What are the alternatives to the canon rf s 3 9mm f 3 5 stm dual fisheye?

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From Canon RF-S 3.9mm f/3.5 STM Dual Fisheye: Canon RF-S 3.9mm f/3.5 STM Dual Fisheye: A prime lens by Canon for APS-C cameras, 3.9mm f/3.5. A twin-lens 3D VR lens for APS-C bodies, with autofocus. Specs - Maker: Canon - Focal length: 3.9mm (prime) - Maximum aperture: f/3.5 - Made for: APS-C sensors - Mounts: Canon RF - Stabilised: no - Autofocus: yes What it frames like - On APS-C (Canon) (Canon RF): like 6.2mm on full frame; f/3.5 gives about the depth of field ...

From Focal length converter: How it works: Multiply the focal length by the sensor's crop factor to get its full-frame equivalent. To go between two smaller formats, multiply by the first crop factor and divide by the second. Crop factors are measured on the sensor diagonal, so formats with a different shape (4:3 against 3:2) frame slightly differently across the width.

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