Why does the left windmill have too many blades?

Question

Ok, check this shot out:

These are the 3 windmills in Marshfield Massachusetts. It was shot from Provincetown, MA about 35 miles away. (I wasn't out to shoot them, I was shooting Right Whales... I know, life is tough...

The shot is a pixel peeped full crop. Canon 40D body, the 24-105mm F4 lens, shot at 1/640sec at f4.0 using aperture priority. ISO 100.

What is going on with the left most windmill? It the blade at around 8pm is doubled. How did that happen?

Asked by Paul Cezanne

Answer

You're seeing the effect of a focal plane shutter (also see Wikipedia on the topic). At ¹⁄₆₄₀th of a second, the whole frame isn't exposed at once, but with a moving slit between the first and trailing shutter curtain. This catches the moving windmill in different places. Helicopter blades are the typical example here, because they often show up distorted in this way.

J. Andrzej Wrotniak has a nice detailed explanation of another possibility: slow readout of the CMOS sensor. I'm pretty sure it's the focal plane shutter in this case, though, as the given explanation there doesn't make sense in this case.

(And there's another nice example in this answer on Stack Exchange, which I mention with the caveat that the person who posted it went slightly insane with unexplained vitriol for no apparent reason and started trolling the site shortly afterwards.)

Answered by mattdm

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