The View Up Here

Random scribblings about kites, photography, machining, and anything else

Tools For Measuring Camera Stability – Part 2

Posted by Tom Benedict on 09/10/2011

In Part 1 of this series I introduced the idea that camera motion on a KAP rig is a measurable quantity, and that in order to understand the effects of changes being made to a KAP rig, this quantity should be evaluated before and after each change. Specifically we are interested in the rates of rotation about each of the three axes of the camera: pitch, yaw, and roll. Pitch is how much the camera tilts up and down. Yaw is how much it turns side to side. Roll is how much it turns about its optical axis. Before launching into the tools to measure these, let’s take a look at some simple solutions that almost everyone uses:

It’s possible to minimize the effect of motion in all three axes by using a faster shutter speed. Let’s say your rig is rotating at a rate of ten degrees per second. With a one second exposure, the rig rotates ten degrees. At a tenth of a second, it rotates by a degree. At a hundredth of a second it’s a tenth of a degree, and at a thousandth of a second it rotates by only one one hundredth of a degree. To give you an idea of what this means in terms of image blur, my camera has about 5000 pixels across the horizontal direction. The field of view of my camera is about 90 degrees. So each pixel represents 90/5000, or 0.018 degrees. A rotation of 0.01 degrees during an exposure means motion blur of just under half a pixel. So a 1/1000 second exposure will save the shot, whereas a 1/100 second exposure would be hopelessly blurred.

It’s possible to minimize the effects of pitch and yaw by using a wider lens.This is obvious to most photographers, so if you find yourself nodding feel free to skip this paragraph. If you find yourself scratching your head, I hope this will help: Let’s say your camera is rotating at ten degrees per second, again. If you have a lens on the camera that gives you a ten degree field of view, then in one second your scene has shifted by the full width of your frame, or 5000 pixels in my case. If you switch to a lens with a 100 degree field of view, a one second exposure will result in features smeared across a tenth of the frame, or about 500 pixels. It’s impractical to go much wider than this, but you get the idea. Wider lenses suffer less from rotation in pitch and yaw.

Home free, right?

Not quite. That only applies to pitch and yaw. But what about roll? Higher shutter speeds still help, but wider lenses don’t. If your camera rolls by a degree while the shutter is open, the center of the image may look reasonably sharp, but the outer edges will be blurred. This happens regardless of focal length. I won’t go into the math, but blur from roll is invariant of focal length. The only easy fix is to increase shutter speed.

Which brings us to the harder fixes for rig motion: active gyro stabilization, passive stabilization, flywheels, etc. It’s possible to spend a lot of time and even more money pursuing any or all of these. But which ones will help, and by how much? The answers aren’t always intuitively obvious, and some solutions that should help don’t. But without tools for measuring camera stability, all you have to go on are anecdotes. It’s imperative to have metrics before starting the engineering.

One solution would be to get an inertial measurement unit (IMU) that logs rotation rates in pitch, roll, and yaw, as a function of time. These are fairly inexpensive these days, and are readily available from places like Sparkfun. But they have limitations in terms of their resolution, sampling rate, drift, and noise. Ask anyone who has spent time working with rate gyros and IMUs and they’ll tell you it’s not as easy as just plugging the thing in. It’s also one more thing to buy, install, and fly in order to gather data. Not everyone has an IMU on their rig, so not everyone can take advantage of the data it offers. Since it doesn’t offer an immediate improvement in the performance of a KAP rig, it can be difficult to justify buying and flying one.

There’s another solution, though, that most people doing kite aerial photography, and really any form of aerial photography, already have: their camera.

In the next installment I’ll discuss how you can use your KAP camera and some readily available software tools to evaluate the stability of your KAP rig.

– Tom

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