Earth rotation limits in-body image stabilization to 6.3 stops (2020)

Original Article ↗ Hacker News Discussion ↗

Camera enthusiasts and engineers examine a claim that Earth’s rotation fundamentally limits in-body image stabilization (IBIS) to about 6.3 “stops” of benefit, because modern gyroscopes are sensitive enough to detect planetary rotation and may try to cancel it like normal hand shake. They unpack what a stop means in terms of shutter speed and light, why stabilization can’t fix subject motion, and how inertial sensors, reference frames, and even navigation concepts like gyro‑compassing relate to the problem. The conversation also explores potential workarounds—using GPS, software filtering, optical flow, or specialized mounts for astrophotography—and questions how some camera makers advertise 8+ stops without obviously violating these physical constraints.

What “stops” mean and how CIPA ratings work

  • “Stops” here are doublings/halvings of light at the sensor, via shutter time or aperture.
  • “Stops of stabilization” are defined via a CIPA test: compare the slowest shutter giving an “acceptably sharp” image to the old 1/focal‑length rule.
  • Several commenters note that real‑world performance beyond ~4 stops often falls short of CIPA numbers.
  • Discussion emphasizes thinking in shutter speed (e.g., 1/25 vs 1/2000) rather than equating stops directly to aperture.

Why Earth’s rotation limits IBIS

  • Modern IBIS uses gyroscopes that can sense Earth’s rotation.
  • A gyro maintains orientation relative to inertial space, not Earth’s surface, so it “sees” Earth rotating under it (Foucault pendulum analogy).
  • For terrestrial subjects, camera and subject rotate together with Earth; trying to cancel that rotation misaligns camera and subject.
  • Earth’s revolution around the Sun and galactic motion also exist, but their angular rates are much smaller, so they’re not the practical limit here.

Proposed technical workarounds

  • Ideas raised:
    • Use GPS + compass + accelerometers + gyros (9‑DoF fusion) to estimate pointing and latitude and subtract Earth’s rotation.
    • Infer Earth rotation from gyro data alone via gyro‑compassing, or by filtering specifically at the known rotation rate.
    • Calibrate by having the camera sit still briefly and integrating gyro output.
    • Software filtering: high‑pass vs low‑pass treatment of gyro signals to separate shake from Earth rotation.
  • Others argue GPS is unnecessary or even useless for orientation; inertial sensors alone suffice.
  • Several note cost/complexity: aircraft/submarine‑grade INS is expensive; consumer MEMS gyros are noisy and drift.

Astrophotography and mounts

  • For stars, compensating Earth’s rotation is desirable; IBIS that “tracks the sky” is beneficial.
  • Equatorial mounts and tracking tripods already solve this for long exposures, sometimes with secondary guide cameras.
  • Some camera systems use sensor‑shift for star tracking when GPS is available.

Debate over limits and products

  • Some suspect regulatory/export issues (e.g., ITAR, missile guidance concerns) as a practical cap on gyro quality in consumer cameras.
  • Others counter that consumer IMUs have surpassed older aerospace mechanical systems and cost is the real driver.
  • Commenters note manufacturers claiming 8+ stops (e.g., certain Nikon and OM System bodies), questioning how this reconciles with a ~6.3‑stop theoretical limit and suggesting test‑optimization “dieselgate”‑style scenarios or narrow conditions.