After driving out to our favorite hot spring in the Eastern Sierra, I was glad I'd brought my camera. The scene had a lot of potential for nighttime photography; an illuminated foreground from the moon, steam from the hot springs, no wind, and semi-willing subjects.

In general, when shooting nighttime photography, the lens should be as “fast” as possible (widest aperture), letting in as much light as possible before getting star trails. When the moon is present and illuminating the foreground and background as much as it does, the lens can be “stopped down” (reduce the f-stop number) since there is so much light.

When a lens is stopped down, the depth of field is increased. This means more of the image will be in focus. Not only will objects in the front of the frame be in focus, but also objects in the “back”. The more a lens is stopped down, the more the “back” of the image comes into focus. So, stopping down was a natural choice for this shot given the ample amount of available light. My lens’ widest aperture was f/1.4, while my final setting was f/2. This effectively allowed half the light to be collected compared to when the lens aperture is at its widest. In hindsight, I probably could have stopped down to f/4, bringing even more of the image into focus and reducing the light hitting the sensor by a factor of four.

In addition to stopping down the lens, I was able to take a relatively short exposure for it being night time. After a couple tests, I settled on a two second exposure. This gave just the right balance of accentuating the steam while being short enough for my subjects to sit still.

Another beneficial consequence when shooting under the moonlight is there isn’t as much of a need to increase the gain of the camera’s sensor — aka increase its ISO. This means you can shoot at a very low ISO, retaining precision without losing too much dynamic range. Precision can be thought of as the noise or graininess of your photo, while dynamic range is simply the ratio of the brightest and dimmest parts of the frame. Having a high dynamic range is often desirable in order to retain the detail in both the highlights and shadows of your image. So, being able to shoot at a low ISO allowed me to capture a photo that wasn’t very noisy and retained an acceptable amount of dynamic range. Normally at night we shoot anywhere between ISO 1600 and ISO 6400, but I was able to shoot this shot at ISO 800.

The most difficult part of capturing this shot was convincing twenty-something hot springers to sit still for the exposure. It only cost me a few beers. My final settings were ISO @ 800, f/2, and a 2 second exposure, shot on a Canon 6D with an H-alpha modification using a Sigma ART f/1.4 35mm lens.

Finally, this image didn’t require much editing. The contrast was increased to make the steam “pop”, the color balance was also adjusted to a more blue tone, giving the picture more of a night time feel. If you put time and thought into capturing a shot, then you won’t have to do much editing and you’ll have more time to enjoy the cosmos.

Samuel Kahn is an astrophotographer living in the Eastern Sierra. He enjoys capturing landscape, panoramic, and deep space astrophotography. (IG: @alpineastro)