# View frustum, “focal length” and near plane distance

I've been reading through "Mathematics for 3D game programming and Computer Graphics" lately and just when I'd got used to author's lack of qualification of important notions, he striked again.

On the topic that interested me the most, the definition of the view frustum and how the APIs generally process it, he introduces the concept of a length e which he classifies as the focal length. And it makes absolutely no sense, isn't the whole notion of focal length incompatible with the pinhole camera concept (inf. small) which is at the heart of 3D APIs as Direct3D and OpenGL? I thought all I need is the aspect ratio, the distance from the near plane and the FoV which can be established from the distance from the near plane.

And then he simply drops it and speaks of near plane distance n, which I thought was the actual value of e. What gives? Isn't the distance from the near plane the same as the "focal length" since we're talking about a pinhole camera? How do they differentiate?

I hope someone can capture my confusion.

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