Categories: Uncategorized

Should I Use a Light Pollution Filter For Night Photography?

Well that depends… are you primarily practicing deep space astrophotography? That’s what they’re designed for: remove terrestrial sources of light so you’re only working with the deep space light.

The vast majority of places we shoot at night with photographers are not truly dark. Even halfway up Death Valley National Park, there’s light from Las Vegas in the sky 110 miles away. In the Eastern Sierra, if you shoot at Mono Lake toward the Sierra Nevada, there’s significant, and growing, light form the Central Valley about 75 miles away. Even up at the Black Rock desert we’ve seen major light pollution, which turned out to be Reno 90 miles away. The “International Dark Sky Park” of Death Valley often also has light domes from Las Vegas and from the greater Los Angeles metropolitan area 100-160 miles away. The dark skies of the Colorado Plateau in Utah? Aren’t all that dark. There are light domes all over from towns nearby.

When you finally get to a truly dark place, like Massacre Rim near the northwest corner of Nevada, your night photography results go to shit. Since everything is much darker, your “signal”, the landscape, is down in the sensor noise. Shadows that you would have been able to recover elsewhere are basically black. It turns out that you’ve been given help all these years by the light pollution. Take that subtle light on the landscape away, and the lighting assist on your landscape goes away. Your night photos can go to shit everywhere!

What are narrow band filters like that good for? Deep space astrophotography, conducted near the light pollution of a city.

The sales pitch says that you can get true colors from the sky. There’s no such thing as true color with human perception. We see white, and colors, differently based on ambient lighting. That’s why we have the concept of white balance. At night, the white balance of the sky can change depending upon which direction your camera is pointing (and how much light pollution is in the sky in that direction). So the logical solution is to always use automatic white balance. Let the camera take a measurement, and provide a vote for the best white balance. In a relatively dark place, the camera will often shift color towards blue if there’s a lot of light pollution. Can it be off? Sure, especially when there’s too much light from aurora or similar phenomena. So it helps to understand what colors do often appear in the sky. See some green at mid latitudes, especially in the lower 30% or so of the sky? It is airglow, and as a chemiluminescence of oxygen, it should usually be “aurora green”. Check out the videos taken from the International Space Station which clearly show the thin layer of green over mid latitudes. Sometimes there’s an orange-ish tan color (I haven’t seen red as an airglow color in those ISS videos, red is most often aurora borealis to the north or a red terrestrial source like brake lights reflecting off of low clouds).

What color is a moonless dark sky? Not blue! Blue skies come from scattered sunlight, including sunlight bounced off the moon, if it’s in the sky. But if there’s no sun, no moon, there’s no light to scatter to blue. Similarly, purple is a mixture of blue and red… scattered blue light with remaining sunset light. If you’re a few minutes into twilight you can get a subtle purplish tint that tells you it’s not really night. Not a desirable thing in a “night” shot, so conduct your night photography outside of astronomical twilight time to avoid it. So if you see “dark sky” “night” images with blue or purple, they’re about as realistic as a daylight shot with a bright pink sky. That’s not a knock against artists who use “creative” colors like Andy Warhol, or photographers who emulate his use of creative colors. Consider deep space objects like nebulae, scientists use false color all the time to represent different elements/gasses. So there may be perfectly good reasons for falsifying colors, or entirely whimsical reasons. Picasso made sculptures out of discarded objects. One man’s trash is another man’s treasure.

When I have to think about it, I know the picture is wrong. And sizing is a form of thinking and coloring is too. My instinct about painting says, ‘If you don’t think about it, it’s right’. As soon as you have to decide and choose, it’s wrong. And the more you decide about, the more wrong it gets. Some people, they paint abstract, so they sit there thinking about it because their thinking makes them feel they’re doing something. But my thinking never makes me feel I’m doing anything.
– Andy Warhol, The Philosophy of Andy Warhol (From A to B and Back Again), 1975

“There are no rules for good photographs, there are only good photographs.” – Ansel Adams
Have fun. By the same token, no one is obligated to like your more creative works, so if you choose that path, develop a thick skin.

What is the true color of deep space objects in the night sky? Most are emitting light. For ones that aren’t, perhaps white balance should be adjusted for the light source nearby, like a blue dwarf star or red giant nearby. For Milky Way images, apparently astronomers prefer a yellowish off-white, as if all of the stars were like our sun. So when submitting a photo to the Astronomy Photographer of the Year competition, judged by astronomers, I make sure that there’s a slight warm bias to my submission if it’s a dark sky night shot with no moon or aurora influence.

Over time you’ll probably notice that in relatively dark areas auto white balance results typically fall in the 3800 to 4000 range, perhaps in the 3600s if you have a big light dome like Vegas or Los Angeles (or both) in your composition. The amount below 4000 seems to be proportional to the amount of light pollution. So we always use Auto White Balance, and let the camera measure what it thinks should be the average white balance for the scene. In post-processing you can see what the camera got with auto white balance, maybe consider trying “auto” in Lightroom as well, then you can adjust the color to whatever looks natural to you. White balance of 3900 to 4000 with a green/purple tint of 10 is a decent place to start for more night photos in a reasonably dark place, like when we’re bringing photographers out to shoot on a dark night in the ghost town of Bodie, California.

If you commonly shot with a filter removing warm light, no doubt all of the prior white balance experience and practices would be counterproductive. Your results would be far too blue. There are huge groups on Facebook that post almost exclusively Milky Way images in bizarrely blue skies. I tried posting Images with green airglow in them to two of those groups recently, and I never saw any response, they were probably rejected. We also don’t see any need for sky trackers (for wide field night landscape shots), and those groups seem to have some weird cultish fads like using sky tracking mounts for everything (when you could skip the sky tracker leveling and alignment setup cost and simply stack multiple photos in Sequator or Starry Landscape Stacker instead). One of the images I post recently was a Milky Way panorama at Mono Lake with green airglow and some faint red aurora borealis to the north. Noting that it was stitched together from single exposures probably antagonized the moderators. Meanwhile, the USFS bought a 5′ metal print that’s now hanging in their Visitor Center at Mono Lake. (I’d rather sell big prints than please misguided petty dictators on Facebook.)

Since we don’t see color at night, intense airglow in the sky painting the landscape green isn’t intuitive to most people (who have never heard of airglow), so you can reduce objections and make a “crowd pleasing” version that looks more natural to yourself as well by selecting the landscape in a mask, and moving the green/purple tint slider more towards purple.

At the end of the day photography can be an artistic pursuit, so do what you want and stick to it. Nothing I say is the “right” way to do anything, it’s just what I choose to do based on my understanding of light/physics and nature, and decades of experience. But regarding those filters, you probably don’t want to cut light reflected off of your landscapes. That light is what enables you to see and photograph the landscapes you’re in, with less noise.

We cover these topics and more in our night photography workshops in the Eastern Sierra region of California, where we shoot in Bodie, the Mono Lake Area, and up in the High Sierra. We cover lighting, exposure techniques, and post-processing tools and techniques to give participants a solid grasp of the science and skills useful for producing outstanding night images with minimal time and effort.