A fixed lens camera comprises a fixed focus lens, an infrared filter, and an optical low pass filter, all disposed in succession within a housing, coaxial with an optical axis. Light from a target passes through an aperture in the housing, and then through the lenses and filters. An image sensor receives the filtered light, and outputs a two-dimensional array signal of corresponding voltages proportional to the incident light striking each sensing element. The voltage signal from the image sensor is filtered using a high pass filter, and the high frequency components of the signal are measured to determine the relative focus of the light from the target. The signal measurement is displayed on a bar LCD to enable focusing of the lens by moving the camera along the optical axis while observing the LCD output.