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Flash Drag

A¬†discussion on flickr’s strobist group forced me to re-examine an image I had recently taken while in Jean, Nevada.

That leading shadow I noticed, but didn’t know the source of.

I’m still not really sure of the cause – it could just be shadow from the bend of her leg, but the flash is coming from that direction and is lighting the lower part of her leg.

Another explanation is a phenomenon known as “flash drag.” Its cause is a slight camera movement during a (relatively) long exposure – in this case 1/30, which is long considering the flash duration of the Quadra A head – upwards of 1/1000 second. This resulted in a shadow forming on her leg as the camera moved ever so slightly. This caused the leg not to be exposed by the flash, but by the ambient exposure, which was about 3, 3 1/2 stops under.

Unlit, she is relatively dark; the sun is actually to camera right and slightly behind.

If the camera moved, why is she sharp? At 3 1/2 stops under the flash is making the exposure, and it is quick enough to stop motion (in the B port the “A” head is capable of 1/6000). The background is out of focus anyway so you don’t notice that. It is also a very slight movement. Steady Shot, which was on I believe (I thought at first it was not) also helps, but it can only help that which is exposed – and the leading edge of her leg was not.

Fixed in Photoshop.

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