Curious Phenomenon

[enki9]

Back over at another forum there was this helicopter picture posted.
http://www.airforce.ru/photogallery/dundin/kuznetsov2004/bpk/page_07.htm

Could anyone explain, Why do two rotor blades seem bent? In reality they are absolutely straight even when in motion. It is a photographic phenomenon, but what causes it? I know something about optics and slow shutter speeds, but this phenomenon puzzles me.

Other pictures show the moving rotor blades just fuzzy, like you'd rather expect:
http://www.airforce.ru/photogallery/dundin/kuznetsov2004/bpk/page_12.htm

(I didn't get the images linked directly, so there are just the links, sorry.)
Any reasonable explanation out there?

 

[Azuth]

http://ludens.cl/photo/gallery/effects/effects.html

Well, I've learnt something

 

[markc]

Andrew Davidhazy exploits the same effect using a modified Polaroid to get images like this:

The lens is replaced by a vertical slit and a string is run from the film tab to a small motor. When the shutter is pressed, the film is pulled past the slit. I was standing on a turntable rotating in front of the camera.

 

[enki9]

Thank you for the replies. I should have thought something like what was explained in the link Azuth posted.

The moving rotors in the second photograph I linked to are, however, not bent. Is this because a slower shutter speed?

 

[hobbes28]

There's a magic number where the blades are running faster than the shutter and it looks like the first one is on that line, where the second one has a slower shutter speed so the whole blade is blurred. Keep in mind that the RPM of the blade is greater the further you go from the center, so that first one, the center part is slower than the shutter but gets closer and passes that toward the edges.

 

[markc]

Oh duh. I forgot to mention swing-lens cameras like the Widelux. Using slow shutter speeds with those can get some weird effects if the subject is moving. I've wanted to own one for years.

 

[DocFrankenstein]

It has nothing to do with slow shutter speed. In fact, the russian photo isn't slow shutter speed at all. It's 1/500 or 1/250


The reason behind this is that it's probably shot with an old zenit. Curtain shutter with really slow curtains. 1/30 of a second to travel across the film plane.

 

[markc]

I didn't say that the original had anything to do with a slow shutter speed, but the Widelux images do. Find the book End Time City by Michael Ackerman and you will see lots of images taken with a slow shutter and a Widelux. The lens swings to cover a panorama. If the shutter is slow and the subject moves, they get either compressed or stretched. You can end up with a very similar effect. This happens with people just walking by, hence the slow, not fast, shutter.

 

[DocFrankenstein]

I didn't say that the original had anything to do with a slow shutter speed, but the Widelux images do. Find the book End Time City by Michael Ackerman and you will see lots of images taken with a slow shutter and a Widelux. The lens swings to cover a panorama. If the shutter is slow and the subject moves, they get either compressed or stretched. You can end up with a very similar effect. This happens with people just walking by, hence the slow, not fast, shutter.

My apologies.

 

[markc]

No problem.

 

[Torus34]

Effect due to focal plane shutter. Think of the shutter as a horizontal slit which moves from top to bottom. Also in the first shot remember that the blades on the helicopter are counter-rotating. When all blades are blurred, chances are shutter was a between-the-lens type which opens and closes radially.

 

[Unimaxium]

There's a magic number where the blades are running faster than the shutter and it looks like the first one is on that line, where the second one has a slower shutter speed so the whole blade is blurred. Keep in mind that the RPM of the blade is greater the further you go from the center, so that first one, the center part is slower than the shutter but gets closer and passes that toward the edges.

Small correction -- the RPM is the same at any point on the blade, but it is the speed (e.g. MPH) that is dependent on the radius. The outside of the blade turns just as many times a minute as the inside, but your point is still valid that the outside turns at a faster speed. Sorry, I'm just in a nit-picky mood today

My only guess as to why the second photo isn't curvy is the same as what the last poster suggested -- it was probably taken with a different camera that uses a different kind of shutter (one that works like an iris rather than a slit).

 

[hobbes28]

Small correction -- the RPM is the same at any point on the blade, but it is the speed (e.g. MPH) that is dependent on the radius. The outside of the blade turns just as many times a minute as the inside, but your point is still valid that the outside turns at a faster speed. Sorry, I'm just in a nit-picky mood today

That's what I get for posting before I've had my coffee.