A method for estimating a portion of undesired particles in crop material includes taking a photograph of the crop material; identifying images of at least one type of undesired particle in the photograph that was taken; measuring the surface area covered by the image of every particle shown in the photograph; and determining the portion of undesired particles as a proportion of the surface area of the images, and may be used, in particular, in a harvesting machine on the crop material picked up in an on-going harvesting operation.