What is multi-cam capture on iPhone? (AVCaptureMultiCamSession explained)

Every app that genuinely shoots the front and back iPhone cameras at the same time is built on one Apple API: AVCaptureMultiCamSession. Knowing roughly how it works explains which apps are real, which are faking it, and why certain limits exist everywhere.

The short version

Introduced in iOS 13 (2019), AVCaptureMultiCamSession lets an app run multiple cameras in one capture session — both are exposed together, frame-synchronized by the system. It requires the A12 Bionic chip or newer: iPhone XS, XS Max, XR and everything after. (Full device list.)

Apps like Selfie and MixCam use it directly. Apps without it — most famously BeReal — simulate dual capture by shooting one camera, then the other, with a visible delay.

How to spot true simultaneous capture

• The reaction test: film a fast event dual. If your front-camera face reacts to the moment, capture is simultaneous; if it reacts a beat late (to the app), it's sequential.
• Video is the tell: sequential apps can't do dual video at all — recording both streams at once requires the multi-cam session.

Why the same limits appear in every dual app

The session has a hardware budget (bandwidth, power, the image signal processor), which produces platform-wide constraints no app can dodge:

• No 0.5x ultra-wide in dual mode. The back camera's virtual multi-lens device can't combine with the front camera; both sides must use their standard wide lens. (The full explanation.)
• Thermals. Two full camera pipelines run hot on long recordings.
• Resolution ceilings. Dual video runs below the single-camera maximum.

When two dual-camera apps differ on these points, one of them is marketing.

What apps build on top

The API delivers two synchronized streams; everything else is the app's work: composing the picture-in-picture, applying filters per-stream in real time (Selfie renders its vintage presets on both streams live), writing combined video with audio, and the layout controls. That layer — not the capture itself — is where dual-camera apps actually differ. (How they compare.)

Common questions

Does Apple's own camera use it? Yes — iPhone 16's native Dual Capture video mode is built on the same engine, in a fixed configuration. (What it can and can't do.)

Can it run the two back cameras together instead? The API supports several combinations (e.g. wide + ultra-wide back) — that's how some pro-video apps record two focal lengths at once. Front + back is the combination dual-capture apps use.

Does multi-cam drain more battery? Yes, roughly double the camera pipeline. For photos it's negligible; for long video sessions it's the price of two cameras genuinely running.