It's the longer shutter speed that creates the effect as the shutter is open long enough to catch the lights

(or whatever)

'moving' through the shot

(same thing you get with camera shake but controlled for effect, like panning shots where you get a blurred background but focused subject).

The smaller aperture is just a function of the camera to keep the exposure correct.

ISO setting makes no difference to the effect.

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it's just there to cope with differing amounts of light.

The tripod though is essential.

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otherwise the long exposure would mean camera shake effecting all of the image

R.
