Substantial difference between declared and real magnification in medical loupes

Background: The use of magnification devices is widely recommended in surgical disciplines, but a validation of the respective optical aids, i.e., medical loupe systems, is lacking. The aim of this study was to objectively assess different medical loupe systems, including their declared magnification. Methods: Six Galilean binocular loupes (Zeiss 2.5×, SurgiTel 2.5×, Merident DCI 2.5×, Orascoptic 2.8×, SwissLoupes 2.5× and 3.2×) and three Keplerian binocular loupes (Zeiss 3.6×, SurgiTel 3.5×, Orascoptic 3.8×) were compared with regard to their optical characteristics. The working distance was 350 mm for all loupe systems except for Orascoptic (370 mm). The magnification factor, field size and distortion were assessed and compared in an independent optical laboratory (NTB, Buchs, Switzerland). Results: The following magnifications (declared/measured) and field sizes (in mm) were obtained: Zeiss (2.5×/2.7×; 50), SurgiTel (2.5×/2.0×; 80), Merident DCI (2.5×/2.6×; 45), Orascoptic (2.8×/2.2×; 85), SwissLoupes (2.5×/2.3×; 50 and 3.2×/3.1×; 30 respectively), Zeiss (3.6×/3.8×; 40), SurgiTel (3.5×/3.6×; 30), and Orascoptic (3.8×/4.2×; 40 [left] and 3.8×/3.8×; 45 [right]). Most loupe systems showed a vertical and horizontal distortion less than 1°. Conclusions: A great variance of magnification and field sizes could be measured in Galilean loupe systems. A lower real magnification than declared is most likely not a problem of manufacturing quality but based on commercial reasons: lower magnification correlates with bigger field size and wider focal range, both of which are good selling points. Keplerian loupe systems showed more consistency, although one loupe system differed between left and right ocular. A test for instant assessment of the effective magnification is desirable and proposed. Recommendations of medical Galilean loupes with certain declared magnification factors and studies based on them up to now must be regarded as biased with respect to the technical details reported here.