## 3 Results
### Investigation of the Influence of Calibration Parameters on the Results of Strinin/Stress Analysis
However, from the 12 th registered position, the plots take on a constant character with low estimated errors and approach those obtained with the largest (3.0 mm) calibration target. When using a middle (2.0 mm) target, the lower precision of its manufacture was reflected in the fact that, in some cases, the camera parameters started to stabilise later (from about the 15 th recorded position of the target) or did not stabilise at all. In addition, several estimated parameters differed significantly from those obtained with the original calibration targets with a higher guaranteed manufacturing accuracy. Also, the correlation system estimated the largest errors in calibration parameters assessment when the 2.0 mm target was used. From the above plots of calibration residua, camera parameters, or lens distortions, it is possible to predict which correlation system calibration should provide the most accurate (and, conversely, the least accurate) results. However, from the mechanics of materials point of view, it is more important to know what effect changing these parameters will have on the results of the strain/strain analyses. Therefore, in the second phase, an analysis was carried out in which the values of the equivalent stresses according to [PERSON] theory were compared in a full-field manner. The comparison was carried out for four specimen loading levels (\(F\) = 400 N, \(F\) = 800 N, \(F\) = 1200 N, and \(F\) = 1600 N), the same as in the previous analysis, always taking as reference the field determined using the calibration file obtained by calibrating the largest (3.0 mm) target at the maximum number of registered positions. The mean values of the relative differences of the equivalent stresses \(\Delta\sigma_{\text{mean}}^{\text{Mises}}\) were obtained based on the relation:  
\[\Delta\sigma_{\text{mean}}^{\text{Mises}}=\text{mean}\left(\left|\frac{ \sigma_{\text{i}}^{\text{Mises}}(a,b)-\sigma_{\text{ref.}}^{\text{Mises}}(a,b )}{\sigma_{\text{ref.}}^{\text{Mises}}(a,b)}\right|\right)\cdot 100\% \tag{6}\]  
where \((a,b)\) represent the facet centres coordinates of the reconstructed specimen surface, \(\sigma_{\text{i}}^{\text{Mises}}\) is the equivalent von Mises stress determined for the \(\text{i}^{\text{th}}\) position of the calibration target, \(\sigma_{\text{ref. }}^{\text{Mises}}\) is the reference equivalent von Mises stress determined using the calibration file obtained by calibrating the largest (3.0 mm) target at the maximum number of registered positions. The MOSTRAN V.1.0 program, developed in MATLAB at the authors' workplace, was used to obtain the equivalent von Mises stress fields. The plots of the mean relative differences in the evaluated equivalent von Mises stresses can be seen in Figure 11. Suppose we disregard the results obtained in calibration with the number of registered calibration target positions less than 8 (which, according to [37], is the minimum at which accurate results should be obtained). In that case, the most significant differences are obtained using the middle (2.0 mm) target with a lower manufacturing precision. However, the percentage differences obtained when using the smallest (1.5 mm) target reach more significant levels for fewer recorded positions, which is again related to the estimation of the calibration parameters. From the 13 th recorded position of the calibration targets, the results obtained when using the original targets (1.5 mm and 3.0 mm) differ only minimally. As the number of recorded calibration target positions increases, it is possible to observe a decreasing character of the relative differences value obtained in the evaluated equivalent von Mises stresses. The achieved levels of differences evaluated for a higher number (at least 13) of calibration target positions do not exceed the value of 2% for all four evaluated measurements.