This photo of the train station in Haarlem has always caught my attention. In trying to figure out why, I noticed that the images contains blur in three flavors: blur because of movement, in the case of the passing train; blur in the case of an area out of focus, as with the distant cityscape; blur because of a translucent surface, as with the station windows.
Each of these elements of blur contributes to the impact of the image. The passing train is captured but transformed to something beyond our reach in the still reference frame. The sense of being inside the station is heightened by both the way the windows diffuse the light, and by the blur of the exterior space. The combination intensifies the feeling of being in a particular place at a particular moment, separated from the rest of the world by distance, glass, and velocity.
All of these elements of blur were unintentional. Their joint effect I only realized months after making the image, but the insight of the power of blur is something I can compose with in the future.
Do you have an example of blur in a photo which contributes to the impact of the image?