White balance night photography

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Focus Blending and Exposure Blending

Often a single exposure white balance night photography of the Milky Way does not have enough depth of field or a long enough shutter speed for ground details. To get around this you have to shoot a longer exposure that is sometimes refocused for a closer object. I often lower my ISO or narrow my aperture by a stop or two and shoot very long exposures of the ground to combine later in Photoshop. The exposure difference varies on light pollution, ground details (woods, snow, grass, water, etc.), and moon light, but sometimes it is 3 or 4 stops more than the exposure of the stars, which can mean a shutter speed of several minutes if I also lowered my ISO for less noise or closed my aperture for more depth of field.

Exposure Stacking and 10x Rule

One method of reducing noise for the night sky is by shooting several [relatively] short exposures at very high ISOs and then averaging them to reduce the random noise. By masking out the ground details, and aligning the stars in each frame, you can achieve a much longer shutter speed without using a tracker and still have pin-point stars. A starting point for this is what I like to call the "10x Rule" to make it easy to remember: 10 exposures @ 10 seconds apiece & ISO 10,000. As with the 500 Rule, you'll want to experiment with this to get the best results for your camera sensor and focal length. If you use the NPF Rule in PhotoPills, choose Accurate instead of Default for the recommended shutter speed.

The more images you use, the more signal-to-noise ratio (SNR) you will get and thus less noise, but you will get diminishing returns--quadrupling the number of images will double the SNR. So the difference between 1, 4, and 16 frames is quite dramatic, but you have to get to 16 to be appreciably better than 4, and 64 to be better than 16, and so on. The sweet spot is somewhere around 9-12 images (without a tracker anyway): too many and you have difficulty aligning the stars, too few and you won't have enough data to average out the random noise.

The post-processing of all this is obviously far more complex than a single image, or blending two exposures of sky and ground. You should still take a separate long exposure of the foreground for this technique, and you can even average several long exposures of the ground if you have the time to take them, but you won't have to worry about aligning the ground shots of course. Fortunately there are two programs that have been made in the last few years that make the post-processing and alignment of the sky images much easier: for Windows and for Mac. There are many tutorials and videos on how to use them that are beyond the scope of this article.

Low Level Lighting

Light painting with a flashlight or bright Xenon light used to be a pretty popular technique for lighting a foreground and avoiding the need for long exposures at low ISO to reduce noise. It is disruptive to wildlife and other photographers however and the practice of light painting is generally frowned upon today, even banned in some places. A much better method with newer technologies like LED panels is called low level lighting, where you use a constant light source turned down low so it is barely perceptible to the human eye but quite dramatic in camera with 15-60 second exposures. If it's done well, you can capture sharp stars and a well-lit foreground in a single image without the need to blend two different exposures. You can still combine it with the above techniques for even better image quality however--stacking or tracking sky shots for less noise and averaging several ground exposures at the same time to reduce noise. Royce Bair and Wayne Pinkston have put together an excellent website and resource on this topic with recommendations for LED panels and other gear: 





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