A total of 350 back‐reflection ZnO powder x‐ray diffraction lines on 25 photographic films have been measured and analysed by the Cohen analytical least squares extrapolation method to yielda=(3.2495±0.00002)A,c=(5.2069±0.00011)A; where the stated errors are the probable errors. The statistical reliability of the results is emphasized.In the measurement of the Debye‐Scherrer ring radii the accuracy obtained with a metric scale and indicator (employing no optical lens system) and with a dividing engine equipped with a microscope are compared and the former simpler method is found more reliable. It is concluded that the use of precision measuring instruments for observing diffraction ring radii are unnecessary and even undesirable apparently because the magnifying power of the optical system of precision micrometers leads to uncertainties in settings on photographic lines.