This expression generally refers to using a slower shutter speed than what normally be the case.
For instance, when shooting fast moving action such as a person running you might generally use a shutter speed of 1/1000th of a second to freeze the action, thereby avoiding motion blur. However if you reduce the shutter speed to something like 1/60th of a second you will most certainly see some blur in your image.
However a shutter speed of 1/60th of a second is perfectly suitable for preventing motion blur when photographing a snail, but if you slow the shutter to 2 seconds or more you will once again introduce motion blur.
Dragging the shutter therefore does not refer to a specific shutter speed, but to a deliberate shutter speed reduction which will introduce some kind of motion blur. Doing this might introduce some dramatic effects if used with care and is often used to great advantage.
Below are some examples of shutter drag :
Dragging the shutter is not only used to introduce blur in an image, but is also extremely useful when shooting in low light to prevent having to increase the camera’s sensitivity (ISO setting). This curbs the inevitable appearance of noise in an image. With care one can curb the noise and still have little enough blur so as not to spoil the image.
Notice in the image below that noise is very low, yet the people sitting at the restaurant tables are still discernible and not overly blurred.
Stay tuned for some even more dramatic examples of shutter drag!
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