TeeJay wrote:
Skip, I'm sure I've seen you comment on this type of isue in another thread - but I've not seen examples before. I understand the "power line frequency" bit (or I thought I did!) but does the type of "gas" in the fitment have any bearing on the "band" - or does it only affect the colour?
TJ
The banding is a function of how the light actually changes over time (too fast for humans to see, of course) and what the exact shutter speed is. Essentially what is happening in the sample pics in this thread is caused by the focal plane shutter. Let me copy something I wrote about focal plane shutters in another thread, then I will continue.
Focal plane shutters (common on SLR cameras) consist of two "curtains", usually made of rubberized cloth (in old film cameras) or very thin metal. The first curtain (which I will call the "leading" curtain) normally covers the film or sensor, hiding it from the light coming through the lens. When you take a photo, the leading curtain moves across the film/sensor to expose it to the light. After the leading curtain has moved, another curtain (which I will call the "trailing" curtain) starts to move, again covering the film/sensor to hide the light from it.
At shutter speeds below the "Max Sync Speed", the leading curtain travels all the way across the film/sensor, fully opening the film/sensor to the light, before the trailing curtain starts to move. At higher shutter speeds, the trailing curtain starts to move before the leading curtain has completely travelled across the film/sensor. What happens to create the very fast "shutter speeds" is that an open slot between the two curtains travels across the film/sensor.
While old focal plane shutters (like in my Nikon F cameras from the 1960's) travelled horizontally, the shutters in most modern SLR's travel across the short distance of the film/sensor frame. The concept of "curtains" turns into one of "blades", but the travel concept is still the same. The leading blade moves first, uncovering the film/sensor, and the trailing blade follows, covering up the film/sensor.
The advantage of the blade style of focal plane shutter is that it can move across the whole film/sensor area faster than the old style curtains. Thus, the maximum sync speed is higher than in the old cameras (max 1/60 for my old Nikon F's, and 1/250 for the 20D).
The concept of a maximum sync speed, however, still applies. If you try to use a flash at higher shutter speeds (faster than the shutter speed at which the leading curtain/blade is fully open before the trailing curtain/blade starts to move), part of the film/sensor will be covered by one or the other of the curtains/blades when the flash (with a very short duration) goes off. Part of the film/sensor will not "see" the light from the flash, and that part of the image will be either black or very dark.
With that little lesson down pat - the photos at the top of this thread must have been taken with a shutter speed in excess of the max sync speed for the camera. Thus, a slit between the curtains was travelling across the image (actually in a top-to-bottom or bottom-to-top direction when the camera is in its normal position). As the slit was travelling across the sensor, the color and intensity of the light was changing. Thus, the image had a different color painted across the image as the slit travelled.
