Bokeh

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A Japanese word that has been adopted as the term for "out of focus blur quality" of a camera lens.
Probably pronounced "bow-keh", "Bow" sound as in bow and arrow and "keh" sound as in Kevin.

When a lens is focussed on a subject, the image on the film of that subject should be sharp. Depending on the depth of field chosen by means of lens focal length and aperture, details behind and in front of the subject will be out of focus to some degree on the film.

If a highlight, like sun reflecting off a rain drop on a leaf, or a distant bright light in a night scene, starts to unfocus then it may adopt unusual forms. The nominal small point of light starts to spread out more into a bigger, blurry disk as it is unfocussed more and more. Now it is the quality of this big blurry disk that gives the lens its Bokeh. With mirror lenses this unfocussed spot becomes a doughnut instead of a disk. In a "perfect" lens the focussed point of light becomes an evenly bright large soft disk when unfocussed, the "perfect" lens does not necessarily have the best Bokeh.
 
 

Blobby night lights in the background.

A very blurred background, Sydney Harbour Bridge pylon on the left and some miscellaneous road lights are the blobs.
This is only a very tiny portion of the print, the foreground was in focus but boring, so I left it out.

The above is an example of bad Bokeh. Notice the bright lights have become over exposed blobs but the interesting effect is when some lights didn't cause overexposure. The blue tinted disks have edges brighter than the centre, this is the potentially annoying bad Bokeh scenario.
Hate to admit it, but this is my luverly Nikon 85mm f/1.8 lens at f/1.8. Extremely sharp lens but bad Bokeh. Now to sift through my junk shots to try and find some good Bokeh effects...... or I may have to shoot some fresh stuff.
 

The ideal Bokeh for portraiture is a soft edged blur with the brighter part towards the centre of the blur disk. The often seen very hard edged disk blur (like the above example)  tends to be a little distracting and may draw the eye away from the subject.

From all the information I have read on the Internet about Bokeh it seems that the main feature of the lens that affects Bokeh is the degree of spherical aberration correction plus a little more effect comes from the lens aperture shape.

If a lens maker hasn't done sufficient correction for spherical aberration then the Bokeh behind the subject will be nice and soft but the Bokeh in front of the subject will be hard edged and unpleasant. Also this nice rear Bokeh lens will be usually a rotten performer due to the under-corrected spherical aberration. A very general statement to make could be that the best Bokeh comes with the worst lenses.

If the lens has been properly designed for general use then the spherical aberration will tend to be over corrected which yields a good image but makes the background Bokeh hard and the foreground Bokeh soft. This decision may be made because foreground details are larger and may have more effect on the image. In extreme cases the hard background Bokeh makes single lines appear double. In this case the unfocussed blur is usually a spot that has a brighter edge than the centre, getting towards the doughnut shape of the mirror lenses. Try focussing (with a single lens reflex camera) on a distant small bright light and then winding the focus back to minimum distance and observe the single point of light become something else. Be aware that the texture of the camera focussing view screen distorts the blur, but some idea of the effect can be seen. Only the final photograph will give a true picture of the Bokeh. Either behind or in front can be "good" but in each case the opposite side will be not so good. It all depends on the degree of spherical aberration correction applied. Use up a bit of film with your different lenses to give an idea of how the Bokeh is with each one.

Be aware that the Bokeh will change for different apertures. As you close down the aperture the edges of the lens are cut off from the light path thereby removing the spherical aberration problems that may be affecting the Bokeh. If you are seriously concerned about your lenses' Bokeh then tests will have to be made at all apertures.

On top of this possible hard unfocussed spot is the effect of the diaphragm in the lens. Its shape is imposed on the unfocussed spot, so a six bladed diaphragm will produce a hexagonal unfocussed blur which will be very evident if the unfocussed blur has a hard edge, it can be very distracting. One way to avoid the diaphragm shape being imposed on the blur is to have a lens that has more blades (say, nine)  in the diaphragm and is designed to give a rounder opening. The other way is to use the lens at maximum aperture where the diaphragm blades are all retracted out of the way and leaving you with the basic lens blur shape.

Some manufacturers try to impress with talk about their multi-bladed diaphragms giving good Bokeh, but that is only part of the story. The basic degree of spherical aberration correction applied is the real Bokeh maker.

A third way is to have a lens that is optically designed to have good Bokeh in the behind or in the front area where it is important to you. One or the other, you can't have both in the one lens. Except for the Nikon 105mm and 135mm DC lenses that have an adjustable spherical aberration control on them. You can choose whether to have the nice quality blur either behind or in front by turning a ring on the lens, but again, never in both sides at the same time. Usually the potentially distracting highlights are only in one place, so that is not a problem.

In the case of Nikon they also make a few other lenses that have deliberately been designed to have good Bokeh, one that springs to mind is the 85mm f/1.4 lens. Notice that the good Bokeh lenses are usually the portrait taking lenses and very expensive as they are aimed at the professional market.

Reputation has it that Minolta has been aware of the Bokeh idea much longer than most companies and their range of lenses is reputed to have better Bokeh.

The logical thing would be to have screw-in filters that disturb the spherical aberration without disturbing the focus point, maybe somebody even makes them. One way to see if you can generate the effect is to put a low strength close-up filter (cheap single element type) on the lens and see if the quality of the background blur changes, of course you may not be able to focus on what you want, but we are trying to see the effect, if any, at this stage.

The situation of making a good lens bad just to get good Bokeh worries me a little. I see it as a useful effect sometimes, but nothing to get worried about in everyday photography.

The good/bad Bokeh issue is not so much a problem with consumer zooms as their maximum aperture is usually not so large that it permits large defocus blurs to be generated. The larger aperture professional zooms and the general line of fixed focal length lenses are the ones that may have the "problem", particularly with the longer focal lengths.



Notes.
Spherical aberration is when the rays of light from the middle and from the outside edges of a lens do not focus to exactly the same point. A major reason is that lenses are cheapest to make in a spherical form and that, unfortunately, is not the ideal shape for a lens. One way to combat this is to use multiple elements with different refractive indexes that tend to overcorrect the undercorrected elements, with luck the end result is good. Future advances in aspherical lens technology may start to make the situation better. Another way is to use only small apertures with that lens, the edge of a lens yields the worst result. The best images are produced around f/8. Larger apertures have the aberration, smaller ones have another problem, diffraction effect from the diaphragm itself (see below).

Bokeh was a term I had never heard of before coming onto the Internet a couple of years ago. It is such a new concept that no text book yet seems to have a mention of it, or maybe they don't consider it important. It really is one of those qualities to worry about only when you have run out of other qualities to fret over. Various magazines have had articles and there are various web pages on the subject. The above article represents my collected knowledge and summary of all the stuff out there and hopefully filters out the rubbish and leaves the facts behind. From information seen on the Internet it appears that Japanese Photo magazines will report on Bokeh as part of their normal testing but the rest of the world doesn't seem to worry much about that particular quality.

When you next go to the movies, train yourself to look at the background and foreground blur whenever the camera operator has used a small depth of field, particularly when there's just a face filling the screen. Recently I saw the film Baraka and it is obvious that the cinematographer has chosen the lens or the degree of spherical aberration carefully to suit the shots. Some full face closeups had nice soft background Bokeh, but a few shots where foreground material intruded the choice was obviously made to go for foreground good Bokeh and forget the background Bokeh.

Some web pages....
 

Bokeh Essays Jakub Trávník's Bokeh resources
Nikon 135mm DC lens examplesTesting the DC lens Bokeh control.
More Bokeh Ken Rockwell on Bokeh
Harold M. Merklinger Bokeh article An excellent article.
Zeiss, about DOF and BokehFrom the lens makers themselves.

Also do a Google search for bokeh and see what you come up with.

I welcome corrections and additions to be sent to me at