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Tech question on a bokeh thing

Jonas

Active member
Here are three different images, taken with three different lenses. They are all 50mm lenses, all images are taken wide open. What you are looking at are distant street lamps with the lens focused pretty close, not MFD but to a short distance.

In the top image we can see a bright ring around the distant "spots" but not a ring completely surrounding the blobs but partially only. Is there a name for this phenomena? I seem to recall somebody once called it crescent type of bokeh but maybe there is a better name for it? And why is it there? ny insight or comments are welcome! Images:






In the center image there is a tendency and the third image is from a lens not making this ugly thing. (The second lens works fine for real life use, the first one is not so nice.)

regards,

/Jonas
 
V

Vivek

Guest
Consult here: http://www.zeiss.com/c12567a8003b8b6f/embedtitelintern/cln_35_bokeh_en/$file/cln35_bokeh_en.pdf

I have posted that link in another thread, earlier.
 

Jonas

Active member
Consult here: (...)
I have posted that link in another thread, earlier.
...and I'm grateful you post it again. I'm all for repeating. It's a pity though, I was hoping there was a particular name for this phenomena. It would have been convenient being able to call it something.

Not more to say, all in Dr Nasse's paper indeed...
Ah, don't let that stop you from adding a post... ;)

regards,

/Jonas
 

aleksanderpolo

New member
I don't recall a name, but isn't it due to vignetting of the bokeh wide open? OM 50/1.4 has it, probably due to its relatively small front element size?
 

Jonas

Active member
Thank you Alex, and Alexander

Yes. I can see signs of the Cat's Eye effect in all three images. At the edges there is none of these lenses making a round blob but there are instead signs of the circle being interrupted from optical vignetting (as I think tothwalker calls it), or the Cat's Eye effect.

Maybe it is Cat's Eye effect after all. Then I think about it as a lens making a bright ring around the blob due to over-correction and then getting a part of it cut off.

Yup. I can settle with that. What i don't understand completely is the fading. At the border blobs there is this bright ring, close to the ring the blob is bright and then is fades a little towards the image edge.

In the Nokton 25/0.95 sample images this effect is very pronounced (and ugly, to my taste). I guess we'll see some images with circular bokeh from the Nokton when more samples get available.

(The top image is taken with an OM50/1.4. The OM50/1.2 and the OM55/1.2 have it even more. On FF cameras it sometimes gets disturbing.)

Thanks again,

/Jonas
 

Jack

Sr. Administrator
Staff member
I am pretty sure it's a result of uncorrected "coma," and the "comets" always point to the center of the lens.
 

Jonas

Active member
Jonas, Here is the thread I could not locate earlier that could be of interest to you (...)
Thank you Vivek. According to the info, and as I understand it I am asking about coma then. Earlier I thought coma was what Brian in the thread describes as saggital oblique spherical aberration. But he does hint that the difference isn't very important unless you are a lens designer and I'm grateful for that. So, Lutz (in the same thread was wrong, just as I were in my post above.

I am pretty sure it's a result of uncorrected "coma," and the "comets" always point to the center of the lens.
Thank you Jack, all this is material for headache so I hope you are right...!
 

Jonas

Active member
Here are two of the available samples taken with the CV Nokton 25/0.95:


To the left a 50% crop from an image taken wide open, crop shows center part of the image.
to the right a 50% crop from the image taken at f/2.8, the crop shows the left most part of the image.

So, in the left one we see a lot of coma with some LoCA added? (Or if it isn't LoCA it is at least the typical green cast often seen behind the focus plane at high contrast areas.)

And the right image shows more of the same I guess.I'm not sure what causes the shape of the projected aperture blades though.

If I'm wrong, or if you have something to add, please feel free to correct me.

/Jonas
 
V

Vivek

Guest
(Or if it isn't LoCA it is at least the typical green cast often seen behind the focus plane at high contrast areas.)



/Jonas

Jonas, That color bleed is a problem that comes from the camera. Pana have hinted that this would be better with the GH2.

=> You should buy a GH2 to use the Cosina lens.

I would expect this would improve further when the GH-3 comes along.
 

Jonas

Active member
Jonas, That color bleed is a problem that comes from the camera. Pana have hinted that this would be better with the GH2.

=> You should buy a GH2 to use the Cosina lens.
It does? If so this already expensive lens will cost even more. Ouch. I'm not sure at all I want to upgrade in the realm of µ4/3.

Thank you,

/Jonas
 
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Jonas

Active member
Can anyone explain why the shape of the aperture blades look this way:


Normally I would have expected the shape to be more or less inverse to what we see: shouldn't the lens barrel (mechanical vignetting) cut off the projection of the shape of the blades to the left? As they don't I guess we don't see any mechanical vignetting here (the image is taken at f/2.8). Then why isn't it a more or less circular projection? Is that the coma again somehow?

Thank you,

/Jonas
 
V

Vivek

Guest
It does? If so this already expensive lens will cost even more. Ouch. I'm not sure at all I want to upgrade in the realm of µ4/3.

Thank you,

/Jonas
Don't know if it does or does not.
 

Jonas

Active member
OK, just a guess then? It doesn't matter really. I need to see a lot more samples, perhaps also with the GH2.
The shape of the aperture blades... I'll have another look at dr Nasse's paper.
 
V

Vivek

Guest
OK, just a guess then? It doesn't matter really. I need to see a lot more samples, perhaps also with the GH2.
The shape of the aperture blades... I'll have another look at dr Nasse's paper.
Jonas, I was just trying to be helpful. Read Panasonic's interview or report on the GH-2 and future. Someone posted a Google translation link, recently, for example. Draw your own conclusions.

My point was that even good lenses may look crappy if the camera has problems.

Oh, yeah, that Zeiss pdf is an useful reference. ;)
 
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