Distortion
Assuming the camera model is pinhole, according to Brown's distortion model, there are two types of common distortions: radial distortion and tangential distortion.
Radial distortion
The most commonly encountered distortions are radially symmetric, due to the symmetry of a photographic lens. These radial distortions can be classified as either barrel distortions or pincushion distortions.
Barrel distortion: image magnification decreases with distance from the optical axis.
Pincushion distortion: image magnification increases with distance from the optical axis.
Mathematically, barrel and pincushion distortion are quadratic, meaning they increase as the square of distance from the center. We can model it with three parameters: .
Tangential distortion
Tangential distortion can happen when the lens is not fully parallel to the image plane.

Mathematically, we can model this distortion with two parameters: .
References
14 Lectures in Visual SLAM
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