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Round bokeh
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PostPosted: Sun Mar 13, 2011 2:01 pm    Post subject: Round bokeh Reply with quote
hello what controls the round bokeh at all the apertures for a lens, is it just a matter of number of blades or just the shape of them?

Is there a list of lens with that characteristic?
Thank you


PostPosted: Sun Mar 13, 2011 4:16 pm    Post subject: Re: Round bokeh Reply with quote
sandro wrote:
hello what controls the round bokeh at all the apertures for a lens, is it just a matter of number of blades or just the shape of them?

Is there a list of lens with that characteristic?
Thank you


Yes, the shape of specular highlights is dependent upon the aperture shape. 9 curved blades is pretty standard these days, resulting in more or less circular highlights at all apertures. If wondering what shape you will get from older lenses, you need only look for the number of blades. For example, 6 blades will provide a hexagon, 5 a pentagon shape, etc, whereas the higher counts such as 12 or 15 will be more likely to give a round shape. It is a matter of taste only.

Something to keep in mind - as the aperture of a lens is opened fully, the blades no longer are positioned in the optical path and specular highlights will be perfectly round, regardless of blade count.

Further, by cutting any shape out of a piece of thick paper and placing it over the rear element of a lens, one can achieve whatever shape they wish (hearts, triangles, stars, etc).

Kelly.


PostPosted: Sun Mar 13, 2011 5:37 pm    Post subject: Reply with quote
Yes, true for the shape of bokeh highlights.
The overall quality of bokeh is influenced by more than the shape of the aperture diaphragm.


PostPosted: Sun Mar 13, 2011 5:56 pm    Post subject: Reply with quote
What happens with 7 blade aperture? will it be harsh?


PostPosted: Sun Mar 13, 2011 6:04 pm    Post subject: Reply with quote
sandro wrote:
What happens with 7 blade aperture? will it be harsh?
Depends on the lens. My 55mm f1.2 S-auto Nikkor has 7 straight blades that give heptagonal highlights, but the Bokeh has a very smooth character. It depends whether the highlights are brighter in the centre or the outlines, which I think is mostly influenced by the lens-design.


PostPosted: Sun Mar 13, 2011 7:10 pm    Post subject: Reply with quote
I would say that at aperture + focal length + focus distance combinations where depth of field is shallow, the number of aperture blades usually has very little effect (and obviously zero effect with the lens wide open).


PostPosted: Sun Mar 13, 2011 7:49 pm    Post subject: Reply with quote
Re: "Specular highlights will be perfectly round": of course this is not always the case. The diaphragm blades may be outside of the optical path, but the lens can still have "optical vignetting". This means that specular highlights will only be perfectly round in the center, but they change in shape to cat-eyes when you go towards the borders. This is most often the case in fast lenses, where the periphery of the film/sensor cannot see the whole front lens anymore.

The Cyclop 85/1.5 (Helios-40 without diaphragm blades) shows this perfectly.



PostPosted: Mon Mar 14, 2011 1:44 pm    Post subject: Re: Round bokeh Reply with quote
thePiRaTE!! wrote:

Something to keep in mind - as the aperture of a lens is opened fully, the blades no longer are positioned in the optical path and specular highlights will be perfectly round, regardless of blade count.

Further, by cutting any shape out of a piece of thick paper and placing it over the rear element of a lens, one can achieve whatever shape they wish (hearts, triangles, stars, etc).

Kelly.


Well actually you can place a peace of cut paper (or something similar) even in front of front element and you'll get the bokeh from the shape the cut. Here is an example where a small cross is cut out of peace of paper and the object is out of focus. Then there is a photo of the motive itself:





/Stanislav


PostPosted: Tue Mar 15, 2011 3:32 pm    Post subject: Re: Round bokeh Reply with quote
Spotmatic wrote:
Re: "Specular highlights will be perfectly round": of course this is not always the case. The diaphragm blades may be outside of the optical path, but the lens can still have "optical vignetting"...


Of course, Peter is correct - it is something you'll discover researching images from a lens. I went too far in my explanation, I should have said simply that since the blades are no longer in the optical path, their shape is of no consequence while the aperture is fully opened.


Stanislav wrote:
...
Well actually you can place a peace of cut paper (or something similar) even in front of front element and you'll get the bokeh from the shape the cut...


Might even shade the lens better this way? I'll give it a shot. Originally, I'd just speculated since the apertures invoked their shape on highlights, that any opening should do so, and placed a cut sheet of card stock popped onto the rear element since it was as close to the actual aperture as I was comfortable sticking a foreign object, but that's even easier. Wonder if the results differ? Experimentation will ensue.

K.


PostPosted: Tue Mar 15, 2011 6:29 pm    Post subject: Reply with quote
Shaped bokeh has been popular lately at some techie gadget sites.

There's actually a kit, essentially lens caps with different shaped holes:
http://www.bokehmasterskit.com/buy/

Looks silly but some striking images can be made:
http://www.bokehmasterskit.com/sample-images/

I made something similar out of cardboard at Christmas time and the effect certainly works. I wasn't really happy with my shots, not so much the bokeh but everything else. Maybe tonight I'll find the least bad ones and post Smile
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