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Thread: Tech question on a bokeh thing

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    Question Tech question on a bokeh thing

    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

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    Re: Tech question on a bokeh thing

    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.

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    Senior Member kds315's Avatar
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    Re: Tech question on a bokeh thing

    Not more to say, all in Dr Nasse's paper indeed...

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Vivek View Post
    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.

    Quote Originally Posted by kds315 View Post
    Not more to say, all in Dr Nasse's paper indeed...
    Ah, don't let that stop you from adding a post...

    regards,

    /Jonas

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    Re: Tech question on a bokeh thing

    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?

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    Re: Tech question on a bokeh thing


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    Re: Tech question on a bokeh thing

    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

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    Re: Tech question on a bokeh thing

    Jonas, Here is the thread I could not locate earlier that could be of interest to you (although many example pictures are now gone):

    http://photo.net/leica-rangefinders-...0EiFs?start=30

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    Re: Tech question on a bokeh thing

    I am pretty sure it's a result of uncorrected "coma," and the "comets" always point to the center of the lens.
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Vivek View Post
    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.

    Quote Originally Posted by Jack Flesher View Post
    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...!

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    Re: Tech question on a bokeh thing

    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

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Jonas View Post
    (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.

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Vivek View Post
    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 4/3.

    Thank you,

    /Jonas
    Last edited by Jonas; 29th September 2010 at 16:17.

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    Re: Tech question on a bokeh thing

    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

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Jonas View Post
    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 4/3.

    Thank you,

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

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    Re: Tech question on a bokeh thing

    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.

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Jonas View Post
    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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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Vivek View Post
    Jonas, I was just trying to be helpful. (...)
    I know you were Vivek. I should have written "educated guess".
    Thanks,

    /Jonas

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    Re: Tech question on a bokeh thing

    Quote Originally Posted by Vivek View Post
    Oh, yeah, that Zeiss pdf is an useful reference.
    Did you meet Dr Nasse at PK?
    Here he is, the guy to the right in image No 6.

    /Jonas

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