Astrophotography with the Zeiss Batis 18mm f/2.8
Astrophotography has two technical ceilings: lens coma at wide apertures, and star trailing from the Earth’s rotation. The Zeiss Batis 18mm f/2.8 addresses the first better than most lenses at this focal length and price point. The second is addressed by the 500 rule — the maximum exposure length in seconds before stars begin to trail, calculated as 500 divided by the full-frame equivalent focal length. At 18mm, that gives approximately 27 seconds.
Coma is a lens aberration that renders point sources of light — stars — as comet-shaped smears rather than points, particularly toward the frame edges. It is the most visible and most disqualifying defect in a Milky Way or star field image. The Batis 18mm f/2.8 exhibits minimal coma at f/2.8. Not zero — at extreme corners, particularly on the Sony A7R V’s high-resolution sensor, some smearing is visible. At f/4, it is essentially gone. The 18mm field of view at f/2.8 provides enough light gathering for a 20-second exposure at ISO 3200 to produce a usable Milky Way core image on a full-frame sensor.
The practical workflow: manual focus set to hyperfocal or slightly inside infinity (use the OLED distance display on the Batis barrel, which is a feature unique to this lens). ISO 3200 or 6400 depending on the darkness of the sky. f/2.8 for the most light, or f/4 if coma at the edges is unacceptable for the specific frame. 20-25 second exposures. The Sony’s in-body stabilization should be disabled — IBIS calibrated for static shooting can introduce small movements during long exposures when the camera is on a tripod, as the system searches for movement that does not exist.
Shoot a sequence of ten or more identical exposures and stack them in Sequator (free, Windows) or Starry Landscape Stacker (Mac) to average down noise without blending subject movement. The OLED distance scale on the Batis is worth the cost of the lens in the dark.