The pinhole camera model describes the mathematical relationship between the coordinates of a point in three-dimensional space and its projection onto the image plane of an ideal pinhole camera, where the camera aperture is described as a point and no lenses are used to focus light. The model does not include, for example, geometric distortions or blurring of unfocused objects caused by lenses and finite sized apertures. It also does not take into account that most practical cameras have only discrete image coordinates. This means that the pinhole camera model can only be used as a first order approximation of the mapping from a 3D scene to a 2D image. Its validity depends on the quality of the camera and, in general, decreases from the center of the image to the edges as lens distortion effects increase.

Some of the effects that the pinhole camera model does not take into account can be compensated, for example by applying suitable coordinate transformations on the image coordinates; other effects are sufficiently small to be neglected if a high quality camera is used. This means that the pinhole camera model often can be used as a reasonable description of how a camera depicts a 3D scene, for example in computer vision and computer graphics.

The geometry related to the mapping of a pinhole camera is illustrated in the figure. The figure contains the following basic objects:

The pinhole aperture of the camera, through which all projection lines must pass, is assumed to be infinitely small, a point. In the literature this point in 3D space is referred to as the optical (or lens or camera) center.

Next we want to understand how the coordinates ${\displaystyle (y_{1},y_{2})}$ of point Q depend on the coordinates ${\displaystyle (x_{1},x_{2},x_{3})}$ of point P. This can be done with the help of the following figure which shows the same scene as the previous figure but now from above, looking down in the negative direction of the X2 axis.

In this figure we see two similar triangles, both having parts of the projection line (green) as their hypotenuses. The catheti of the left triangle are ${\displaystyle -y_{1}}$ and f and the catheti of the right triangle are ${\displaystyle x_{1}}$ and ${\displaystyle x_{3}}$. Since the two triangles are similar it follows that

A similar investigation, looking in the negative direction of the X1 axis gives

This can be summarized as