Originally posted by nanok do you think the discussion is interesting enough still to move to a separate thread? i'm still curious what controlled testing will show, but i'm definitely not about to continue this here. beginner thread or not, this is clearly offtopic for this thread.
I think it's at least interesting enough for some test shots. I don't see the point of "internet-arguing" in a beginner's thread, but if everyone can learn something, that's relevant.
You don't really lay out your theory in one post. It is apparently based on this: The shutter has two parts called the front and rear curtains. When you take a shot, the rear curtain starts out fully open, the front fully closed. The front curtain opens, the sensor is exposed to light, the rear curtain closes. The curtains are mechanical so they can only move so fast, and it is often useful to have a fast shutter speed. So when you want speeds faster than 1/180 sec., the rear curtain starts to close while the front curtain is still opening. In other words, at 1/180 sec. or less, the sensor at some point is completely exposed to light. At faster speeds, it is only partly exposed, by a traveling slit, the gap between the front and rear curtains. To get really fast shutter speeds, the rear curtain just gets very close to the front one. A mechanical trick.
It sounds like you think this has some effect on shots and everyone else doesn't.
I decided a ceiling fan was a good test subject. I put my camera on a tripod, put on a Pentax-F 50mm f1.7, focused in the center of the frame, in Tv mode. Some test shots showed I could use ISO 3200 for a useful range of shutter speeds. I took shots at 1/3000, f2.0, then pressed the AE-L button to keep that exposure. Then I stopped down a full stop in shutter speed, took a shot, etc. So my shots were:
1/3000, f2.0
1/1500, f2.8
1/750, f4
1/350, f5.6
1/180, f8
1/90, f11
1/45, f16
1/20, f22
Here is the first shot 1/3000:
IMGT5727 by
just1moredave, on Flickr
And here is the shot at 1/180:
IMGT5731 by
just1moredave, on Flickr
I have to say, all these shots look exactly as I would expect. The fan blades have more motion blur as shutter speed decreases, perfectly consistent with the shutter speed changes. With five blades, you can find one moving pretty close to parallel with that travelling opening. There's no panning and the blade speed is the same. So it's a decent test.
Unfortunately the fan is white, the ceiling is white, so the contrast isn't great. I raised it some in Lightroom, both shots the same. Also the fan wobbles slightly, and in Tv mode/fixed ISO obviously the aperture will vary, so DOF changes. I think this is visible in the first shot. I haven't thought of a better test and I'm not painting the ceiling black. Anyone else have a ceiling fan?