Night Sky Photography: How Far Away Are the Stars?

The nearest star outside our solar system is Proxima Centauri, at about 4.24 light years or roughly 40 trillion kilometres. The stars of the Milky Way core are 25,000 light years away. For photographers, this immense distance has one practical consequence: stars are, for all photographic purposes, at infinity. And that changes everything about how you focus and think about depth of field in night sky photography.

What Infinite Distance Means for Focus

When a subject is at infinity, all parallel light rays entering the lens converge at exactly the focal plane (the sensor). No depth of field calculation is needed because there is no near limit to worry about. Your focus ring set to infinity captures stars, the moon, and distant mountains equally sharply.

The practical implication: focusing for night sky photography is a solved problem technically, but often a frustrating one in practice. The difficulty is not the concept but the execution, finding true infinity focus in the dark with a lens and camera system that may not cooperate cleanly.

Why "Infinity" on the Focus Ring Is Often Wrong

Most modern AF lenses can focus past the optical infinity point. They do this intentionally, to compensate for thermal expansion (the lens elements shift slightly as temperature changes) and to give the AF system room to confirm focus without hunting past its limits. The result is that the infinity symbol on the focus ring does not reliably correspond to true optical infinity focus.

This is a critical practical problem for night sky shooters. A lens set to the infinity mark on the ring may be slightly past true infinity, producing softly unfocused stars. In darkness, on a small LCD, this can be impossible to detect without magnifying the live view significantly.

Methods for Achieving True Infinity Focus

Live View Magnification on a Bright Star

The most reliable method. Point the camera at a bright star (Sirius, Vega, or any first-magnitude star). Activate live view. Zoom in to 10x magnification on that star. Slowly rotate the focus ring until the star is the smallest, sharpest point it can be. Lock or tape the ring there. This is your true infinity position for this lens, at this temperature.

Focus on a Distant Daytime Object

Before your night session, focus on the most distant object visible from your location during daylight, a distant mountain, a tower on the horizon, or anything clearly beyond 1km. At these distances the subject is effectively at infinity for any normal lens. Use AF to lock focus, then switch to MF to hold it. Tape the ring if needed.

Hyperfocal Distance for Night

For images that include a foreground element (a rock, a tree, a person under the stars), pure infinity focus will leave the foreground soft. Hyperfocal distance gives you the focus point at which everything from half that distance to infinity is acceptably sharp.

For a wide-angle Milky Way shot with foreground, focus at the hyperfocal distance rather than infinity. Stars will be acceptably sharp (they are at infinity, within the far limit of your DOF), and foreground elements beyond half the hyperfocal distance will also be acceptably sharp.

The 500 Rule and Star Trails

Stars are not stationary in long exposures. The Earth's rotation causes stars to trail across the sensor over time. Whether this trail is visible as a streak depends on exposure length, focal length, and whether you consider it acceptable for your image.

The 500 rule gives an approximate maximum exposure. In practice, at the edges of the frame where stars move faster (the corners of a wide-angle shot), trailing begins sooner. For the sharpest possible stars at any focal length, an equatorial tracking mount eliminates trailing entirely by rotating the camera to match the Earth's rotation.

Star Distance by Object Type

For a subject distance calculator, the distances to common night sky objects are worth knowing, not because they change any photographic technique, but because they illuminate why infinity focus is so absolute:

The Moon: Technically Calculable

The Moon is the one night sky object where the subject distance calculator could theoretically apply, because the Moon has a known physical size (3,474 km diameter) and a known distance (averaging 384,400 km). A full moon subtends about 0.5° in the sky.

If you photograph the Moon and want to know what focal length you need to fill a certain percentage of the frame, you can reverse-calculate it. A 500mm lens on full frame renders the moon at approximately 4.8mm on sensor, which is about 20% of frame height. A 1000mm equivalent is needed to fill roughly 40% of the frame. This is why dedicated lunar photographers use 1000mm or longer.

Depth of Field in Night Sky Shots: A Non-Issue

Because all night sky objects are at infinity, depth of field in the sense of near and far focus limits does not apply to pure sky shots. The entire sky, from horizon to zenith, is at the same photographic distance (infinity), and any lens focused at infinity captures all of it equally sharply.

DOF becomes relevant only when you include a foreground element. In that case you have two subjects at very different distances, and the challenge is finding a focus distance that keeps both acceptably sharp. The solutions are:

• Hyperfocal focus: For wide lenses at f/2-f/2.8, the hyperfocal distance is often close enough to bring the foreground into acceptable sharpness while keeping stars sharp.
• Focus stacking: Two exposures, one focused on the foreground, one at infinity for the sky, blended in post. This is the highest quality approach and avoids the sharpness compromises of hyperfocal.
• Wide aperture compromise: At f/1.4 or f/1.8, hyperfocal distance is much further and foreground sharpness is harder to achieve. Many astrophotographers shoot foreground and sky as separate exposures regardless.

Night Sky Settings at a Glance

Final Thoughts

Stars are so far away that the concept of subject distance becomes almost philosophical rather than practical. They are not just far, they are incomprehensibly far, separated from us by distances that dwarf any terrestrial measurement. For photography, this resolves cleanly to one setting: infinity focus.

The interesting photographic challenge in night sky work is everything else. How long can you expose before stars trail? How do you include a foreground and keep both sharp? How do you find true infinity in the dark? The distance to the stars is fixed and vast. What you do with the foreground, the exposure time, and the focus technique is where the craft lives.