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Northern Lights - Washington State

Why your camera sees the night sky better than your eyes

Have you ever photographed the northern lights with your camera and wondered why the colors are so much more vivid on the camera? Or perhaps you’ve looked at a star-filled sky where the stars all appear white to your eyes, yet your camera sees various star colors? All those colors are there, but you don’t perceive them all because of the biology of the human eye.

The human eye is remarkable in adapting to various light conditions. It is often superior to camera sensors because it can see scenes with a very large dynamic range, where a camera needs multiple exposures to capture those considerable differences in brightness. However, under dark skies, cameras can see colors that the human eye struggles to see, and they can gather more light by using longer exposures, leading to brighter and more vivid pictures than you saw with your eyes.

There are two types of photosensitive cells on your retina: cones and rods. Under well-lit conditions, your eyes use cones to process light, known as photopic vision. The cones are responsible for color vision but are not responsive to low light. In very dark environments, rod cells dominate what you see, known as scotopic vision. Colors are barely perceptible under scotopic vision, leading to almost black-and-white, desaturated vision. Mesopic vision is when your vision is a mix of rod and cone cells during periods of intermediate lighting.

Your eyes adapt to a low-light environment by transitioning sensitivity from the cones to the rods, which takes 20–30 minutes to complete. As your eyes transition during mesopic vision, they experience the Purkinje effect, where reds appear darker than other colors. Using red light at night activates the cones for photopic vision yet doesn’t deactivate the rods, which are insensitive to red light, helping you see at night while maintaining your eyes’ dark adaptation.

Viewing faint aurora borealis is the most extreme example of how your camera sees things differently than your eyes. Very faint auroras will look like a barely perceptible milky haze in the night sky to your eyes. Snapping a photo with your phone or camera will suddenly reveal the auroras’ characteristic lime green, even when very dim. The green color is there, but your eyes can’t see it because it is too faint. As the intensity of auroras increases, so does color perception in your eyes, and you will see more and more colors, similar to the camera.

Faint aurora borealis
Even faint auroras are a vibrant green color when photographed.

Geography also plays a role when viewing northern lights. Light pollution reduces the contrast you and your camera see, so viewing them from dark sky areas is beneficial. In addition, auroras at higher latitudes, like in the auroral zone in the far north, display higher intensities than at lower latitudes, leading to stronger colors for your eyes.

The Milky Way and stars on a moonless night also show the same effect. The stars are primarily pinpoints of white light to your eyes, but to the camera, they are a fantastic array of colors. Star trail photos show this effect quite dramatically.

Star trails over Mt. Rainier
Star trails show a wide variety of star colors in the night sky.

Your camera also has another advantage over your eyes at night. With a camera sensor, you can set a very slow shutter speed to gather the available light over a more extended period. For example, you can keep your shutter open for 20 seconds, 5 minutes, or even longer. For example, pointing a camera at a dark scene using a slow shutter speed of 10 minutes would reveal much detail in dark areas. Our eyes don’t have a shutter speed, but they have a timeframe needed to capture and process the light they see. If the darkness exceeds the low-light sensitivity of our eyes, staring at the same dark scene for 10 minutes doesn’t reveal any of the details that the camera saw.

Milky Way over Mount St. HelensMilky Way over Mount St. Helens
With a 10-second shutter speed, details in the land are black. With a shutter speed of 6 minutes, the camera gathers more light and reveals the dark landscape’s details.

There is a night sky phenomenon where the human eye is superior to camera sensors: the moon. The moon is dramatically brighter than the surrounding landscape at night. With your eyes, you will see crater details on the moon, but you can also perceive details in the dark areas of the land because your eyes have a better dynamic range than the camera. If you set your camera exposure to capture the details of moon craters, the surrounding landscape will be pitch black. If you set your camera’s exposure for dark landscapes, the moon will be overexposed, showing only a bright white blob of light lacking detail. A camera can only capture scenes like these by combining multiple exposures.

Full moon rises over a mountain
With a camera exposure set for the moon’s brightness, the surrounding details in the land are pure black.

One of the wonders of night sky photography is revealing all of these details our eyes don’t always perceive. Cameras capture a more vivid and colorful version of the night sky than our eyes, and understanding the human eye’s biological limitations explains why your camera sees things differently.

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