Abstract: In a device for ascertaining the actual feed transmittted by an feed device f an industrial sewing machine to a material to be sewn, in which the number of the threads moved across a sensor arrangement is counted and the feed is calculated from the previously detected thread density, it is provided, for obtaining an exact thread count which is as independent as possible of the orientation of the sewn web, to place ahead of the photocell (10) of the sensor arrangement a rotating slit diaphragm (7) with at least one radially extending parallel limited slit (8). Preferably a second, rotationally adjustable, but fixed slit diaphragm (5) is provided, having slits (6) opening across a set angular sector, the rotatitng slit diaphragm (7) having a larger number of slits (8). Because of this the required rpm of the slit diaphragm (7) can be reduced. To avoid errors caused by the pattern of the material it is possible to operate in the relatively longer wave infrared spectrum.