How to calculate a unified continuous zoom range using only physical cameras (like the default camera app)?

[stopkite]

Background

The default camera app provides a seamless continuous zoom experience from ultra-wide → wide → max zoom. The starting zoom ratio varies by device — some start at 0.5x, others at 0.6x, etc.
Notably, even on devices that don't support a logical multi-camera, the default camera app still provides this smooth transition between physical cameras.
I want to replicate this exact behavior in my app using the Camera2 API.

Goal

Combine the zoom ranges of multiple physical cameras into a single continuous zoom range — starting from the correct device-specific starting ratio (0.5x, 0.6x, etc.) up to the maximum zoom — just like the default camera app does.

Example

Physical Camera 0 (Ultra-wide): focal length 2.0mm, zoom range 1.0x–8.0x
Physical Camera 1 (Wide):       focal length 5.0mm, zoom range 1.0x–10.0x

Default camera app behavior:
0.5x (or 0.6x, etc.) ────── 1.0x ────── max
[Ultra-wide camera zone]      [Wide camera zone]

→ Starting ratio varies by device
→ Smooth transition to wide camera around 1.0x

Question

1. How is the starting ratio (0.5x, 0.6x, etc.) calculated?

• Is it the focal length ratio of ultra-wide / wide? (e.g., 2.0mm / 5.0mm = 0.4x?)
• Is the reason it differs per device simply due to different focal lengths?

2. What is the formula to calculate the switch point (crossover) between physical cameras?
3. How should the remaining zoom range of the wide camera be mapped into the unified range after the switch point?
4. Since the default camera app handles this even without a logical multi-camera, is there a recommended approach at the Camera2 API level, or is manual calculation the only

Environment

• Camera2 API directly (not CameraX)
• Targeting devices without logical multi-camera support