Jakub Trávník's resources

On Bokeh

The first part of article - On Bokeh Character - is here. I plan to produce a part about bokeh size and a part with some of my pictures related to bokeh theme later. I also plan to add more illustrations. This is work in progress.

What is bokeh?

Bokeh is a word which photographers use for an appearance of out of focus parts of the picture. Each bright element in scene that is out of focus contributes to overall appearance. Bokeh can be observed best on bright dots which are out of focus and surrounded by dark background. Basic shape of such highlights is a filled circle. There are some geometry rules that tell how big it will be. Those are generally known. So I will skip this part for now. What I find interesting is character of bokeh.

On Bokeh Character

What affects bokeh shape?

The character of bokeh is best observed on small light sources. This character is mainly given by optical construction of lens. Most of modern lenses have rounded aperture blades. Older straight aperture blades produced polygonal out of focus highlights (if aperture was slightly closed). While the aperture blades are rounded in current lenses they may still be slightly visible in some aperture settings. Sometimes a single blade is slightly misaligned and more visible than others. I have seen too often marketing claim "rounded aperture blades produce pleasing bokeh". But rounded aperture blades are not guarantee of pleasing bokeh.

So what else goes into a character of bokeh? Sometimes what would be a circular out of focus highlight in the center of the image has a lemon shape outside of center, gradually more near borders and edges. This is caused by mechanical vignetting because the optical path in the lens is to narrow. Resulting shape is often a intersection of two non-concentric circles, often described as "lemon shape" or "cat eye".

Distracting bokeh or not?

While shape of bokeh can be slightly distracting if it is not circular, there is another issue with out of focus highlights. It's a distribution of light inside of them. The differences in light density around edges of out of focus highlights have these basic types (red graphs show light intensity from the marked areas):

bokeh types and light intensity graphs for each type

Why there is such light distribution? There is a nice explanation here (see section "Aberration and the Defocused Image"): http://www.sony.ca/html/uf/microsite/Capturing_Depth_Minisite/2.html That section of that article is really important for understanding rest of the notes here. I will probably attempt to draw similar diagram here sometimes later (with emphasis specific to what I find important). That page describes special lens called a Minolta/Sony 135 STF f/2.8 t/4.5 lens. It also shows how aberrations affect light density in out of focus areas in classic lenses. Alternatively you can see the same explanation at http://bokehtests.com/Site/About_Bokeh.html.

What happens when highlight is not a single small point? What about a line? A line will carry still some pronounced edge on both sides. It will look like a double line. What if the highlight is some big blob? You will get some bigger blob.

With such knowledge following types of classic lens behavior will not surprise you (numbering is mine, non-standard, but I will use it for the rest of the article):

Bokeh Samples

Samples of Sigma 50/1.4 which mostly belongs to type 3 - the rear bokeh has softer edges and front bokeh has more noticeable edges (note: both pictures are JPGs in landscape picture control with sharpening value 5 of range 0 to 9):

sigma 50/1.4 bokeh example overview
sigma 50/1.4 bokeh example profile



Samples of Sigma 85/1.4 which mostly belongs to type 1 (shorter focus distances), and to type 2 (longer focus distances). Both rear and front bokeh is fairly "flat" or normal at shorter distances, but front is slightly more smooth. At longer focus distances you can see edges very clearly. I would rather prefer more smooth rear bokeh, like I have seen in pictures of cheap Samyang (or Vivitar) 85/1.4, but that one does not autofocus. These were captured as JPG in portrait picture control with sharpening 5 of range 0 to 9.

sigma 85/1.4 bokeh example overview
sigma 85/1.4 bokeh example profile

Comparison of Sigma 85/1.4 background (rear) bokeh at shorter and longer focus distances:

sigma 85/1.4 background (rear) bokeh for shorter and longer focus distances
These statues are in my favorite location for photography practice at the Vinohrady Cemetery in Prague. You can also see high amount of purple fringing near highlight-dark object for f/1.4 (left picture), somewhat less for f/2.0 and none for f/2.8.




Samples of Sigma 30/1.4 which mostly belongs to type 3 - the rear bokeh has softer edges and front bokeh has more noticeable edges (note: both pictures are JPGs in portrait picture control with sharpening value 5 of range 0 to 9):

sigma 30/1.4 bokeh example profile Unlike Sigma 85/1.4 and Sigma 50/1.4, this Sigma 30/1.4 has noticable polygon bokeh appearance at f/2 due to shape and position of aperture blades. The aperture blades are slightly rounded.

Exceptions to above lens types

DC lens examples

Defocus Control (DC) is a name which Nikon uses for special lenses that allow to adjust bokeh type through variable correction of spherical aberration. It is a rare opportunity that allows to study transition between lens type 2 (front creamy), type 1 (neutral) and type 3 (rear creamy), all in one lens.

Some notes

External Links

Some articles that go deeper into character of bokeh than just tell the rules for size of bokeh:

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