While I spend more than my fair share of hours on airplanes, the vast majority of that time is spent on jet airliners. When I do end up on a smaller propeller-driven plane for shorter flights, I generally don’t end up with a seat that looks out at the propeller. But when I do, I like to have a little fun with my hallucinating camera.
Perhaps you’ve seen this effect before. And perhaps you’ve even photographed it yourself. I have too, but I never seem to get tired of it. I sit there in awe, watching the LCD display on my camera (or, in this case, my iPhone), watching the “magic” of the camera seeing things that my eyes can’t.
There are actually two cool (at least to me) things going on here.
First, of course, is the rather wild effect you see with the propeller blades. This effect is two-fold. There is both an apparent bending and warping of the propeller blades, as well as an apparent increase in the total number of blades. Both go beyond reality, and are the result of the scanning type of sensor used in the camera. While the camera is taking the photo very quickly (ranging between 1/900th of a second and 1/1800th of a second in the photos shown here), it isn’t actually capturing all of the pixels at the exact same moment. Instead, it is capturing the images in a scanning fashion. For example, many sensors capture the image data from top to bottom, or from left to right. That process happens very quickly, but with a fast moving subject the speed of the subject exceeds the scan speed, and you get some wild effects.
The second thing that is happening here, which is a bit less impressive but still pretty cool (I think) is capturing something that we simply are unable to see (or at least see in the same way) with the naked eye.
I’m no expert on the Bombardier (née De Havilland) Dash 8 that was my platform for the photos shown here, but I figure the propeller spins at a speed of around 1,000 revolutions per minute (RPM). That would suggest around 15 or so full turns every second, which means each blade of the propeller goes around in a full circle in about 1/15th of a second. While the speed of that propeller blade will vary from hub to tip (since different points of the blade have a different distance to cover in order to make a full circle), let’s just assume a speed of around 250 miles per hour (for those interested, this is based on a 13-foot diameter or 7.5-foot radius, measuring the speed at the mid-point of the propeller blade).
Anyway, what that effectively means, if my math is even close to accurate, is that the shadow of each propeller blade passing across the engine nacelle is moving at about 250 miles per hour. That’s pretty darn fast, and of course all things considered a shadow is relatively faint, not providing a huge amount of contrast. So with the naked eye you can perceive the lines of the shadow, but you can’t really “see” them, exactly. They appear as ever-so-brief blurs of slight darkness flashing across the engine nacelle, not actual strips that match the shape of the shadow.
But with an adequately fast shutter speed, those shadows, much like the propeller blades, are frozen in time, providing a clear view of them. Of course, those shadows are also multiplied in the same way the propeller blades are, so the effect is way beyond what we could ever hope to see with the naked eye.
We generally think of a camera as more or less recording the scene as it appears in reality (with some variation based on color and exposure issues, and of course based on shutter speed effects). As such, I suppose the artifacts shown in images like those shown here would be considered something of a defect in a camera sensor that allows this sort of artificial effect to be created. And yet, the effect can be pretty cool. And mind-bending.