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Wednesday, June 8, 2016

Why are pictures of props so distorted?

Here's a short video of what it looks like skimming the tops of a cloud layer. Check out what the propeller seems to be doing:

That prop disc in the video looked almost hypnotic, didn't it? I also have tons of pictures with crazy-bent props like these:

Waves splashing over the Blue Ridge Mountains.
Washington-Dulles peeking out of a cloud layer.
Power plant plume rising on an exceptionally calm day.
Pilot's glory 1.
Pilot's glory 2.
Sunset coming from Buffalo.

Crepuscular rays.
 (I did an entire post on crepuscular rays before.)
Flying past a building thunderstorm over North Carolina.

In the soup 1.
In the soup 2.
In the soup and icing up.
Departing Newark with New York City in the distance.
Departing Newark with New York City in the distance.

The propeller isn't made of rubber, but the camera doesn't know that. You can't read an entire page of a book at once; instead, you scan through the page line by line working your way from the top of the page down to the bottom. Unless you have a very expensive professional-quality camera, your camera does the same sort of thing as it captures a picture: it "reads" the scene from one end to the other.

This is called "rolling shutter", and works quite well in most cases. However, when the thing you're taking a picture of is rotating at 900 RPM (as was the case in all but the last two pictures above—those were at 1050 RPM), by the time the camera makes it to the next line, the prop has moved a bit.

This incredible animation lets you see what's happening as a digital camera takes a picture of a moving propeller:

Original image by Hunter5625 on imgur.
You can see the scan line moving from the bottom to the top. As the red "propeller" rotates, the bent blue shapes show where the scan line and the propeller meet at that instant. If the propeller wasn't moving, the red shape and the blue shape would overlap perfectly, and there would be no distortion. That's why 99.9% of the time, this method of taking pictures works just fine. There are camera sensors that work on a different principle that aren't subject to this distortion, but they are so expensive that the cost outweighs the benefit many times over.

As a bonus, here's a different kind of distortion. This one has nothing to do with camera sensors, but instead is how much change an aircraft tire distorts when inflated to a couple hundred PSI. (That's 6-7 times the tire pressure in your car!) The tire on the left is at normal pressure, and the one on the right is flat due to the big piece of metal in it. They're both the same kind of tire, but one is almost twice as big as the other!

Flat tire on a Dash-8.
See you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a Master-level participant in the FAA's WINGS program and a former FAASafety Team representative. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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