Photographing the night sky

With a phone

Every recent phone has a mode for the dark, and each asks for the same thing, a phone held still, which is why a tripod, or a rock, makes the difference between a smear and a photograph. What the makers say about their own modes, from their own pages, and no more than they say:

iPhone. Night mode is on iPhone 11 and later and switches itself on when the camera sees a dark scene. Apple's page suggests a solid, secure surface or a tripod, and the slider above the shutter offers Max, which extends the capture and becomes a countdown while the phone works.

Pixel. Astrophotography lives inside Night Sight. With the phone set down on something steady and pointed at the sky, the shutter changes to Astrophotography by itself, and Google's page asks for stillness until the photo completes and suggests the three-second timer so the tap does not shake it. The same page says the stars want the camera at least 45 to 90 minutes after sunset, and that on the Pixel 4a (5G) and later the mode only works at 1x zoom or more.

Galaxy. The Expert RAW app carries an Astrophotography mode on the S22 and S23 series, with a Sky Guide that draws the constellations over the view, a shooting duration of the photographer's choosing, and a RAW file saved beside the JPEG. Samsung calls it a labs function behind the app's Special photo options, which only switch on at 12-megapixel resolution.

The limit is the lens. A phone's is a few millimeters across, so the Milky Way comes out soft and grainy beside what a camera makes of the same sky, and the phone's own processing decides how much to smooth and sharpen. It still shows the arch of the galaxy over the formations, which most people have never seen in a photograph of their own, and that is worth the tripod.

With a camera

Manual mode, and four settings. The aperture as wide as the lens goes, because the faint light is the whole subject. The ISO high, well past daylight habits, because the short exposure the rule below allows has to be made up somewhere; the grain that comes with it is cheaper than a blurred star, and how high a given camera can go before the grain wins is the one thing its owner learns by trying. The white balance fixed rather than automatic, so every frame of the night matches; a daylight setting reads warm and a tungsten one reads blue, the sky sits between them, and processing settles it. And RAW, because the shadows are where the picture is and JPEG throws them away.

Focus is the thing that ruins more frames than any setting. Autofocus finds nothing in the dark, and the infinity mark on a lens is rarely infinity. What works is manual focus on the brightest star in the live view, magnified as far as the screen allows, turned until the star is the smallest point it will make, and then left alone for the night; a strip of tape across the focus ring is the cheapest insurance there is.

How long the shutter can stay open

The sky moves. Past a certain exposure each star draws a short line instead of a point, and the wider the lens the longer that takes. The rule everyone quotes is the 500 rule: 500 divided by the focal length, in full-frame terms, gives the seconds. It is older than today's sensors, whose small pixels see the movement sooner, and it trails visibly on them. The rule that holds is the NPF rule, worked out by Frédéric Michaud for the Société Astronomique du Havre, which counts the aperture and the sensor's pixel pitch as well as the lens. In its simplified form it is 35 times the f-number, plus 30 times the pixel pitch in microns, divided by the focal length; worked against the cited article's own example, a 15mm f/2.4 on a Nikon Z 6, it lands within a quarter of a second of the figure the article's app gave. Three ordinary setups, both ways:

Computed from the formulas above. The pixel pitch used for each row is typical of that sensor class and not the figure for any one camera; a camera's own pitch is its sensor width divided by its pixels across, and the apps that plan night shoots carry a table of them.

One frame, many frames, or trails

A single frame at the NPF limit is the honest starting point and, on a dark night with a fast lens, often the finished photograph. Stacking goes further: a dozen or more identical frames, aligned on the stars and averaged by software made for the job, come out with the noise of one frame divided among them and the faint dust lanes of the core showing through. The cost is the foreground, which moves between frames as the sky is aligned and has to be taken separately and combined. Star trails are the opposite choice: the same frames, or an exposure of many minutes, combined so that every star keeps its line, and the lines wheel around the north star, which at the Badlands' latitude sits about 44 degrees above the northern horizon.

What to bring

A tripod that will not walk in the wind, because the Badlands has wind. A remote release or the two-second timer, so the shutter is not pressed by a hand. The widest and fastest lens on hand. Spare batteries, kept warm in an inside pocket, because a cold battery gives up long before an empty one and the nights at these sites turn cold even in August. Something warm strapped to the lens barrel, a hand warmer will do, because a lens that cools below the dew point fogs over in the middle of a sequence. A red light rather than a white one, for the reason the dark skies page gives, and gloves that leave the dials workable.

The formations and the core

What makes a Badlands photograph a Badlands photograph is the ground. The Milky Way core sits low in the southern sky and never climbs higher than about 17 degrees at this latitude, roughly a fist and a half at arm's length, so a formation to the south puts the two in the same frame without a wide lens straining for it. The pullouts that look south are the ones worth the drive, and the first-hand notes on the Badlands National Park page say which they are. A thin moon lights the formations and leaves the core bright; a fat one lights the formations and takes the core away, which the forecast's moon column shows night by night, and the moon calendar shows for the months ahead.

Lights, and the other people out there

A flashlight played across a formation makes one striking photograph and appears in every exposure pointed its way from the pullout. It also resets the dark adaptation of everyone there, twenty minutes of it each time, on a night they drove a long way for. Moonlight and a longer exposure do the same job for the foreground at no cost to anyone else, and a headlamp pointed at the ground on its red setting gets a person back to the car without lighting anyone's frame.

Afterwards

A RAW frame from a dark sky looks flat and dim on the back of the camera, and that is normal. The picture comes out in processing: the white balance settled, the shadows lifted, the noise reduced with a light hand, and the color left alone, because the saturated purple sky in so many night photographs was never in the sky. Free software does all of it, and a phone edits its own frames well enough for most of it.