This doesn’t actually increase the amount of light on the scene, but since the shutter is open
longer, there’s more time for more light to get to the sensor.
Using this method – like every other choice in photography – has both good and bad aspects.
On the good side... There is no expense for additional equipment. It’s just a setting on the
camera. Plus, you can leave the shutter open as long as you want. (Leave a shutter open long
enough and you can get a pretty decent shot in a very dark scene!)
On the other hand... A long shutter speed can cause camera shake - and camera shake is a photo
killer.
In most of our general photography, if there is no movement in the scene, we will want to use the
fastest shutter speed we can – to eliminate camera shake - unless there is a specific reason to use
a slower one.
Generally our reason for using a slow shutter is because we WANT a bit of blur to indicate
movement in our scene.