Effects of magnification methods and magnifier shapes on visual inspection

This study investigated the effectiveness of the novel differential linear and differential nonlinear magnification methods, and the effects of magnifier shape and magnification power on visual search performance. With the differential magnification, objects that were more peripheral were scaled at a level higher than those in the vicinity of fixation. It was hypothesized that 1) owing to the ineffectiveness of scaling stimulus in the vicinity of fixation, the differential magnification methods would enhance magnification effectiveness and thereby improve inspection performance, and 2) because an elliptical lens shape resembles the shape of human visual field, for the same area of magnified view, an elliptical magnifier would be more effective than the circular one. In this study, objective performance measures of speed and accuracy and subjective workload evaluation using the NASA Task Load Index paradigm were used. The results showed that 4x magnification increased the overall inspection time as compared with 3x magnification, suggesting that a trade-off might have occurred between magnification and field size. The two hypotheses were not confirmed; however, a number of questions arose which need further investigation. Discussions of the concerned questions were made on i) the unfamiliarity of subjects with the new differential magnification methods, ii) perceptual discomfort due to image duplication, discontinuity, and distortion resulting from stimulus scaling, iii) the movement of fixation points, and iv) the competition for attention between foveal and peripheral magnified objects. This study prompts the authors to consider a training programme on the differential linear and nonlinear magnification methods in future studies. With proper instructions regarding the nature of lens and magnified images, subjects' performance with the novel differential magnification methods may be improved.