Shadows of an Eclipse

Eclipse 2012- 5

There were a lot of amazing things that we saw this weekend at Maker Faire— everything from live demonstrations of snails creating artwork (by Presley Martin) to the DIY pick and place machine (from buildyourcnc.com).

There was also something special in the sky: the solar eclipse on Sunday.

Eclipse 2012- 9

Viewed elsewhere (e.g., further north in California) this was an annular eclipse, where the sun does not disappear entirely, but instead becomes a ring of fire (since the apparent size of the moon is not large enough to block the full disk of the sun).

For us at Maker Faire in San Mateo, it was a spectacular partial eclipse, which we were able to view through solar viewing filters, kindly handed out by the Exploratorium.


Eclipse 2012- 8

Of course, it turns out that you don’t actually need a solar filter to watch the eclipse. Any little aperture— in this case the cap between my hand and the camera —can act as the pinhole in a pinhole camera and project the image of the sun onto a surface.


Eclipse 2012- 4

Eclipse 2012- 2

So if you’re not sure if an eclipse has started, or how much of an eclipse it is, just hold out your hands and make some little apertures; the shadows will show up with little bright spots in the shape of the sun, whether that’s a circle, ring, or crescent.

Eclipse 2012- 1

Stranger yet is to look around at all the shadows that you see every day. Even the shadow of your hand takes on an unexpected shape when the sun is anything other than round.
There are actually five outstreched fingers on my hand here, but you can hardly tell that when every bit of light that seeps through (or around the edges) projects a crescent-shaped image.

We take for granted that the shadow of an object will the same shape as the object, but as you can see, that isn’t necessarily the case when the light source isn’t round.

Visualizing image stabilization

EclipseLast month I went outside at 3 AM to photograph the eclipse. But I ended up having a hard time getting a good picture of the moon. The pictures were turning out unreasonably blurry– much more than I’d expect from just the moon’s apparent motion during the exposure time. The problem turned out to be the “IS” in my Canon S3 IS– the image stabilization– which apparently needs to be turned off for this sort of thing.

But why? Isn’t image stabilization supposed to take out blur?

Pointing elsewhere in the sky, you can sometimes (depending where you live) see small point-like sources of lights that can provide a useful tool for figuring out what the bleep your camera is actually doing. In the land of moderate light pollution one thing that we can see through our electronic viewfinder is the Pleiades star cluster, so let’s point our lens at that. The parameters for the two photographs below are identical, except that they were taken with image stabilization off and on, respectively: 15 second exposure, aperture wide open, zoomed in like crazy.
No IS.jpg
IS.jpg
In the top photo the stars each look like clean, easily distinguished stripes.

(In order to reduce vibration due to the button press, these two pictures were taken with a timer delay of a few seconds. However, there is still a small, squiggly tail at the base of each star track, presumably due to residual vibration of the tripod.)

In the lower photo, with image stabilization turned on, you can really see a significant difference. The initial squiggly tail at the lower left of each star track is still present, but is now smaller– thanks to the IS no doubt. Otherwise, the shape of the tracks is quite different. Let’s zoom in:

At the lower left is the initial squiggle from the tripod. In the middle is a large almost triangular structure where the star light was initially steered to. Then, there is an additional, wandering shape starting towards the upper right that bends down the right side.The net effect is that the trail of the star is not reduced to a point– just bent around into a loop.

Conclusion? It looks as though the image stabilization works well for short times– maybe up to a second– but lacks accuracy for longer exposures. And *that* is why you might want the IS off to take a clean picture of the moon.

Update: From what I can see online, there’s no image processing component to the image stabilization process– it’s based solely on sensing acceleration of the camera. Possible answer: Could the slow wandering of the image result from noise in the accelerometer signal?