'spherical' bokeh

Discussion in 'Beyond the Basics' started by Jayme Cowley, Jun 14, 2010.

  1. Jayme Cowley

    Jayme Cowley TPF Noob!

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    I have noticed in some photographs that the bokeh has a 'spherical' quality, like the background is curling around the subject. As in this picture

    [​IMG]
    "The image has been taken from wikipedia.org and the original author is carlosluis. This file is licensed under the Creative Commons Attribution 2.0 Generic license.

    In fact, I've even taken a couple shots with my Sigma 30mm f1.4 that have given me this effect, but I don't know what I did to get it. What causes this effect, and how do you intentionally setup a shot to reproduce it?
     
    Last edited: Jun 14, 2010
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Notice how in the middle they are perfectly round? The football shapes are the result of light hitting the aperture blades on one side, and yet hitting the lens edge on the other side.

    Easy visualisation: Grab a tube and look through it, now rotate it slightly and what you should be able to see through is a football shape where the front edge blocks one part of your view and the back edge blocks the other.
     
  3. Jayme Cowley

    Jayme Cowley TPF Noob!

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    hmm, I believe I understand that. So how would that apply to taking a shot? I should not aim my camera directly at a light source, but to the left or the right?
     
  4. Dwig

    Dwig TPF Noob!

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    The action you took to get the effect was to use a Sigma 30mm f/1.4.

    This circular bokeh ("the swirlies" as some call it) is a property of the lens and, for the most part, it the result of optical flaws. Some of it comes from the non-round appearance the aperture when seen from the point of view of the edge of the image field. Some of it comes from incomplete optical corrections. Balancing all of a lens' optical aberrations for all colors and doing so evenly across the whole image is impossible. All designs are compromises.

    The amount of swirl, at any one mount of blur, in the bokeh is a fixed quality of the lens (or fixed for any one focal length in a zoom). Since some "prime" lens focus all or in part by changing focal length, they may exhibit different bokeh characteristics at different distance. Other than those changes, the only thing you can do either get swirlies or not get them is to change lenses.
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    Unfortunately it's just a design element of the lens. There only way to avoid it is to not shoot into point sources of light that generate big bright circles, or to stop down the lens. When you stop down the lens the amount of light that bends in at extreme angles is reduced by the aperture blades so as you stop down all point sources will not only become smaller and more in focus, but also tend more to their round shape.
     
  6. Jayme Cowley

    Jayme Cowley TPF Noob!

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    cool, thanks for the info, it seems this swirl bokeh is really mainly evident when trees are out of focus in the background, but at least now I understand how to create it/ avoid it.
     
  7. djacobox372

    djacobox372 No longer a newbie, moving up!

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    I think it looks cool... gives the background a sense of motion, perfect circles would feel too static.
     
  8. Petraio Prime

    Petraio Prime TPF Noob!

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    It's simply the vignetting inherent in the lens.
     
  9. Derrel

    Derrel Mr. Rain Cloud

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    The spherical bokeh shapes are know as the cat's eye effect, and it is due to vignetting inherent within some lens designs. You can read about it here and see an example of the swirling bokeh effect.

    Bokeh
     
  10. GeneralBenson

    GeneralBenson TPF Noob!

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    So I feel like there are two things sort of being talked about. One thing is that the specular circles are rounder in the center than towards the rame edges. But when you all say 'swirlies', you mean the fact that the individual bokeh circles as a group look like they are swirling around together, almost like a galaxy, right? Because I think that looks pretty cool.
     

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