A maximum a priori (MAP) estimation technique for the detection of focus aberrations in electro-optical imaging systems is developed. The technique simplifies the equipment required in focus aberration detection over previous methodologies. The magnitude of the focus aberration is estimated from a single image. The MAP estimation technique uses a Poisson distribution of the photons arriving at the detector from the object. A Gaussian distribution is added to the statistical model to account for the focus aberration. Using the imaging system statistical model and real laboratory images from a charge-coupled device (CCD) camera, the focus aberration detection (FAD) algorithm produces estimates of the focus aberrations. The results demonstrate a viable approach for estimation and potential removal of focus aberrations in electro-optical systems, without the need to divert any light from the primary channel, or for additional complicated equipment and associated calibration requirements.