How is Bokeh Formed?

September 8, 2010 10:47 PM

Can anyone explain these odd bokeh effects I saw in the film "Adventures In Babysitting?"

I recently watched "Adventures In Babysitting" and noticed some odd bokeh effects in the background during first night driving sequence in the film. Sometimes it is perfectly round, which makes sense to me. Sometimes it is rectangular, or shaped like to differently sized semicircles, or trapezoidal.

Here is a crappy composite of such images. Are there lens elements, hoods, or other doodads used in film that aren't used in still images that would explain this?

posted by chairface to Technology (6 answers total)

Best answer: The shape of the lights in the bokeh is usually determined by the shape of the aperture, but perhaps it's an artifact of the rotary disc shutter used in motion picture cameras in combination with the shutter angle.

BTW, you can make your own fun shapes.
posted by zsazsa at 11:15 PM on September 8, 2010

Total guess but:

I notice the square ones are mostly on the tight close-up shots. I'm guessing they're shaped like that because there's actually a light back there shining through something colored and square-shaped. A lot of times close shots like that are "faked" on a soundstage on in front of a greenscreen, and they might have used pieces of fabric or whatever to simulate lit windows or whatever was supposed to be back there.
posted by drjimmy11 at 11:15 PM on September 8, 2010

Best answer: I see the phenomenon with 35mm SLRs all the time. I’m not an optical engineer, but my understanding is that it is caused by obstructions between the lens and the film plane. The flipped‐up mirror is the primary obstruction in an SLR; I’m not sure what would obstruct the out‐of‐focus light in a motion picture camera.

Here’s a citation.
posted by Fongotskilernie at 2:14 AM on September 9, 2010

The recent Scott Pilgrim movie had intentionally shaped bokeh.
posted by Mwongozi at 4:21 AM on September 9, 2010 [6 favorites]

Nothing to add, but thanks for teaching me this word!
posted by kimota at 7:11 AM on September 9, 2010

Response by poster: Figure 3 in Fongotskilernie's link exhibits similar spatially varying bokeh, as does zsazsa's DIY pic. So I'm guessing there is something like a rectangular framing element in front of the lens, which explains some of the weirdness. I'm still curious what went on in the lower right frame which exhibits the strangest bokehs. I've only seen stuff like that when experimenting with dual aperture discs on my Lens Baby.
posted by chairface at 8:02 AM on September 9, 2010