Bokeh is a Japanese word used to describe the visual character of the out-of-focus areas of a photographic image, especially those captured at very wide apertures with narrow depth of field. Fast lenses, those with wide apertures, are more expensive to make and how good or bad the Bokeh a lens produces is sometime a factor so photographers use when deciding whether to spend $500 for an f/1.8 lens or $1,500 for an f/1.2. Then after spending the money to buy the more expensive glass so seem to justify the purchase by taking lots of photos with shallow depth-of-field (DOF) and large expanses of each frame filled with out-of-focus background. That is a good way to show off the Bokeh, but is it a good way to deliver the message in at photo? That is what we will explore here.

Perception and how to translate the emotional reactions we have when viewing a scene in person into a photograph facsimile which is able to evoke a similar emotional reaction in the mind of the viewer is something which interests me. The physical properties of the photographic equipment are important and quite helpful to know, but what really matters is how the viewer will react it results it produces. So first lets explore human vision and perception.

When looking at things in person our stereo vision provides most of the subconscious clues about shape and size relationships. For example in person we can detect the shape of things and their distance even in uniform flat light, where in a photograph every would blend together. Another thing which happens on a subconscious level is that our eyes, which have relatively shallow DOF, really only focus and tunnel in on what is in the very center of our vision. Everything on the periphery is unsharp and the brain tends to tune it out until some form of contrast - usually movement - attracts attention. The physiology of the of the eye consists of color sensitive cones concentrated in the center focal area of the retina and rod cells which cover the outer edges. The rods are about 3000x more sensitive to light than the cones and also only sensitive to a relatively narrow range of wavelengths in the blue-green range of the visible spectrum. That explains why at night we are more likely to notice detail and movement out of the corner of the eye: the peripheral vision is much more sensitive.

When we look at a wide scene the mental image we get of it is actually a panorama our brain stitches together of the small areas we've focused on as our eyes have darted around the scene. Wide apertures on a camera lens do in a photo what our brains do in nature. Contrasting sharpness within a photo sends a subconscious message to the brain of the viewer regarding what is most important. Sending that clue via shallow DOF in a photo is important because when we look at a photo all of it is usually in focus at the same time and absent any other visual clues such as tonal or color contrast, or leading lines we will have trouble identifying what is most important in the photo or the message the photographer is trying to deliver. When we look at a photo with selective DOF we subconsciously know where the important things are and go there instinctively.

The most interesting and effective photographs tend to be those with a center of interest which contrasts strongly with the background. If the area of strongest contrast isn't the most interesting, the intended message of the photo can be missed or misinterpreted by the viewer. If no particular area of the photo contrasts in sharpness, tone, or color from the background the viewer may have difficulty deciding what in the photo is most important.

What defines good Bokeh?

When we look at an image with selective focus we will be subconsciously drawn to the area which is in focus where we then consciously observe and react emotionally to it. How we react depends on whether or not we recognize it - a pattern matching process in which occurs in the brain. A familiar face will create a different reaction than a unfamiliar one. At some point we get tired looking at the center of interest and decide to explore the rest of the photo. If the sharp center of interest is surrounded by a lot of negative space the subconscious reaction is that there must be something of interest in that space for it to have been included in the photo. So more often than not the eye will wander off to the wider side of the image. What it wanders off towards is usually the area on the background which contrasts most strongly with it. What "good" Bokeh makes distractions in the background less distracting by blurring them more and lowering their contrast. The less distracting a highlight tone in the background is the longer the attention of the viewer will be held on the center of interest until those pesky sensitive rods in the periphery of the field of vision catch some contrasting tonal detail which can't quite see clearly but the brain wants to go check out, just in case it contains something interesting. The eye reacts to tonal contrast in the edges of the field of vision in a photo much like it does to movement at the edge if the field of vision in nature.

Can there be too much Bokeh?

Bokeh is a very useful method to help minimize distractions from the sharply focused center of interest, but the fact it is needed to eliminate distractions invites a question: if a effective image is one which keeps the viewer's attention on the sharply focused center of interest in the photo, wouldn't a more effective strategy be to simply crop the photo tighter and eliminate the distraction entirely if possible rather than including extra space with distractions it it? The best answer to that question is, "It depends on what is in the photo".

Empty out-of-focus areas are considered to be "negative" space in the composition of a photo which balance the "positive" space containing the message or main area of interest. At some point the visual mass of the negative space will match the visual mass of the center of interest and the photo will feel balanced. For example, when a person is looking to the right in a photo putting more empty negative space on the side of the photo they are looking towards gives it better balance than if the head were centered in the frame.

I find the best way to determine when a composition is balanced is to crop from the inside-out. I pull in tight on the center of interest, such as the eyes and mouth in a portrait, then expand the frame outward watching out of the corners of my eye for any distractions entering the frame. If I see a distraction enter the frame I'll make the view a bit wider, then smaller again to judge if what caught my eye is important to the message or not. If not I will stop cropping there, or plan to tone it down by using a wider aperture with more Bokeh, or a bit of darkening and blur in post processing. My goal isn't to show off the Bokeh, it is to use it only to the extent it helps clarify the message. If someone looks at the photo and their first comment it is, "Wow, great Bokeh in the background!" it is a clue there is too much background showing in the photo and it is dominating the balance.