also have many examples with varying light temperature and color. The only thing in common for smooth is natural or DC light and AC light for the onion-rings.


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## neuroanatomist (Apr 14, 2014)

Interesting. No idea why AC versus DC would result in this observation. It's subjective, but I have noticed a trend that smaller light sources (Christmas tree lights versus incandescent bulbs, for example) tend to exhibit onion bokeh more frequently.


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## Sporgon (Apr 14, 2014)

Does this mean Zeiss test their lenses by candle light ?

Like DxO

:-X

If I was the owner of an Otus I'd drop Zeiss an e mail asking why they think this happens.


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## Eldar (Apr 14, 2014)

Another difference is that the AC light sources are more concentrated (point source) than the DC/candle. That may well be the reason.

But next question then is how a lens manufacturer can overcome this problem on a super sharp lens like the Otus. Is it at all possible?


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## neuroanatomist (Apr 14, 2014)

Eldar said:


> Another difference is that the AC light sources are more concentrated (point source) than the DC/candle. That may well be the reason.



That was my point above about the size of the light source. A smaller point source would be more likely to result in diffraction effects.


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## Eldar (Apr 14, 2014)

neuroanatomist said:


> Eldar said:
> 
> 
> > Another difference is that the AC light sources are more concentrated (point source) than the DC/candle. That may well be the reason.
> ...


But can they design their way around it?


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## Eldar (Apr 14, 2014)

Back to square one ... This is a small (halogen) 25W bulb, DC. It is smaller than the Christmas tree lights. No onion rings ...


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## mackguyver (Apr 14, 2014)

I'll avoid making a reference to a side of filter fries to go with you rings (oops, I just did!), but my next thought would be the intensity of the light. A candle is (obviously) very dim, while even a 25w bulb is significantly brighter. My bet is that the bokeh aberrations will increase with the brightness of the light source. If you have a dimmable light (or similar lights of varying brightness) try shots at different brightnesses.


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## Eldar (Apr 14, 2014)

mackguyver said:


> I'll avoid making a reference to a side of filter fries to go with you rings (oops, I just did!), but my next thought would be the intensity of the light. A candle is (obviously) very dim, while even a 25w bulb is significantly brighter. My bet is that the bokeh aberrations will increase with the brightness of the light source. If you have a dimmable light (or similar lights of varying brightness) try shots at different brightnesses.


The last post is from a dimmable light. I shot loads of images from full trottle to darkness, using several shutter speeds (same ISO and f-stop) on all. The one I posted is representative. There are no onion rings in any of them.

Needless to say, I am quite puzzled with this ...


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## Sporgon (Apr 14, 2014)

mackguyver said:


> I'll avoid making a reference to a side of filter fries to go with you rings (oops, I just did!), but my next thought would be the intensity of the light. A candle is (obviously) very dim, while even a 25w bulb is significantly brighter. My bet is that the bokeh aberrations will increase with the brightness of the light source. If you have a dimmable light (or similar lights of varying brightness) try shots at different brightnesses.



That's a good point because one candle is, after all - 1 cp ( candlepower) or about 17 milliwatts.


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## mackguyver (Apr 14, 2014)

Eldar said:


> mackguyver said:
> 
> 
> > I'll avoid making a reference to a side of filter fries to go with you rings (oops, I just did!), but my next thought would be the intensity of the light. A candle is (obviously) very dim, while even a 25w bulb is significantly brighter. My bet is that the bokeh aberrations will increase with the brightness of the light source. If you have a dimmable light (or similar lights of varying brightness) try shots at different brightnesses.
> ...


Hmm, could it be the color rendering index (CRI)? Here's an interesting article (on its own), but take a look at the spectrum graphs under "The Light That Was": Why Hollywood Will Never Look the Same Again on Film: LEDs Hit the Streets of LA & NY.

A candle has a CRI of 100 (just like sunlight), BTW, according to this link, whereas a "cool white" bulb is only 62.

Perhaps the differences in CRI (and thus wavelengths/spectrum) affect the bokeh differently. That might make sense (and I'm NO scientist) if certain parts of the spectrum were more sensitive to the aberration than others. Just another guess


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## sagittariansrock (Apr 14, 2014)

As danski said, it might have to do with the varying intensity due to the AC current cycles.
What was the shutter speed?
AC current is usually 60 Hz (might be different in Norway)- so it swings from one extreme to the other in 1/60 of a second, passing zero. So it only needs 1/120 of a second to go from one extreme to zero.
So, if slower than 1/120 (or Norway equivalent), can you try higher shutter speeds?


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## Eldar (Apr 14, 2014)

sagittariansrock said:


> As danski said, it might have to do with the varying intensity due to the AC current cycles.
> What was the shutter speed?
> AC current is usually 60 Hz (might be different in Norway)- so it swings from one extreme to the other in 1/60 of a second, passing zero. So it only needs 1/120 of a second to go from one extreme to zero.
> So, if slower than 1/120 (or Norway equivalent), can you try higher shutter speeds?


Norway is 240V/50Hz. 

For a lot of other things that has to do with light, the AC frequency of 60/50Hz comes into play. The Christmas candle picture is shot at 1/25s, with very clear onion rings. If the 50Hz theory was correct, it should have been clean. This one is shot at 1/40s (handheld)


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## philmoz (Apr 14, 2014)

I'd be interested to see how the light sources compare in size when they are in focus - some of the descriptions of this type of bokeh say it occurs when the light source is very small compared to the OOF area.

Phil.


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## mackguyver (Apr 14, 2014)

I fear that we may need to find Eldar a good marriage counselor or psychiatrist, or both, if we encourage him to keep going down this path 

I'm kidding of course and I'm glad to know that I'm not the only one who obsesses over things like this from time-to-time!


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## Eldar (Apr 14, 2014)

mackguyver said:


> I fear that we may need to find Eldar a good marriage counselor or psychiatrist, or both, if we encourage him to keep going down this path
> 
> I'm kidding of course and I'm glad to know that I'm not the only one who obsesses over things like this from time-to-time!


He he, Luckily I have two days alone in my cabin (my wife´s at work), so I can be a lunatic all by myself :

First picture is one of the chandelier light bulbs, (bulb hight is 7cm. Light source is very much a point source) (look at the gorgeous bokeh from the 85 1.2L II ). 

The other one is the dimmable DC bulb (light source app. 3-4mm in diameter). This is a more concentrated source than the christmas light shown in an earlier post.

We have some optical wizards out there in CR. It´s time for you to come and enlighten us!!!


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## danski0224 (Apr 15, 2014)

Eldar said:


> It´s time for you to come and enlighten us!!!



I see what you did there...



Do you have any DC powered LED lights you could photograph? Most of what I have for DC LED sources (flashlight, bike light, head lamp and undercabinet lighting) has some sort of frequency to it- video taken with my phone has a flickering effect.


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## sagittariansrock (Apr 15, 2014)

Eldar said:


> sagittariansrock said:
> 
> 
> > As danski said, it might have to do with the varying intensity due to the AC current cycles.
> ...



Ah, I see. I must have misunderstood- I thought you got onion rings with AC and not with candle light or DC.


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## Eldar (Apr 15, 2014)

Case solved and closed! Jrista posted a couple of very clarifying posts over at the Otus thread:

http://www.canonrumors.com/forum/index.php?topic=18169.msg387409#msg387409


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