Why the Dark Skies?

Posted October 6th, 2009 by   |  Photography, Travel  |  Permalink
Notice how the sky changes in brightness from the left side to the right side. Read on for an explanation of why this happens. Nikon D700, 12-24mm, from moving Alaska Railroad train.

Notice how the sky changes in brightness from the left side to the right side. Read on for an explanation of why this happens. Nikon D700, 12-24mm, from moving Alaska Railroad train.

I received a great email question yesterday from a newsletter reader, Jack. He says, “Mike, in my landscape shots, sometimes one side of the sky is darker than the other side.  This can happen whether I’m using a polarizer or not.  Do you know what causes that?”

Here’s my answer:

This is a common issue with wide angle lenses when you are photographing blue skies. These lenses take in so much of the sky, that one side is bright and the other is dark based on that areas’s proximity to the sun. You’ll notice that the area next to the sun is brighter while the area of sky opposite the sun is much darker. This is a natural atmospheric phenomenon and is more pronounced at sunrise/sunset than during mid-day. If you use a polarizer, the effect can be much more pronounced.

I see this effect all the time in my photography. For example, look at this photograph above, taken just outside of Anchorage Alaska. Our Nikonians photo group was riding the Alaska Railroad on our way to Denali National Park a few weeks ago when we saw this beautiful lake with mountains behind it. I wanted to capture the expansiveness, so I used my 12-24mm and composed photograph so that I could include the foreground, lake, mountains, blue sky and white puffy clouds. The variation in sky brightness from the left to the right is definitely visible. The sun was rising on the right, so that side of the sky is brighter.

The next time you are taking photographs outside on a sunny day, do this test:
1. Set your camera for spot meter
2. Point it at the sky in the right side of the frame and note the exposure value.
3. Then, point the spot meter at the sky in the left side of the frame and note the exposure value.

You’ll generally see a one, two or three stop difference depending on time of day.

One more thought on this. You’ll also see this brightness shift vertically, from the horizon to the azimuth. In fact, Singh-Ray has made a filter to combat this vertical brightness difference called a reverse neutral density filter. http://www.singh-ray.com/reversegrads.html



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