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Wednesday, August 1, 2012

Bokeh-ness, which lens to use?

If all you care is bokeh, how to decide which lens to use?
I've come across a number of articles and videos that talk about bokeh and depth of field in photography. But, I could never find a easy-to-access simple rule-of-thumb answer to a key question that I think one should ask and know the answer.

I assume the reader already knows what Bokeh in photography means, if not see here. A related, and more scientific terminology is Depth of Field. I am using the term Bokeh here in the usual sense, meaning “blurr”. Photographers love to have nice good bokeh in their photograph, and they seek out for expensive lenses, perhaps something line 85mm f/1.2 or 50mm f/0.95, and so on. It is somewhat well known in the community that bokeh-ness depends on the subject distance, the focal length, and of course the f-number or the aperture number. So, for a particular shoot, if all I care about is bokeh, and I have a number of lenses to choose from, how do I decide which lens to use? I answer this here:
How does various lens compare in terms of bokeh-ness?

For example, will a 50mm f/1.4 lens produce more (or less) bokeh than a 200mm f/2.8 lens?
The short answer is: yes and no!

The long answer is: A 50mm f/1.4 lens will produce more blur or bokeh of objects that are in immediate vicinity of the focus plane than a 200mm f/2.8 lens. However, a 200mm f/2.8 lens will produce more blur/bokeh of objects that are infinitely farther away from the subject. Note, I am assuming both the lens are operated at their maximum aperture opening (f/1.4 for 50mm lens, and f/2.8 for 200mm lens), and that we are shooting at the same magnification – meaning, the subject size remains the same.

Consider this example. You are shooting a portrait or a head shot with some distant background, perhaps a tree which is 100s of feet away. With a 50mm lens at f/1.4, you'd probably be about 3-5 feet from the subject, while with a 200mm lens, you should be about 12-20 feet away so that you have the same magnification, and hence the same subject size in the photographs in both cases.

50mm f/1.4 Lens – Focus set on the eyes, one can see the ears and nose starts to blur off gradually. The background is even more blurred.

200mm f/2.8 Lens – Focus set on the eyes, the the nose and ears start to blur, however, not as much as in the 50mm lens case. But, the distant background is far more blurred than the 50mm lens case.

Takeaway message: 50mm f/1.4 lens produces more blurr immediately around the subject, however not as much to the distant background. However, the 200mm f/2.8 lens produces less blurr around the subject, but produces more blurry background.
The Rule of Thumb

The amount of blur in the immediate vicinity of the focus plane depends purely on the aperture number or the f-number. So,  f/1.4 will produce more blur than  f/2.8

The amount of blur of the distant background depends on the ratio: (focal-length-of-lens) / (f-number). So, (50/1.4) produces less blurry background than a (200/2.8).
Because 50/1.4 < 200/2.8
Assuming same magnification (subject size remains the same in the photograph).
The Right Choice
One of the main benefits of having a large aperture lens like f/1.4 or f/2.8 lens is that the background can be made blurry so that the subject stands out. So, what lens to use in a given situation: it depends, of course!
If you are interested in a very dreamy portraiture, where the eyes are in focus, and rest is blurred out shoot with a small f-number lens, like f/1.4 or f/1.2.
If you want the portrait of the subject to be more sharp overall but want the distant backgrounds to be dramatically blurry, shoot with a 200mm f/2 lens or a 400mm f/2.8 lens or any bigger white lenses.

A side note: If you have a 2X extender, this would make, for example, a 200mm f/2 lens into 400mm f/4 lens. With this modification, the background bokeh-ness will not change because the ratio is still the same. However, the subject will be more sharper because of f/4 aperture.
Mathematical Derivations


Plot shows the bokehness on the y-axis, and distance _from_ the focus plane or the subject plane on the x-axis.

Plot of Bokeh-ness




Some Useful References


[2] Bokeh from Wikipedia http://en.wikipedia.org/wiki/Bokeh


[3] Circle of Confusion: http://en.wikipedia.org/wiki/Circle_of_confusion -- this is the place I referred to the equations for my derivation and plots.




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