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Brushstroke Bokeh
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PostPosted: Wed Sep 15, 2010 11:47 pm    Post subject: Brushstroke Bokeh Reply with quote
WHile photographing the kittens with the Contax Planar 1.4/50, I tested some wide open bokeh on flowering plant.
Something which I rarely do because I usually dislike bokeh shots of green plants. (maybe too much exposure?)
Anyway, I got surprised by the results, which are much more "painterly" than I expected from a Contax lens.
In fact, the bokeh wide open looks much alike to the typical bokeh of Schneider small format lenses, like the Tele-Xenar.
It is sufficient, however, to stop the Planar just one stop down, that all the brushstrokes are gone, and a beautiful but normal bokeh remains.

The first two images are wide open, and you can see the "brushstrokes" clearly; the third image is f/2, and as you can see, just one stop down and it's all gone:

01



02



03


PostPosted: Thu Sep 16, 2010 1:23 am    Post subject: Reply with quote
The pictures have a very unique and distinct look,very nice


PostPosted: Thu Sep 16, 2010 4:53 am    Post subject: Reply with quote
I like it... Orio!!
Can you control the bokeh?

Last edited by mo on Thu Sep 16, 2010 6:55 am; edited 1 time in total


PostPosted: Thu Sep 16, 2010 6:38 am    Post subject: Reply with quote
mo wrote:
I like Orio!!
Can you control the bokeh?


Yes Mo, to an extent, you can.
Bokeh (or should we type it Boke? Since it is my understanding that the original Japanese word is composed by two parts, Bo-Ke) is the result of the addition of all the rays that come through the parts of the glass that is not blocked by the iris.
This means that when the lens is wide open, the whole surface of the glass contributes to the boke. While, when we stop the iris down, this progressively excludes the rays incoming from the outer edges of the glass, until, at maximum stop down, only a small portion of rays coming from the very centre of the lens will form the image.

Now, as you probably know, most glass surfaces in a lens (and we can safely say all the surfaces in lenses that are older than 50 years) are shaped as section of a sphere, either external (convex), or internal (concave).
This spherical characteristic of the glass has advantages and disadvantages. The most obvious disadvantage is that the nearer you get the the edge of a spherical glass, the most difficult is to control astigmatism and aberrations.

Astigmatism and aberrations are the reason why photo lenses require so much calculations: lens designers through combined use of different glass shapes and chemical composition must correct as much as possible the defects related to the spherical surfaces of the glass.
This is also the reason why the technology of aspherical glasses has much progressed in the last 50 years: aspherical glasses allow to control or even virtually eliminate the astigmatism and aberrations.

So what happens when you focus a lens that uses spherical glass elements?
The point of focus of your lens will (hopefully, if the lens builders are accurate) focus all light rays coming from all the accessible parts of your glass in the same exact point: when this happens, you have the perception, through your viewfinder, that your object is in focus.

But what happens to the objects that are not in focus?
Their image will be composed, like for the focus point, from the rays that come from all the exposed parts of your glass. The problem is that being out of focus, those points will not fall on the same place, but they will spread on different points over the surface of the focal plane (i.e. your film surface, or the digital sensor surface), and the more distant from the focal point the object, the wider the spread. By the way, this is the reason why things that are out of focus in your image appear larger than real: try framing a candle light and focus on the flame: the flame will appear in it's natural proportions. Now progressively shift the focus away from the candle flame, and you will see it getting progressively larger. This happens for the reason explained above: the light rays coming through different parts of your glass surface fall apart at a spread that becomes larger as the object gets defocused.

Now you will also understand why stopping down a lens produces an apparent extension of the focus depth: the more you stop your iris down, the narrower is the glass surface that is used to produce the image. This means that the light rays that are used originate progressively closer to each other as you stop down the iris. And since they originate closer, they also end closer, and their spread over the focal plane is more limited. And this is what giver the impression of a deeper focus.
I say 'impression' because in reality, the true focus always happens on a single point over the focal plane, no matter how stopped down is the iris.
With a stopped down iris, you get the impression that the focus is wider because the out of focus objects have the light rays spread progressively narrower, and your eyes (whose resolvance ability is not infinite) get your brain tricked into believeing that they are seeing focused objects even where there really aren't.

Now back to the boke, what does all the above have with it?

Simply, as we learned, the out of focus image (the boke) is formed by all the light rays that through the iris you allow to enter your camera and reach the focal plane.
When you set your iris wide open, all rays contribute to the image, from the most central ones to the most peripheral ones.
But as we learned, peripheral areas of a spherical surface introduce astigmatism and optical aberrations. These defective rays (if I should call them so) coming from the edge of your lens mix up with the cleaner rays that come from the center of the lens.

In the focus point, all these rays coincide, so the result will be a focus object whose resolvance will be determined by the addition of all those rays: and it comes intuitively that the 'defective rays' will lower the resolvance that the cleaner rays coming from the center would produce. This is the reason why the lenses used wide open appear usually less sharp than when you stop the iris down.

In the out of focus parts, the rays (the 'defective' rays and the cleaner rays) still mix up, but they do so, like we explained, not on the same point, but spread wide, the more distant from the focus point, the wider.
So the bokeh of a lens is created by this particular mix of defective edge rays and cleaner centre rays, and the mix varies as you move farther from the focal point.

This will make you understand that, by controlling the iris, and stopping it down where you want, you are able to control the mix of the rays: the more you stop the iris down, the more you leave out the defective rays from the edges of the lens and the more you allow only the cleaner rays from the centre to create the image.
So the answer is: yes, you can control the lens boke. You do that by setting your desired aperture.

Of course, you can do this only to an extent. And the extent is that you can not change the nature and quality of the light rays that your lens projects: that is determined by the lens design, build, and materials.

Every lens model is slightly or not so slightly different from another.
Mostly what determines this difference is the quality and type of the peripheral rays, i.e. of the edges of the glass.

Lenses that share the same optical design (say, for instance, two Tessar scheme lenses) are likely to have similar optical "fingerprint". But there are more variables to the resulting image than the simple scheme: the optical characteristics and quality of the glass that is used, the precision in the manufacturing of the glass surfaces, these are factors that influence the result. This explains why for instance a Zeiss Tessar 3.5/50 and a Meyer Primotar 3.5/50 create a bokeh that is similar but not identical. The optical schemes of the two lenses are very similar if not identical, but the manufacturing and the materials are not.

We can also understand a few other things now. For instance, we understand why crop-factor cameras are not reliable tools to evaluate the performance of a lens: by their nature, crop-factor cameras cut off by default the projected image by a significan amount around the edges.
This means that only the central part of the frame is used by a crop factor camera. And as intuition suggests, the central part of a frame uses more central rays and less peripheral rays than the outer parts.
So, not only crop factor cameras limit your field of view: they also use more the central part of your glasses, and by doing that, they narrow the differences between different lenses, because, as we saw above, the character of a lens (it's fingerprint) is mostly determined by the unique nature of the light rays coming through the edges of the glass.
So crop factor cameras alter the perception of the character of the lenses, and make all lenses, the good and the bad ones, look more similar to each other than they actually are.
This is the reason why all photographers that were used to using crop factor cameras are so astonished when they move to a full frame camera: they find out that the lenses that they always used, look not just wider, but also different.

So in the end, you have control over the aspect of your boke by setting the iris aperture: as you can see from my examples above, stopping the iris down always produces a cleaner boke, because you cut off the more nasty peripheral rays.
You can change the amount and the mix, but you can not change the nature of the bokeh: there are lenses that wide open produce that "brushstroke" bokeh that I show above, and there are lenses that don't. If you have a lens that doesn't, you will never get that type of boke, no matter how you set the aperture. You will get a boke with a different character, that is determined by your lens, and that you can not change. You can just "water it down" by determining the mix percentage via the lens iris.

I hope this long verbose message helps explaining a bit of the subject Smile


PostPosted: Thu Sep 16, 2010 7:04 am    Post subject: Reply with quote
Shocked It's a lot to take in all at once but I have it as a reference now! Very Happy And I am sure everyone will find it handy. I do understand a lot more now you have explained the mechanics of it.
Thanks Orio for the detailed explanation. Very Happy


PostPosted: Thu Sep 16, 2010 9:56 am    Post subject: Reply with quote
Very interesting and very well described Orio.
Personally I am not a all a friend of changing
or modifying well established names or terms
(esp. technically ones) as it usually leads to
unnecessary confusion (sorry for being such
a technical engineer that is) - it would be like
chopping off parts of my or your name... Wink
So let's stay with "bokeh" please....

edit:

Definition "boke"

"Often mistakenly defined online as the act itself,
in Scottish and Northern Irish slang boke refers to
the gagging stage when you feel you're about to,
just before actually puking."

I have a feeling you did not know that Orio... Wink


PostPosted: Thu Sep 16, 2010 11:12 am    Post subject: Reply with quote
kds315* wrote:

Definition "boke"

"Often mistakenly defined online as the act itself,
in Scottish and Northern Irish slang boke refers to
the gagging stage when you feel you're about to,
just before actually puking."

I have a feeling you did not know that Orio... Wink


Some lenses actually cause that Razz
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