The great yet difficult thing about photography is that it can be used to capture a single moment in time and/or it can be used to capture a span of moments in a single photo.
You can set your shutter speed very high and freeze a millisecond in time – a racecar as it zooms through tracks, rain drops as they fall from the sky, and you can also freeze flowing water.
You can also set your shutter speed low and capture a sequence of events – fireworks as they streak through the sky, people or cars busily moving along streets, and not just water but the flow of water.
The above two photos are from one of the pedestrian bridges in Spokane, above the Spokane River. I was here first in mid-day and captured the first photo. I didn’t have my ND filter on me so I couldn’t smoothen out the water during the day. So I later went back to that spot during sunset and captured the flowing water. Since during a long exposure, you capture not only the water at a single moment in time but also its movement down the river, which smoothens out the water to give it a milky/foamy look.
The reason I said this is an aspect of photography that can be difficult is that it is up to the photographer to view a scene and determine which settings would give the best photo. There are different situations where long shutter speeds would look better and situations where shorter speeds would look better.
Which do you think was better in this scene?