Originally posted by jaieger I remember reading up on ND filters somewhere and the author said that a trick of architecture photographers was to slap on an ND with a larger stop reduction (like, -3 stops or more. ish.) and to do a long exposure, which would render the masses of tourists so blurry that they were invisible. He included an example picture where a .5 second of a tourist walking down a halfway produced a strong blur, whereas a 4 second exposure of the same hallway produced...just the hallway.
You can use this phenomenon to play around altering the lighting (or even the objects) in a scene.
For example:
Pentax K20D
Tamron 17-50/2.8 Pentax K20D
Cosina MF 20/3.8
The first photo is a 15 minute exposure, during which I walked up to the lantern and removed a flash unit and also crossed the bridge twice in order to hang out over the edge with a flash on a monopod, illuminating the bridge by firing the flash a total of twenty times. It is sort of fun moving around in the scene of your photograph during the exposure, knowing you won't show up in it.
The second is a 13 second exposure. I used the self-timer to get myself into the frame, illuminated myself with an LED flashlight, jumped out of the frame, and illuminated the binocular stand with the flashlight.