Bokeh is a term for the quality of out-of-focus light in a photograph. One interesting effect is that points or small areas of light that aren't in focus take on the shape of the aperture they pass through on the way to the film (or digital sensor). This instructable will show you how to easily make the highlights into interesting shapes.

Camera lenses control the amount of light that passes through them by making a hole (the "aperture") larger or smaller. They usually do that by moving a set of metal blades (check out this trippy animated gif ). You can count the number of blades for many lenses by looking at the shape that a point of light becomes when it's out of focus. My 50mm lens produces a 7-sided figure (a heptagon).

What if we want something more interesting than a heptagon? It would be tricky to change the shape of the aperture inside the lens, but it's easy to make the light pass through another aperture as well...

Step 1: Materials

All you'll need is:
- A sheet of paper. You should get more pronounced effects if this is opaque, but I just used thin paper that's black on one side and white on the other. A friend had good results with a sheet of aluminum foil.
- Cutting tools. I used scissors to cut the outside shape and a knife to cut the inside shape
- A screw-in filter (semi-optional)
- A camera with a "fast" (large maximum aperture) lens and a screw-thread for a filter

I found I had pretty good results with my lens aperture set to f/4 or larger (that is, closer to f/1). It may work better with telephoto lenses. Please comment if you find particularly good (or bad) effects with various lens settings.

Step 2: Cut Paper to Fit Inside the Filter Ring

Trace the shape of the filter screw threads on a sheet of paper, and then cut it out.

If you don't have a filter, you can trace the front of your lens and then cut a bit inside the line.

Step 3: Cut Shape

Cut a shape into the center of the paper

Step 4: Pop the Paper Into the Screw Threads

Because you traced the edge of the threads to cut it, the paper should be a little bit bigger than the threading, which will make it stay snugly in the filter threads.

I recommend that you put the filter on the lens and put the paper in the screw threads of the filter instead of the screw threads of the lens. Much better to scratch or get fingerprints on an easily cleaned/replaced filter than your lens.

If the shape screen doesn't fit snugly, you can put it between the filter and the lens, and screw the filter into place, trapping it. But then you won't be able to easily adjust its direction.

Step 5: Shoot

Try it out. Set the camera to the maximum aperture (smallest f-number), and use the shutter speed to control the exposure. It may help to focus manually. Shooting highlights or small light sources against a dark background is the easiest way to see the effect.

One neat effect is that if some highlights are out of focus because they're too close and others are out of focus because they're too far away, they'll have bokeh shapes that point in opposite directions.

Step 6: Learn More About Bokeh

The shape of highlights is only one aspect of bokeh. Some other important technical aspects are:
- the pattern of light intensity and its tendency to form hard or soft edges (see Fig 1-3 here)
- the tendency of the blur to solidify or double lines (see Fig 6 here)
- the tendency to create chromatic aberration (see the second set of figures here).

And it's important to consider the artistic impact of out of focus areas. Do they distract from the foreground? Do they hide distractions? Is the pattern of light pleasing?

If you want to read more about bokeh, here are some links:
A short article about the history of the term, and about the use of bokeh
An explanation of the optics of bokeh