It Moves

Tripod-mounted exposure of the full Moon at mid-eclipse on September 27, 2015. Image made with Nikon D610, Nikkor 200-500 f/5.6E at 500mm, f/5.6, ISO 3200, 1-second

Tripod-mounted exposure of a full Moon at mid-eclipse on September 27, 2015. Image made with Nikon D610, Nikkor 200-500 f/5.6E at 500mm, f/5.6, ISO 3200, 1-second (Bill Ferris)

On the night of September 27-28, 2015, the Moon passed through the densest, darkest portion of Earth’s shadow, an event known as a lunar eclipse. Normally, I wouldn’t publish or share a photo like this. It’s just a tad soft, not rich in fine detail. I tried to make a sharp, detailed photo at mid-eclipse but the forces of nature intervened.

How is it that we’re able to see the Moon? Well most of the time, the Moon is exposed to the Sun. Despite being a relatively dark object, enough sunlight reflects off the lunar surface to make Earth’s largest natural satellite the brightest object in the night sky…when it’s up and when the side of the Moon that faces Earth also happens to be facing the Sun.

When photographing the Moon, you can use a normal daylight white balance setting (reflected sunlight) a reasonably large aperture (f/5.6), a not-too-high ISO (400) and make a proper exposure at about 1/500-second. That’s when the Moon is near its fully-illuminated best.

During a lunar eclipse, the Moon is not directly exposed to the Sun. It’s hiding in the Earth’s shadow…but not totally dark. You see, Earth’s atmosphere acts like a lens. It scatters and refracts sunlight. Short wavelengths (blue light) are scattered in all directions by the atmosphere. Longer wavelengths (red light) are refracted so that this light passes through the atmosphere, travels through space and falls on the Moon.

This is why the Moon looks red during an eclipse. Only the red light which passes through Earth’s atmosphere falls on and illuminates la Luna. If you saw the September 2015 eclipse, you probably noticed how dark the Moon looked. Earth was blocking most of the sunlight that normally paints the lunar surface. The rest was mostly scattered. What little passed through Earth’s atmosphere to fall on Luna’s surface was the long wavelength red stuff. As a result, the Moon looked dark or blood red.

So, what does this have to do with slightly unsharp photos of the Moon taken during mid-eclipse? Well, with less light to work with, your camera needs to do one of three things to make a proper exposure:

  • Use a larger aperture to collect more light
  • Use a higher ISO to be more sensitive to faint light
  • Use a longer exposure to collect more light

Two of those three options have nasty consequences for your photos.

Handheld exposure of a waxing gibbous Moon on September 24, 2015. Image made with Nikon D610 and Nikkor 200-500 f/5.6E at 500mm, f/5.6, ISO 400, 1/800-second.

Handheld exposure of a waxing gibbous Moon on September 24, 2015. Image made with Nikon D610 and Nikkor 200-500 f/5.6E at 500mm, f/5.6, ISO 400, 1/800-second. (Bill Ferris)

A few days before the eclipse, I shared the above Moon photo taken at 500mm, f/5.6, ISO 400 and 1/800-second. The Moon is a moving object. It orbits Earth, moving west-to-east about 13 degrees (1/2-degree per hour) through the sky, each day. Much of its motion through the sky is the result of the fact that Earth rotates about an axis. Due to that rotation, the Moon moves east-to-west covering about 15-degrees per hour.

If you take a picture of the Moon using an exposure of 1/500-second, your photo will record the Moon and its motion over a distance of about 0.03 arcsecond. The full Moon is about 30 arcminutes in size. There are 60 seconds of arc in each arcminute so, that gives the Moon an angular diameter of 1,800 arcseconds. Divided by 0.03, that 1/500-second exposure records motion spanning 1/60,000th the diameter of the Moon. Yes, that is incredibly tiny and is imperceptible to the eye.

If you take a picture of the Moon during mid-eclipse using a the same focal length and aperture, and an ISO of 3200, you’ll need about a 1-second exposure to make a proper image. That’s 500-times longer than an exposure when the Moon is illuminated directly by the Sun. Your exposure will record the Moon and its motion across a distance of 15 arcseconds.

Now, 15 arcseconds is also a small distance. But it is large enough that the exposure you make will look slightly soft. If your goal is to achieve critical focus on the Moon shooting at 500mm, you’ll need to open the aperture or increase the ISO to use an exposure of 1/2-second or faster. Modern digital cameras are certainly capable of working at ISO 6400 and higher. But unless you’re using a really long lens, you’ll end up cropping the resulting image significantly just to make the Moon fill the frame. This not only makes the Moon look bigger but also emphasizes the digital noise in the photo. The resulting image will look grainy and, as a result, even more soft.

The one sure way to make a sharp photo of the Moon during an eclipse such as the one we enjoyed in September 2015, is to attach your camera to an astronomical mount. The mount will need a motor drive that rotates one axis to effectively move the camera opposite Earth’s rotation during the exposure. This rotation cancels the east-west motion of the Moon through the sky so, in essence, you’re photographing a static object. Among the many benefits will be that you can use longer exposures (2-3 seconds) at lower ISO’s (under 1000) to make properly exposed images that are sharp and detailed.

That’s not what I used during the September 2015 lunar eclipse. I set up my camera on a tripod, zoomed in to 500mm, opened the aperture as wide as it can be, jacked up the ISO to 3200 and started making exposures. Unfortunately, without the right equipment, all my photos from mid-eclipse – when the Moon looked its most devilish and eerie –  look just a tad soft. The photos are soft because, as Galileo Galilei would have observed, “It moves.”

Now, get out and shoot.

Bill Ferris | September 2015

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