One thing I can’t settle is the pronunciation of the word ‘bokeh’. I use it as I use the word ‘okay’ – Bokeh is okay.
This article builds on the following:
The shape of light
From the above links, you’ll have learnt that any light that hits a sensor is never a point, but always a disk-shaped thing. In fact, the shape of this disk-shaped thing depends on the shape of the aperture:
Cut a hole in a piece a paper and shine light through it, the light takes on the shape of the hole. Same for windows. So, why should it be different for lens apertures?
What is Bokeh?
Every image has a plane that is in focus. If you’re not using a tilt-shift lens, then the plane is perpendicular to the ground, parallel to the image sensor. What is outside this plane of focus is said to be out of focus. This area is both in front of and behind the plane of focus.
This is the Out of focus area – OOFA.
Many people confuse the out of focus area with blurriness. When you stop down a lens to increase the depth of field, you also increase the range of the focus zone so objects in this area appear sharp, sometimes as much as the objects on the actual plane of focus. Now, these are still out of focus areas, but they don’t have to be blurred.
The word bluriness is itself a blur. Rather than think of bluriness or out of focus areas separately, we have a word called Bokeh that marries them into one no-less-confusing concept.
What is bokeh? Simple – it’s the blurred portions of the out of focus areas.
But wait, what about the circular or other patterns in the out of focus areas? Why aren’t they blurred? Take a look:
The answer is simple really. Contrast. When we have light sources in the scene, they are so bright as to stand out from the surrounding areas. Hence, the circle they produce stands out. The wall on the back is also producing circular bokeh, it’s just that they blend in together. The same happens with green leaves against a blue sky, etc.
This is the source of confusion: How can circular blobs be blurred when they’re not? Don’t look at the circle and assume they are not blurred. They are not blurred as circles (they have definable edges so they are circles) but blurred as what they are really – bulbs or point sources of light or leaves, etc. E.g., in the above image, can you make out what the shape and make of the bulb is? Of course not. It’s blurred. Blurred into a circle that’s not blurred.
It’s your call. When using the word ‘bokeh’, you can use it to mean all the blurred portions in a scene, or just the ‘sharply-blurred’ geometrical shapes in a scene. They’re the same thing, but try explaining that to a person who simply refuses to believe it.
I use it to mean both.
Are all bokeh created equal?
Nope. Take a look:
Because a lens is spherical, light travels a bit differently at the edges than the center. Furthermore, most lenses vignette, and are brighter at the center than at the edges. This affects the bokeh as well. In the above image, the first bokeh is brighter at the edges. In the second bokeh, the light is even. In the third bokeh, the light is brighter in the center and gradually drops off.
Now try mentally mixing these bokehs with each other. The first one won’t mix very well, which is why you see individual circles overlapping each other without mixing. The third will blend in very well, while the second one can go either way (in theory it should work but in practice nothing is that even – the edges are still discernible).
The edges can also be brought into sharp relief with chromatic aberrations in the lens. The question of bokeh might as well be reduced to: Do you like edges in your bokeh?
And then there’s fall-off.
What is Fall-off?
The boundary between the sharp in-focus portions to the bokeh is never a strict line. Usually it is a region where the focus gradually blurs into the bokeh.
Depending on the focal length, distance to the subject, plane of focus, aperture and optical design, the region can be long or short. E.g., if you’re using a telephoto lens with a wide aperture, the change is abrupt:
Or it could be gradual:
This zone, from sharp to bokeh, is called the Fall-off.
What is good or bad bokeh?
The way a lens goes from sharp to bokeh is aesthetically important. E.g., look at the above image. The bokeh is really weird (look at the red light near the left shoulder) – it’s like a doughnut. That’s because the lens used was a mirror lens, which has a mirror in front of the aperture. Doughnut in, doughnut out.
In contrast, look at how the bokeh behaves in this image:
The point is not whether or not the bokeh looks beautiful. The point is: how well does it serve the subject that is in focus?
When thinking about good or bad bokeh, you need to consider three things:
- Shape of highlights or patterns in the bokeh
- Overall aesthetic impression of the whole image
This is why I define bokeh generally as I have. Bokeh is relative. Even a doughnut bokeh can produce acceptable aesthetic results.
It is shots like these that give rise to confusion:
Mind you, the shot is beautiful, and this is not a commentary on its aesthetic quality. Rather, shots like these are used to define bokeh. A newcomer looking at the above image will only see the colorful patterns, and assume that’s all bokeh is. As we have seen, bokeh is a whole lot more.
Therefore, when selecting lenses for good bokeh, look for all the three things I’ve outlined. No matter which direction you point the camera in, and no matter what aperture you set it at (bokeh characteristics change according to aperture) – the lens should deliver beautiful bokeh. One such lens known for this is the Leica Noctilux 50mm f/0.95:
For an excellent in-depth and technical overview of bokeh, read this excellent guide (PDF) from Zeiss.
I hope this simple explanation has provided a bit of clarity about bokeh. Now, go out and shoot, but don’t come back and show me the bokeh beacuse I’ll be too busy looking at the sharp portions of your imagery. That’s what interests me the most.