Severed propeller?

[Graeme]

This photograph was posted on this site...

Google Image Result for http://www.boingboing.net/IMG00005.jpg

...asking for an explanation...

"I was travelling in a Bombardier turboprop plane recently, looked out my window and saw the propeller. Thought it would make a nice picture, so I took out my cameraphone.
The picture is attached. Unphotoshopped. There are propeller blades that appear to be floating, and that appear in a path that is outside the normal path of the propeller. Not sure what to make of it."



One 'answer' is given, from this site...

Google Image Result for http://www.boingboing.net/IMG00005.jpg

"The effect of the turboprop picture is caused by the propagation of the charge across the sensor field. All sorts o "ghost" and "UFO" proofs are starting to show up from digital pictures."

What is he saying in laymans terms?

 

[A4K]

I can't explain that myself mate, but that makes for an interesting picture...

 

[Henk]

It does look as if the blade is not attached to the prop, but it is. To the human eye and if you take a picture it looks the way it looks.

 

[Matt308]

BS

 

[Graeme]

BS

The photo, the explanation, or both?

 

[Matt308]

Oh sorry Graeme. Didn't see the explanation. I thought your post was asking if this was a pic of an actuall prop separation. Read it too fast. Yeah, some sort of artifact related to camera speed being a harmonic of the prop. Kinda like watching a wagon wheel where the harmonics of shutter speed of the camera are such that you see the wheel slow down, stop, and then speed up again. That would be my guess.

 

[Graeme]

Thanks.

Possessing no photographic knowledge, I thought the 'Wagon Wheel Phenomenon' died in the 'digital age'.

 

[Matt308]

Maybe it did. But the digital cameras still have variable shutter speeds and processor clocks controlling the CCDs, so I'm thinking that a comparable speed between the object being photographed and camera speed (or a harmonic thereof) is likely what is causing the stroboscopic effect.

 

[evangilder]

Nope, still happens in digital. I have a shot of the Bearcat where the prop blade is near the numbers on the nose, and the numbers literally look torn.

 

[Graeme]

It that the result of the stroboscopic effect through the propeller arc, or the combination of it, plus the forward motion of the aircraft?

 

[evangilder]

It's actually a great example of the Nyquist Theorem. There is a great discussion on this subject on Fencecheck about this very phenomenon.

Fence Check | What's going on with this photo?????

For the Math afficionados, here is Nyquist's Theorem, with all the math and formulae.

Nyquistâ€"Shannon sampling theorem - Wikipedia, the free encyclopedia