.. _distortion_causes:

Causes of distortion
====================

In a lens-coupled camera or detector, there are two major types of distortion
which may often be seen in an acquired image: radial distortion and perspective
distortion (:numref:`fig_1`).

.. figure:: figs/causes/fig1.jpg
    :name: fig_1
    :figwidth: 98 %
    :align: center
    :figclass: align-center

    (a) Non-distorted image. (b) Barrel type of radial distortion.
    (c) Pincushion type of radial distortion. (d) Perspective distortion.

**Radial distortion** is caused by the increase or decrease of the magnification of a
lens with respect to the radius from the optical axis as shown in :numref:`fig_2`.

.. figure:: figs/causes/fig2.jpg
    :name: fig_2
    :figwidth: 70 %
    :align: center
    :figclass: align-center

    Cause of radial distortion.

**Perspective distortion** can be caused by two sources: a lens-plane is not parallel
to a camera-sensor plane, known as tangential distortion, (:numref:`fig_3`) and/or an
object-plane is not parallel to a camera-sensor plane (:numref:`fig_4`).

.. figure:: figs/causes/fig3.jpg
    :name: fig_3
    :figwidth: 70 %
    :align: center
    :figclass: align-center

    Lens-plane and sensor-plane are not parallel.

.. figure:: figs/causes/fig4.jpg
    :name: fig_4
    :figwidth: 70 %
    :align: center
    :figclass: align-center

    Object-plane and sensor-plane are not parallel.