In day time photography we often use neutral density filters and HDR to control the range of light in an image. Nighttime photography takes this in the opposite direction. There is so little light the photographer is challenged to capture as much light as possible. Long exposures are required, and often we crank up the ISO to make the sensor in our cameras more sensitive to light.
Noise
Noise is generated by the heat in the sensor in your camera. The longer the exposure the more heat is generated and at higher ISO’s the sensor is more sensitive to light but is also more sensitive to the noise in sensor. This is why long exposures combined with high ISO can make an image very noisy. At night against a very dark sky the noise generated by your camera shows up as multi-colored dots mixed in with the stars, as well as the “oatmeal” we are used to seeing in noisy images.
Try this some night; take a 30 second exposure at ISO 200 with a wide or normal lens (25mm to 50mm will work fine.) Then repeat this same exposure at ISO 400 and 800. Look at these images in on your computer as it is tough to judge night images using the LCD on the camera. You will get a feel for how high you can push your camera in ISO before a lot of noise shows up in the image. The goal is to keep noise to a level that does not require a lot of noise reduction in post process.
In post process we can try and remove the noise using noise reduction software (some cameras have this built in.) In night photography this can be a challenge. Noise reduction is a “destructive” process; the image is changed and data is removed. If we over noise reduce a night image this will remove many faint stars in the image, leaving us with an image of only the very brightest stars and this can ruin the look and feel of a night image. This can be acceptable however if your goal is to capture mountains, waterfalls or something other than stars in a night image.
Focus
One of the challenges taking night exposures is focus. The camera has little to focus on and most often you will use manual focus. In night sky photography it is important to have the stars to be in crisp focus. The temptation is to manually set the lens at infinity. Unfortunately nearly all lenses are not in focus at infinity when pushed against the infinity stop.
Here’s an easy way to focus on the stars. Manually turn the focus ring up against the infinity stop and take a 10 second exposure (tripod required here.) Magnify the image in the LCD as much as possible. The stars will likely look a bit soft. Manually change focus away from infinity just a very tiny bit and retake the image. The stars will likely look a bit sharper. Keep trying this – moving the focus manually in very small movements back and forth until the stars are as sharp as you can get them at high magnification in the LCD. This seems a bit cumbersome at first, but after you get the hang of it you will be able to do this in just a few minutes. Now the focus is set and you can concentrate on exposing the image.
The next step in noise
The two items discussed here are advanced techniques. I used them both extensively shooting astrophotography. Neither is hard to learn but they are both time intensive and require some planning and forethought, and, both are techniques that are beyond the scope of this article. If you are interested in learning the techniques please let me know.
Sometimes we can remove noise by taking a “dark frame.” This is an image taken with the shutter closed or a lens cap in place. The dark frame image will show only the noise being generated by the camera, and this can be digitally “subtracted” from the light image; this can be done with Photoshop. Some cameras have this feature built in and some do this type of noise reduction very well. Results vary however so test by comparing images with and without this feature turned on.
We can also reduce noise by taking multiple images and “stacking” them; this can be cone in Photoshop. What this does is to “average” the data in the digital image. In this way the noise in the dark sky parts of the image is reduced while the parts of the image we want to see are enhanced. I have created astrophotography images that have several hours of combined exposures!
Black sky
We often think the night sky should be black, but it never is. Even way out in a rural area with no city lights around there is some light in the atmosphere. The stars themselves make quite a bit of light that is refracted in the earth’s atmosphere, and the moon of course will do the same. In post process the temptation is to darken the sky to near black; many people use the black point slider in Photoshop to this. But this can make the sky look artificially dark. Try putting a tree or mountain in one of your night shots; the dark black of the tree or mountain will show you how much light is really in the night sky.
Include the earth in the sky
Wide angle night images are often more interesting if we include some earth bound object. Shooting the ocean, mountains and waterfalls at night (especially under a full moon) can produce spectacular results. Including a mountain range or tree in the image can add depth and interest.
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