## 4 Discussion  
This paper describes the influence of calibration parameters and image brightness established when strain/stress analyses are carried out on a small area of a planar specimen with a hole using stereoscopic low-speed digital image correlation. The experiments aimed to obtain this information to improve the accuracy of the results of the analyses related to the quantification of residual stresses by means of the hole-drilling method combined with DIC, which has recently received increased attention. By such measurements, it is possible to obtain relatively quickly and efficiently information on the strains relieved during the formation of the hole or groove in the analysed specimen, which is the principle for determining the residual stresses by the mentioned semi-destructive approaches. In addition, the measurement is easily repeatable, which leads to at least a saving of time related to the preparation of the strain gauge measurement. However, the disadvantages of DIC over conventional methods of measuring the strains relieved are:  
* the problem of applying a standard digital image correlation system with commercially available drilling equipment;
* the lower sensitivity of the cameras compared to specialised strain gauge rosettes. Problems associated with the application of digital image correlation systems with commercially supplied drilling equipment (e.g., the body of the drilling equipment not being adapted to the application of optical methods, the principle of which is to acquire the surface of the analysed object repeatedly) are often overcome by the installation of the optical system at greater distances, or by the development of custom drilling/imaging equipment. However, the lower sensitivity of correlation system cameras compared to strain gauge rosettes is guaranteed to lead to some measurement inaccuracy. However, to avoid further adverse effects, it is essential to be aware of all possible aspects that could adversely affect the results of strain/stress analyses performed by correlation systems. The paper describes several analyses processed in MATLAB or MOSTRAN V.1.0 program, developed at the authors' workplace. Based on the obtained results, it can be concluded that the size of the calibration target, the accuracy of its manufacture, as well as the number of its registered positions in the process of calibration of the stereo-camera DIC system can affect the results of strain/stress analyses. To minimize their influence, the authors of the paper suggest:  
* if possible, use calibration targets whose size corresponds approximately to the field of view of the cameras used in correlation mode;
* if the calibration target used is significantly smaller than the field of view of the cameras, there is a risk that some of the calibration parameters will not be estimated correctly if the number of registered target positions is less than 10; it is, therefore, \begin{table}
\begin{tabular}{c c c c} \hline
**Tensile Force** & **Mean Relative Measurement Errors and Their Standard Deviations (\%)** \\ \hline  & Calibration target & Calibration target & Calibration target \\  & 1.5 mm & 2.0 mm & 3.0 mm \\ \hline \(F=400\) N & 6.65 \(\pm\) 0.31 & 6.70 \(\pm\) 0.33 & 6.66 \(\pm\) 0.32 \\ \hline \(F=800\) N & 3.05 \(\pm\) 0.14 & 3.07 \(\pm\) 0.14 & 3.04 \(\pm\) 0.14 \\ \hline \(F=1200\) N & 2.17 \(\pm\) 0.10 & 2.17 \(\pm\) 0.09 & 2.16 \(\pm\) 0.09 \\ \hline \(F=1600\) N & 1.63 \(\pm\) 0.08 & 1.65 \(\pm\) 0.08 & 1.64 \(\pm\) 0.08 \\ \hline \end{tabular}
\end{table} 
Table 4: Mean relative measurement errors and their standard deviations obtained for individual load levels.