Why do the branches on this motion-blurred picture look like swiss cheese?

Question

I took this picture the other night while my wife was driving down the highway. The effect is somewhat apparent looking at this smaller size, but at full size, you can see this bizarre pitting effect on the larger branches of the trees, making them look like swiss cheese.

I've seen a funky effect where the bottom of the picture is at a different location than the top due to the lag time as different parts of the sensor are exposed, but this doesn't seem to have that same "bleeding" effect. It seems like something else is at play. What is it?

This was taken with a Canon Rebel T5 with 75-300mm lens at f8 1/100 640ISO 75mm... and about 45 MPH, I would guess 20-30 feet away from these trees.

Answer 1

I think the effect is caused at least in part by the many small branches that you don't see in the image. As you're moving along in the car, the dark twigs and small branches moved across the frame at various rates depending on their distance from the camera. Some points on the sensor happened to image various branches and twigs throughout all or most of the exposure, while other points happened to image sky the entire time, and still others got half branches and half sky. You were traveling horizontally, so branches that are horizontal or nearly so are most likely to be cause dark pixels, but there will also be points where different vertical and diagonal branches all happened to land at the same point as the camera moved. That explains the random, snowy effect.

Here's an illustration: Spread the fingers of both hands and position them in front of your eyes so that one hand is at arm's length and the other is 3 to 6 inches closer to your face. Now move both hands back and forth horizontally, looking through your fingers at the sky or other bright background. As you move, you'll see some areas remain dark and others are mostly light. The faster you move your hands, the more obvious the effect is. Vary the orientation of each hand and watch the effect change:

• fingers all horizontal: you of course get horizontal lines of light and dark.
• fingers all vertical: you again get bands of light and dark, but each band is caused by different fingers blocking the same area at different times.
• fingers at different angles: you get a splotchy effect.

Answer 2

The key information is that you took this picture from a moving car. That implies lots of vibrations over a wide frequency range.

What happened is that the camera "bounced" (was shaken back and forth) during the relatively long 10 ms exposure time. For a little while, it was sortof "tracking" the motion, then jumped quickly to a different angle, then "traked" the motion again. This may have happened different amounts at different angles at different frequencies. Consider that vertical angular motion is independent of horizontal angular motion.

The visible effect is caused by this wobbling being larger than the detail of the picture. Therefore, instead of large blurred objects, you have smaller objects that appear in several places.

Try reproducing this by taking a bunch of pictures from the same moving car. You will see quite a variation in effect as the camera bounced differently for different shots. Leaning the lens against a piece of the car body will make a large difference. All effects will be reduced with faster shutter speed.