Long Exposures on Film

By Dylan McGrory — 2025

Astro on film is half physics problem, half ritual. This post documents my experience with Ektachrome E100: reciprocity behavior, exposure decisions, and a growing set of examples with meter notes under each frame.

Why Ektachrome for astro

E100 is fine-grained and surprisingly stable in color for long exposures, but it’s still slide film: your highlight placement and overall exposure matter more than they do on negative stocks.

E100 astro example 01 — “Example 01” — E100 • 4×5 • Lens: ___mm • f/___ • ___s • ISO 100 • Meter: (spot / incident / phone app) • Notes: baseline exposure

What I’m watching for

Density loss, color shifts, and how quickly stars/skyglow clip on transparency film compared to my memory of the scene.

Reciprocity: Kodak’s guidance vs my notes

According to Kodak, E100 does not require reciprocity correction for exposures under ~10 seconds. Past that, I’m building my own notes from repeated scenes and consistent metering.

E100 example 02 — “Example 02” — E100 • ___ format • f/___ • ___s • Meter: ___ • Notes: (color stable / slight shift / density drop)

E100 example 03 — “Example 03” — E100 • ___ format • f/___ • ___s • Meter: ___ • Notes: (compare to Example 02)

Reciprocity test log (draft)

Frame	Metered Time	Applied Time	Aperture	Result	Notes
01	10s	10s	f/5.6	OK	Baseline
02	30s	30s	f/5.6	OK	Minimal shift observed

Graph placeholder (export from Excel / Python and drop in as an image)

Final thoughts (for now)

The best part of shooting E100 for astro is that the final object exists as a transparency. Scans are useful, but holding a 4×5 slide of the stars is the real payoff.