ARRI Alexa 35 — Enhanced Sensitivity (ES) Mode Kills Sync Output¶

Camera Triggering Resolved All rigs BOTH

Summary¶

The ARRI Alexa 35 stops outputting a sync signal when Enhanced Sensitivity (ES) mode is active. ES mode is enabled when the camera is set to an ISO above approximately 1250 — the camera switches to a different sensor readout path that does not support external sync. When this happens, Flair's sync-based triggering fails silently: the camera records but does not lock to the robot's timing. The fix is to disable ES mode, or to genlock the camera externally if high ISO is required.

Symptoms¶

Alexa 35 sync was working, then stops after a camera setting change
Camera appears to be triggering normally (the TRIG light on the MRMC trigger box may still pulse)
However, sync is not phase-locked — the camera is running at its own internal timing
Issue appears when shooting above ~ISO 1250 (the ES threshold)
The problem is invisible unless you check sync status directly or notice frame-rate drift

Root Cause¶

The Alexa 35 has two ISO modes:

Mode	ISO range	External sync
Normal sensitivity	Up to ~ISO 1250	Supported
Enhanced Sensitivity (ES)	Above ~ISO 1250	Not supported — sync output is disabled

When ES mode is active, the camera uses a different internal signal processing path that cannot accept or output a sync reference signal. The MRMC trigger box has no way to detect this — it continues sending pulses, but the camera ignores them for synchronisation purposes.

"Did Arri ever address dropping sync out when the camera goes into Enhanced-ISO mode on Alexa 35?" — James G (2025-02-21)

"Make sure 'enhanced sensitivity' is off." — MRMC Support (2025-06-28)

"Check the camera to not be in ES mode on ISO." — Cosmin Fericean (2025-06-28)

Resolution¶

Option 1 — Disable Enhanced Sensitivity (preferred)¶

On the Alexa 35, reduce ISO to 1250 or below.
The camera will exit ES mode automatically.
External sync will be restored.

Confirm sync is working by checking: - MRMC trigger box TRIG LED pulses in sync with the move - The camera frame count increments in step with robot frame counter

Option 2 — Use genlock instead of direct sync¶

If the shot requires ES mode (high ISO is essential):

Use an external genlock generator instead of the MRMC sync box.
Connect the genlock output to the Alexa 35's reference input.
Adjust the robot trigger timing to match the genlock phase.

"I always bring a genlock box in case. So far I haven't had a camera body that can neither sync nor genlock." — Riley Morgan (2025-04-25)

Confirming ES mode status¶

On the Alexa 35 menu:

System → Sensor → Enhanced Sensitivity shows whether ES mode is active
The camera display may also show "ES" as an indicator when in ES mode

Evidence from Community Chat¶

"Did Arri ever address dropping sync out when the camera goes into Enhanced-ISO mode on Alexa 35?" — James G (2025-02-21)

"Make sure 'enhanced sensitivity' is off." — MRMC Support (2025-06-28)

"Check the camera to not be in ES mode on ISO." — Cosmin Fericean (2025-06-28)

"I always bring a genlock box in case. So far I haven't had a camera body that can neither sync nor genlock, but with the Raptors and Komodos I frequently get cameras that won't work for one or the other." — Riley Morgan (2025-04-25)

WhatsApp Excerpts¶

2025-02-21 03:44 - Gordon Eschke: Should be fine with your standard Alexa adapter on universal sync box
2025-02-21 04:00 - ~ James G: Did Arri ever address dropping sync out when the camera goes into Enhanced-ISO mode on Alexa35?
2025-02-21 04:03 - Gordon Eschke: https://talk.mrmoco.com/t/alexa-35-sync/1094/3
2025-02-21 04:07 - Timothy Heys Cerchio: ARRI didn't ... but this cool group discussed it 👆🏼 And Gordon reposted to the Forum 👍🏼😎