Pixiq

What on earth is ‘Bokeh’ ?

Putting the word Bokeh into Google will generate “about 7,490,000 results (0.05sec) or at least it did at 10.22am GMT this morning as I wrote this introduction.

 ‘Bokeh’ is essentially a Japanese word (meaning blur or haze) used to describe the character of the ‘out-of-focus’ background produced by a lens…The word 'bokeh' is bandied about and used with gay (in its original sense, that is) abandon as fashionable words tend to be. As far as I can determine, it first appeared in relation to photography in recent history – 1997 - when Mike Johnston as editor introduced it in Photo Techniques magazine.

I have been revisiting familiar territory for my latest Lark book Digital Close-up Photography Q & A accept no substitute: AND coming very soon a series of ebooks). I do this often, even without a prompt of a book to be written. It never does to be complacent about things, especially ideas, and we all need, from time to time, to get back to basics…questions, particularly from kids can floor you. This post is, in fact, adapted from the very book itself  Digital Close-up Photography Q & A...just to mention it again, ever so subtly!

My fellow Pixiq blogger, Haje Jan Kamps, made a noble attempt recently to look at f-numbers and explain them. Suddenly, the 'debate' was hi-jacked to embrace ‘elitism’. Oh how I hate that word and I thought of posting a comment about it, but could not make it polite enough…unless I left a completely blank space.

Today, I am going back to basics, too, and for those who regard this approach as ‘elitist’ don’t read it…simple. Knowledge and thought must never be regarded as the province of an elite – through that route humankind has fostered the darkness of ignorance and the blindness of religious dogma. It helps some of us to know 'why' and, clearly, others not to. If there is something of which we are ignorant then it is crass stupidity to label it ‘elitist’. Ignorance is not something to be celebrated – though few politicians seem able to appreciate that. It makes me weep when I hear people who make a virtue of their ignorance of 'science' when they should bow their heads and run to a library, not wear it as a badge of pride. Rant over...for a moment or two.

So, back to Bokeh this is a useful concept, particularly with large aperture lenses of all sorts where the depth of field will inevitably be shallow and large areas may be out of focus. Long telephotos and macro lenses are often used at wide apertures for practical or artistic effect as are medium telephotos when used for portraiture: anywhere, in fact, where shallow depth of field eliminates background clutter and emphasizes the sharpness of the subject.

Whereas a wide-angle lens will render foreground subject sharp as well as retaining detail in the background, telephoto lenses tend to isolate subjects from the background. The two different types of lens treat spatial separation of elements in the picture differently and this has to do with perspective and angle of view. With wide angle lenses you are close to a subject than with a telephoto to get a subject occupying the same portion of a viewfinder.

Many photographers prefer a smoothness in the background that gentle blur that is termed ‘good bokeh’ and several factors affect this:

 the focal length of the lens (the longer this is the more blurred the background) 
 the iris diaphragm in the lens - how close the aperture is to a perfect circle 
 How well the lens is corrected for spherical aberration
 

  The basis of ‘good Bokeh’

The sharp parts of an image can be thought of as ‘points’ but away from these – the out of focus areas the light spreads slightly and the points become tiny circles - those blur circles or circles of confusion. As long as these circles are no larger than 1/30mm the human eye cannot distinguish them from a point - which is why this measure was chosen. It is worth noting this is a subjective value but at least it offers some sort of standard measure.

These blurred areas comprise overlapping images (actually diffraction patterns) whose edges assume the shape of the aperture in the iris diaphragm. If the aperture is a perfect circle then overlapping images  blend beautifully and backgrounds look smooth – ‘good bokeh’. 

If the diaphragm is not circular then the angular edges do not produce perfect circles and the ‘bokeh’ is slightly ‘harsh’. The more diaphragm blades (7-9) then the closer to the perfect circular becomes the aperture: if these blades are slightly curved  the smoother it is. Lens specifications nowadays state the number of blades in the diaphragm as a selling point. 

The diagram below shows how a greater depth of field results from smaller apertures - you can move further either side of the sharpest focus and still get what looks like a 'point'

Whether these tiny image discs are uniformly illuminated or not also affects ‘bokeh’. Disc brightness can vary from edge to centre depending on how effectively spherical aberration is controlled – discs can be brighter at centre than edge (and vice-versa) and there is a difference in appearance of these discs behind and in front of the plane of focus.

Some lenses have been designed with the capacity for changing the way the out-of-focus areas appear by increasing the degree of spherical aberration and there are add-ons to cameras within the ‘lens baby’ series that do the same.

An unmistakable ‘bokeh’ occurs with mirror telephotos lenses (catadioptric lenses) where out of focus highlights become ‘donuts’. The centre of each light disc is blocked out because it is imaging the mirror in the lens which is, in turn, blocked by the secondary mirror inside the lens. Some like it and some do not.

And the best 'bokeh' I can get from any lens I own ?

It has to be the Sigma 150mm f/2.8 EX DG IF HSM  macro - thoroughly assessed some four months ago on PIXIQ...check out both parts

In recent work using Image Stacks I have been using this lens but at f/5.6 - f/8 and that bokeh is a major factor for the soft background blur at these apertures is in stark contrast to the amazing depth of field created for the subject via an image stack.