Why does the left windmill have too many blades?

Question

Ok, check this shot out:

These are the 3 windmills in Marshfield Massachusetts. It was shot from Provincetown, MA about 35 miles away. (I wasn't out to shoot them, I was shooting Right Whales... I know, life is tough...

The shot is a pixel peeped full crop. Canon 40D body, the 24-105mm F4 lens, shot at 1/640sec at f4.0 using aperture priority. ISO 100.

What is going on with the left most windmill? It the blade at around 8pm is doubled. How did that happen?

Answer 1

Despite what it might look like, this is not a focal plane shutter effect. A focal plane shutter effect causes distortion because different parts of the frame are exposed at different times. For a spinning blade that moves a fraction of a rotation during the entire exposure, the blade would appear curved and its thickness wrong. However, that is not what we are seeing. In no case will this effect cause two distinct images of a smoothly moving object like that blade.

I see two other artifacts in this picture:

1. Horizon diffraction. The subjects are 35 miles away very low to the horizon. The air near the ground (or water in this case) has various temperature gradients and hot and cold pockets. Hot and cold air has a slight difference in its index of refraction. The hot and cold air layers and pockets then bend light as a result. This is a slight effect, but over 35 miles can certainly cause these kinds of artifacts and much more. This is the same effect that causes mirages over much shorter distances in deserts where the ground is warmer than the air. It would be very suprrising if the see temperature and air temperature were well matched that day, especially over such a long distance.

   Since there are pockets of hot and cold (relatively) air, distant object can apear multiple times or not at all at any one instance. It is no stretch at all to believe that the double image of the blade could be caused by this.

2. Compression artifacts. There is a lot of stuff around the windmills that look like JPEG compression artifacts. This image is at full resolution, so it's no surprise these artifacts are visible even at a "hiqh quality" setting in the camera. I don't think these artifacts are so strong as to cause the double blade image, but they certainly don't help. I really don't understand why people shoot in anything but raw if they are going to pixel peep afterwards. It makes no sense. Also, you want as much information as possible going into post-processing, which certainly does NOT mean adding compression artifacts and squasing the image to only 8 bits per color per pixel. However, that is another discussion.

Added:

For those who are still thinking this might be a focal plane shutter artifact, here is some math. The picture was taken at 1/640 second shutter speed, which means each pixel was exposed for about 1.6 ms.

I don't know what the X-sync speed is of that camera (the maximum shutter speed at which the whole shutter is open at once), but let's be generous and say it is 1/100 second. That's pretty slow by today's standards. Even if so, it means the shutter traversal time accross the whole image is no more than 10 ms. In other words, the center of exposure time varies by 10 ms accross the picture. This will be true regardless of shutter speed.

However, this 10 ms time lag is spread accross the image. The center is only 5 ms off from either edge, for example. I looked at the picture above, and the windmill blade in question is only 12 pixels wide at most. A full resolution picture on that camera is about 3900 pixels accross, so the time skew across the width of the artifact is 10ms(12/3900) = 215 ns. That is tiny compared to the exposure time, but adding the two still comes out to less than 1.6 ms.

Now let's say the windmill was rotating at 1 Hz. That would be pretty darn fast for such a large machine. 1.6 ms would be only .6 degrees of rotation. Considering a radius of 12 pixels, that would mean a motion of .12 pixels at the tip of the blade, which represents the maximum size that any focal plane shutter artifact can be in this case. That's simply not what's going on.

Answer 2

My guess (as an imaging scientist, remote sensing) is that what you might be seeing is an artifact of the Fresnel Effect. Light is an electromagnetic wave, and the sea water is a conductor (salt water). There is interference between the light reaching the lens, with some of it reflecting off the water (or not).

Surface texture plays a role as well, and sea state will impact the Fresnel Effect coefficients.

Sometimes Fresnel effects are very short lived in time, and our eye compensates for them. However in a still photograph it will not. Furthermore, the path from the windmill to the lens is different for each windmill, with constantly varying surface textures, with constantly varying reflective and polarizing properties.

A practical example of Fresnel effect is when traveling at the edge of a FM radio station, the signal will "picket fence" where it fades in and out rapidly. Another example is a mirage, where the mirage is more persistent than the picket fence on the radio station.

Now, I could be wrong, and I frequently am, but my considered opinion is that this is not a shutter induced artifact. @Olin Lathrop's suggestion of atmospheric influence is not inconsistent with the Fresnel effect, and can contribute to it. Keep in mind that reflection, refraction and dielectrics all can play a role with Fresnel Effect.

In conclusion, I would submit that the most likely explanation of the seemingly extra blade on the left turbine is that it is an artifact created by Fresnel Effect, and that distance, low angle over a conducting media (salt water) and the temporal changing scene (which the camera captured a short instant of) are all contributory factors.

Answer 3

Seeing the poor quality of the image it can just be an artifact which the mind reads as a blade.

Answer 4

Two aligned windmills? One behind the other?