I am developing CineZoom, part of CameraControl, a remote camera-direction system (macOS app controlling an iPhone capture engine). Its core requirement is a smooth, repeatable cinematic zoom.
On iPhone 17 Pro (iOS 26.5.1), using the physical Back Ultra Wide camera at 3840x2160, 60 fps, a single native ramp(toVideoZoomFactor:withRate:) from 1.7x to 2.1x over 5 seconds shows a visible stutter. I isolated it with a minimal diagnostic app (no networking) and analysed four tripod recordings frame by frame, in two locations with different lighting. The results are consistent across all four:
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Jitter below 2.0x: while the zoom factor is below 2.0, the image centre shifts about 2.8 px RMS between consecutive frames (peaks ~5 px), alternating almost every frame (26-28 Hz). Above 2.0x, during the same ramp at the same speed, it drops to 1.2-1.5 px.
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Sharpness step at 2.0x: centre-crop sharpness drops by 23-31% within a few frames exactly when the zoom factor crosses 2.0. There is no consistent effect at the reported secondaryNativeResolutionZoomFactors value (1.7838).
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The zoom command path itself is correct: the zoom measured from the image matches videoZoomFactor throughout, with no dropped or duplicated frames.
Questions:
- Is this expected behaviour for the Ultra Wide camera in this zoom range? Is the jitter related to video stabilization or to how the sensor crop is positioned in this readout mode?
- Is there a public API that reports or controls the zoom factor at which this transition happens, so an app can plan around it?
- What configuration do you recommend for a smooth continuous video zoom across 1.7x-2.1x on this camera (format, stabilization mode, constituent-camera switching)?
- Are there existing APIs that would let an app detect or compensate this (per-frame stabilization/crop metadata, consistent processing during zoom, or a reduced-processing video stream)?
I have a full technical report, per-frame measurements, charts and the original recordings, and I am happy to share them.
Thank you!