Tuesday, March 27, 2012
Mirror slapping the moon around until it disappears altogether
Usually when I take a photograph of the moon, it's a sudden impulse shot with what I have at hand. Often I do everything wrong, shooting telephoto handheld at a slow shutter speed at an insanely high ISO. Sometimes it sort of works, in a blurry, expressive sort of way.
Last night, the moon and Venus (cropped out here) were close together, and the shadowed part of the moon was illuminated beautifully by earthglow. I thought I'd do it up right this time -- but everything went wrong.
I set the D90 up on a tripod with an 80-200 zoom. I turned the ISO way, way down. I turned off image stabilization, the way you're supposed to on a tripod. Turned on Long Exposure Noise Reduction, because I was tired of noisy night skies. I wanted a rich, deep black. Well, I got the black background. That's about it.
The multiple ghost moons are the result of "DSLR mirror slap." That's when the mirror in a DSLR swings out of the way of the film or sensor at the moment of exposure. It comes to rest against the pentaprism housing with a loud slapping sound. The resulting vibration during a time exposure -- especially with a bright subject against a dark background -- creates ghost images like this. Or in softer lighting, just a general softness. That's why most serious SLRs used to have a way to lock up the mirror after you composed your picture. Now many DSLRs don't, including my D90. So I experimented with different shutter speeds to no avail.
Then I remembered that the D90 has a workaround, buried deep in the display menu -- Exposure Delay Mode. When the shutter is pressed, this holds the mirror up for a couple seconds, letting the vibrations settle down, before taking the picture. I tried it. It works. But I noticed the moon still looked pretty soft, and Venus looked more like a micro star trail than a point of light. I was using such a long exposure -- 20 sec., don't ask why -- that the moon and Venus were visibly moving across the frame, visibly streaking and blurring.
Finally figured that out. I reset ISO, shutter speed, and aperture. I was set to take a technically flawless tripod shot of the moon and Venus. Looked up to frame the shot, but there was no moon. Clouds had moved in for the night.