24 sep 2008 kl. 21.24 skrev ADavidhazy:
Hi, I was given a sample of some photographs taken of the propeller of an airplane made with, I believe, a phone camera. There is an obvious anomaly present. Although I suspect the reason I am not really sure (really I do not know!) how this came about. If you can shed some light on the effect for me it would be much appreciated. See the images here: http://people.rit.edu/andpph/a-misc/phonecamera-prop-effect-1.jpg Thank you for taking the time to give me a hand! cheers, andy
Well, I don´t believe in either strobe lights or corrections for turbulence (Mark, with all respect for his flying experience), nor polarization in the windows (Stephen).
The pattern in both images is absolutely regular, and pin sharp, with the same "line frequency". Only, the pattern is horizontal in the horizontal image, and vertical in the vertical one, i. e. the lines are roughly parallel to the long side of the image.
Also, the "lines" are not perfect; they broaden and bend close to the hub, where the linear speed of the propeller blades are lower.
These observations seem like a dead giveaway that we do have some kind of scanning effect within the camera itself. Only, the scan isn´t just sequentially one line at a time like a flatbed scanner; there are several parallel channels at work simultaneously, spaced a certain fraction of the image width apart. AFAIK, many CCD devices do work that way.
When watching a dull TV show next, try waving your hand rapidly in front of the screen, with fingers spread (works well with an old- fashioned CRT TV; don´t know about modern flat ones). You´ll be surprised by the apparent form of your fingers.... ;-) It´s a similar phenomenon of scanning interference.
Humbly waiting to be proved wrong.... Per Öfverbeck http://ofverbeck.se "In a world without walls or fences, who needs Windows or Gates"
