The error is investigated which results from the employment of tangential approximation in the calculation of line shift caused by specimen displacement from the recording circle in focusing systems (Guinier, Seemann–Bohlin). After an exact expression has been deduced and compared with the approximate formula in a numerical example, it is concluded that the error caused by the approximate formula may be important only in exceptional cases. The deduced exact formula is also compared with that given by Rafaja and Valvoda [Powder Diffr. 6, 200–203 (1991)] with the conclusion that both formulas are mathematically equivalent and complementary with respect to the theoretical and measured values of the diffraction angle 2θ.
