Bokeh refers to the out-of-focus shapes formed in a photograph by the camera's aperture, particularly those of background features. I had found various instructions on the web on how to construct your own shaped filters, but none that would allow me to easily change these filters on the go.
After some thought I came up with a useful, reusable method for easily switching out shaped filters. This method is documented here.
You will need:
* A camera. Any DSLR camera should work; point-n-shoots tend to have quite small lenses so may not work so well.
* A lens for which you know the key measurements: focal length, and maximum aperture. I used a 50mm f/1.8 canon lens. Something with a large aperture relative to its focal length will probably work best.
* Black construction paper
* White glue (Elmer's or similar)
* Scissors & sharp craft knife.
* Pencil, ruler, and (optional) pair of compasses.
* A cheap UV or similar filter to fit your chosen lens (optional) Why? I wanted to be able to permanently stick the filter holder in place without ruining a good lens. I bought the cheapest UV filter I could, about 99 cents on eBay!
Step 1: Calculations
The first step is to determine the maximum size of your bokeh filter design. This size will be determined by the maximum aperture of your lens. Take the focal length of your lens, and divide it by the maximum aperture size, for example:
50mm / 1.8 =~ 27.7
I went with a 26mm diameter to be safe.
You'll also need to measure the inner diameter of your filter or lens body.
Ok, maths over, on to construction.
Step 2: Aperture Holder
Cut 5 discs the size of the inner body diameter of your filter. From the centre of three of these discs cut out a circle the diameter of your calculated aperture.
In one of the discs, cut a 1mm wide slot a little longer than this centre diameter - for my measurements I used 1.5mm either side.
Glue the two unslotted discs together.
Step 3: Make the Insert Template
The two remaining discs should now be glued together - do not cut out their centres.
Next, trace the slot and cut-out circle from the previous step onto one side, then mark and cut out the shape shown in diagram 2 below.
The measurements do not need to be exact, but the width of the tab should be less than the width of the slot, and greater than the diameter of the inner circle. If you happen to make the width too narrow, you can reinforce the template with another layer or two of construction paper.
Glue the slotted disc on top of the (glued) discs with centre holes, being careful only to glue around the edge (see diagram 4).
Step 4: Create Some Inserts
Trace out the template onto more construction paper, and cut it out. You can then make and cut your own designs into the scribed circle.
Here are some samples - feel free to use them, or design your own.
Step 5: Let the Fun Begin!
To use, insert one tabbed insert into the holder, and put the holder in place in the front of your lens/ filter. You'll probably need to slightly overexpose any shots by a half-stop or two due to the darkening effect of the bokeh filter.
At this point you can glue the filter holder in place if desired, but it's probably better to make a few test shots and adjust everything as needed before committing. Anyway, add any interesting shots you make using this technique to the comments, and happy shooting!