A method that estimates the 3D trajectory of a projectile, such as a golf ball or baseball, by analyzing a sequence of images from a single-lens, 2D camera. Image analysis may be used to locate the projectile in the camera images, using motion detection and filters for the projectile's expected size and shape. A physics model may be used to calculate the projectile's trajectory as a function of its initial position and velocity (for example, just after impact with a golf club or bat). A camera projection transform may map this trajectory into predicted pixel locations, which may be compared to the observed projectile locations in the camera images. The projectile's trajectory may be estimated by finding initial conditions that minimize differences between observed and predicted pixel locations, using a nonlinear least squares solver for example. The method may be extended to multiple cameras.