If you’re fortunate enough to be outside after the sunset color has subsided, but before the world is enveloped in total darkness, you may find yourself basking in the rarified hues of the “blue hour*.” It’s called the blue hour because, wait for it…, the landscape is indeed bathed in blue. (And also because it just rolls off the tongue better than “the blue 20-minutes,” which would actually be more accurate.)
The greatest joy I get from photography is the connection it gives me to the natural world. Whether it’s lightning, a celestial event, a geological feature, some technical aspect of photography, the qualities of light, or whatever, whenever possible, in my blog posts I try to share something of the science behind my subject or its capture. The goal of this sharing isn’t entirely altruistic—it’s also a great excuse to dig deep into things that absolutely fascinate me. And as it turns out, the science behind the blue hour is especially fascinating.
The blue hour actually last less than one hour, with its duration decreasing with latitude because come and go faster the lower the latitude because the sun ascends and descends at a steeper angle. A lower latitude also speeds sunrise, sunset, and the other stages of twilight for the same reason. This means that in Hawaii, if I’m on location 30-minutes before sunrise, I can probably catch the entire show, whereas in Iceland, even if I’m there 90-minutes before sunrise (brrrr), I might miss something. In the mid-latitudes, 20-minutes is a reasonable rule-of-thumb blue hour duration to rely on.
The cool thing about blue hour color is that, unlike daylight blue sky and sunrise/sunset color, it’s not a function of scattered sunlight. (Read my Sunset Color Photo Tips article for more on this.) Instead, until the sun drops about 8 degrees below the horizon, its blue wavelengths are absorbed by ozone in the upper atmosphere. Though this absorption (Chappuis absorption, if you must know) happens when the sun is above the horizon too, its effects are completely overpowered by direct sunlight. But as direct and scattered sunlight fades with the sinking sun, about the time the sun is about 4 degrees below the horizon, the blue ozone sky enjoys its 20-minutes of fame. (In other words, the blue hour reigns when the sun is between 4 and 8 degrees below the horizon.) Because the cones in our eyes require a certain amount of light to register color, this twilight blue is fairly subtle, but still noticeable, to human vision. A camera, on the other hand, with its ability to increase its light sensitivity (ISO) and accumulate light over time (shutter speed), has no problem capturing the dominant blue cast.
Nature photographers love the blue hour, both for the otherworldly hues that aren’t possible any other time of day, and for the exquisitely soft, shadowless light that’s a joy to photograph. The key for getting the most out of blue hour photography is understanding that the camera sees the world differently than you. Some of the best blue hour photography happens after the color is nearly gone from our eyes, making it easy to pack up and go home. But as I said earlier, the camera can see color too faint for our eyes to register. The color camera might pick up can range from a purplish mix of the longest waves of sunlight and the beginning of the blue hour light, to the absolute deep-blue that soon fades to night.
The blue hour is also absolutely the best time to photograph the moon, which is why you see so many moon images in the Blue Hour gallery below. While fitting the dynamic range of a daylight-bright moon above virtually dark landscape is tricky, it’s definitely worth trying because the darker the sky, the more dramatic the moon will appear in your image. (Check out my articles on moon photography in the Photo Tips menu above.)
One of my favorite blue hour subjects is the ocean, because the low light means long exposures that turn the surf into a gauzy haze. In Iceland last January for a workshop, I was photographing the surf washing up onto Diamond Beach in the fading twilight. As the blue hour descended, I was drawn the way the distant snow-covered peaks (that’s Vestrahorn on the right) seemed to glow in the bluish twilight. Looking for something to put in the foreground, my eyes landed on a pair of (relatively) large icebergs just offshore and I positioned myself to align them with mountains.
Though I’d spent most of the evening shooting with my widest lenses, for this shot I switched to my Sony 100-400 GM to compress the distance between the mountains and nearby ice. Before dialing in my exposure settings, I studied the icebergs to make sure they were stationary enough to not blur in a long exposure, and decided it would be work if I could expose between waves. It took a few tries, but I finally managed this 15-second exposure.
* The blue hour of course happens in reverse before sunrise (from dark to light); for simplicity sake, I’ll just describe the evening half of the phenomenon.