Long Exposure Mistake #2: Using the Wrong Aperture
In order to extend the shutter speed to the appropriate time you need to blur movement, you have to decrease how much light enters the lens.
Of course, to do that, you have to use a smaller aperture.
But there is such a thing as using too small of an aperture…
Most lenses have an aperture range that goes to f/22, which is a really, really small aperture.
The problem, though, is that at f/22, most lenses – even very expensive ones – show signs of diffraction, which reduces the sharpness of the image.
A better method is to shoot in the sweet spot of your lens.
Finding the sweet spot is easy, but it’s generally somewhere between f/8 and f/11.
Doing so will ensure you get the sharpest image and also have good depth of field.
And since these apertures are middle-of-the-road, they also help limit the amount of light entering the lens to facilitate those longer shutter speeds that you need.