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Bokeh - the quality of blur

Light is a straight line What is Bokeh?
Bokeh describes the rendition of out-of-focus points of light. Bokeh is different from sharpness. Sharpness is what happens at the point of best focus. Bokeh is what happens away from the point of best focus.

To understand bokeh you need to understand how light gets focused through your camera lens. It is important to understand that light travels in a straight line. You can only see light that is directed straight at you. If you shine a flashlight in the dark you cannot see the beam of light unless it is pointed right into your eyes. You can only see what the beam of light hits. Once a ray of light hits the surface of an object, it bounces off in an infinite number of directions, dispersing the light so everybody around can see it. This is due to the imperfections of the surface material the object is made of (see figure 1).

Light is a straight line This is where your camera's lens goes to work. Its job is to collect as much of that light as possible and focus it onto the camera's light sensitive chip (CCD). Every ray of light that hits the subject bounces in a different direction. There are trillions of light-rays shooting off the object you are photographing. Only a small portion of that light actually makes it directly into your camera but even that small percentage of light rays is a very large number. This light enters through your lens as cone shaped points of light which take the size and shape of your aperture and focus down to a tiny point on the CCD. The smaller the point, the sharper the image (see figure 2).

Light is a straight line Every tiny area of your subject is sending light to your camera, from your main subject that you focused on to background and foreground subjects that are out of focus. Those points that are out of focus miss the CCD in your camera and actually focus in front of or behind the CCD. This means that the cone from those points of light did not come to it's smallest point on the CCD. These points of light get spread across the CCD in a larger area known as the circle of confusion. The further the point is from sharp focus, the larger the area of confusion. These out of focus cones of light overlap each other causing what we know as blur. The larger the area of confusion, the more blur (see figure 3).

It is important to note here that this area of confusion takes the exact shape of your aperture. If your aperture is wide open they will be perfectly round. This is not usually noticeable because there are millions of these points overlapping each other in the blurred parts of your photo usually resulting in a pleasing blur. However, when there is very high contrast in your blurred area such as a small point of light, the area of confusion for that point will be very distinct.

A high quality lens has the ability to create sharp images by focusing on a very tiny tip of these cones of light while at the same time producing a soft blur around the larger, out-of-focus points. Some low quality lenses are unable to obtain such a sharp focus due to imperfect lenses which cause spherical aberrations. This prevents the lens from reaching the finest points on each cone of light. Your camera may be able to capture an extremely high resolution but without a lens capable of focusing down to a small enough point you still cannot obtain the sharpest image your CCD may be capable of.
Poor Bokeh Fig. 1. Poor Bokeh. This is a greatly magnified blur circle showing very poor bokeh. A blur circle is how an out-of-focus point of light is rendered. Note how the edge is sharply defined and even emphasized for a point that is supposed to be out-of-focus, and that the center is dim.
OK Bokeh Fig 2. Neutral Bokeh. This is a a technically perfect and evenly illuminated blur circle. This isn't good either for bokeh, because the edge is still well defined. Out-of-focus objects, either points of light or lines, can effectively create reasonably sharp lines in the image due to the edges of the sharp blur circle. This is a blur circle from most modern lenses designed to be "perfect"
OK Bokeh Fig. 3. Good Bokeh. Here is what we want. This is great for bokeh since the edge is completely undefined. This also is the result of the same spherical aberration, but in the opposite direction, of the poor example seen in Fig. 1. This is where art and engineering start to diverge, since the better looking image is the result of an imperfection. Perfect bokeh demands a Gaussian blur circle distribution, and lenses are designed for the neutral example shown in 2.) above.
OK Bokeh

The photo to the right was taken with a low to mid quality lens. Notice the out-of-focus branches in the background of the photo. Even though the branches are out of focus, the edges are still distinct. The branch simply got larger. This happens because the area of confusion in the out-of-focus points of light has a distinct edge as seen in figure one above.

This sharp edge is not usually noticeable when there are millions of points mixed together. It is only noticeable when there is a large contrast difference between points such as small lights or light reflecting from shiny surfaces.

Out-of-Focus Points of Light

Bokeh Lights
Those small high contrast, out of focus points of lights in your photograph can be used to your advantage in an artistic way. Because objects that are out of focus keep a sharp edge around them, small points of light become large orbs with a soft, creamy body.

These out of focus points of lights can be an unusual, pleasing backdrop to a photo. Strings of Christmas lights are commonly used to create large orbs of light in the background of your photos. With a little creative composition you can make the orbs in the background appear to be interacting with your foreground. The lights in these photos are about 20 feet behind the foreground subject.

Bokeh Lights

The trick to creating these orbs is 1) Make sure there is a large distance between the foreground and background. 2) Use the largest aperture your lens is capable of. The best way to achieve this is to use aperture priority mode. 3) Focus on a subject as close to the lens as possible. Remember, every lens has a minimum focus distance. If you get too close to your subject the lens will not be able to focus. Only macro lenses can get up close to your subject. If your lens won't get a focus lock, back up a little and try again.

The Shape of Bokeh

An interesting thing about bokeh. The out-of-focus lights take the shape of the lens' aperture. When the aperture is wide open it creates a perfectly round circle but when the aperture is closed down, several leaflets fold in among themselves creating a hexagonal shape. Each point of light will become this shape. Wouldn't it be great if we can make the aperture any shape we wanted?

Bokeh home made aperture Old time violin bokeh

There is a way you can make your own aperture. All you have to do is make a paper cover for your lens with the shape you want punched right in the center. First, Cut a strip of black paper, wrap it around the lens and tape the ends to form a loop. Second, cut a round circular disk from the black paper the exact diameter of the loop you made. Next, punch a shape in the center of the disk. It is critical that the shape is absolutely in the center of the disk. Tape the disk to one end of the loop and push it onto the lens.

That sounds easy but there are a few catches. How are you going to punch a hole in the center of the disk when most hole punches only punch close to the edge of the paper? You'll have to cut a crude hole with scissors then punch the edge of a smaller piece of paper that you will tape over the hole.

Where are you going to get those hole punches? That's an easy one. Those chain craft stores have tons of them.

How large should the hole be? That depends on your lens. For most lenses 10mm will work well but there is a formula if you want to get technical: lens focal length divided by the lens' maximum aperture. Example: a 50mm f1.8 will be 50 / 1.8 = 27.7mm or smaller. Lenses with a larger aperture work best.

Bokeh Lights Some important details:
1) Keep the aperture as wide open as it can go.
2) Prime lenses work best but if you use a zoom, make sure you are zoomed in all the way.
3) Place your foreground object as close to the lens as possible and still be able to focus on it. This will give you the largest shapes.
4) When making your aperture lens cover do not use ordinary construction paper. There are too many loose fibers that show up in the photo. Use scrapbooking card paper.
5) Make sure the hole you punch is exactly in the center.
6) Don't make the hole too large. If it is too large, it will not work.
7) Dim the room lights and use a tripod

Bokeh Lights Hearts Setup Bokeh Lights Violin Setup