iPhone在AVCaptureDevice.Format中报告的焦距和值之间存在差异

Question

为了计算出iPhone相机的35mm等效焦距，我有以下功能。

func get35mmEquivalentFocalLength(format : AVCaptureDevice.Format) -> Float {
    // get reported field of view. Documentation says this is the horizontal field of view
    var fov = format.videoFieldOfView
    // convert to radians
    fov *= Float.pi/180.0
    // angle and opposite of right angle triangle are half the fov and half the width of 
    // 35mm film (ie 18mm). The adjacent value of the right angle triangle is the equivalent 
    // focal length. Using some right angle triangle math you can work out focal length
    let focalLen = 18 / tan(fov/2)
    return focalLen
}

iPhone 7有一个不到60度的videoFieldOfView，它的焦距等于31mm。这并不是苹果公司报道的价值在他们的媒体中的等效焦距。他们报告28毫米。我将相机与其他一些相机进行了比较，iPhone 7相机确实看起来有28mm等效镜头，所以上面的计算出了什么问题呢？