The authors analysed the methods for determining the focal length in digital cameras (DC) and proposed a method, the implementation of which is to survey the correct triangular metal prism from the left and right points of the basis. Next, on the stereo pair of digital images obtained from the surveying, the corresponding coordinates of the points on the near and far edges of the prism are measured, and the focal length of the non-metric DC is determined. To test the method, the focal length of the Sony ILCE-7R and Canon EOS 450D cameras was determined. In order to establish the precision parameters of the method, the following parameters were taken into account: determining the accuracy of measurment on the basis of surveying, the accuracy of measuring the heights, the accuracy of calculation of the longitudinal parallax, the accuracy of calculating the difference between the parallaxes.