[empw]

In my experience there are two mistakes people sometimes make that leads to the fisheye lens look. The first, as parent mentioned, when calculating the ray directions you should linearly sample a line rather than interpolating view angles. Second, when doing per-column perspective division, you should divide by dot product of the forward vector and the difference between the ray intersection and the camera location. Often if someone is making the first mistake they are also making the second.

Following this will give you the normal pinhole camera 3d projection that we all expect to see from a 3D game.

Rule of thumb I found as a beginner in 3D graphics: any time polar coordinates seem like the obvious solution, there's usually a better way.