Abstract: The invention relates to a method for determining the validity of a measured sharpness of a fixed-focus camera (8), in which a first image of an object (13) is captured by the camera (8) and the sharpness of the image is determined, wherein an additional first optical element (15, 15?) is introduced into the optical path of the camera (8) and a second image of the object (13) is captured and the sharpness of the second image is determined, wherein depending on the comparison of the sharpnesses of at least the two images, the presence of an imaging error of the camera (8) is identified. The invention also relates to a device for testing the sharpness of a fixed-focus camera, a fixed-focus camera as well as the use of a test device for a fixed-focus camera in a vehicle and a method for assembling a fixed-focus camera.