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
- 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
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
Step 4: Pop the Paper Into the Screw 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
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 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