It has long been advocated that the accuracy of the synoptophore is affected by the artificial viewing conditions it provides and the patient's awareness of its proximity, which results in an increase in the angle in ET and a decrease in XT.

The aim of this retrospective study was to investigate the agreement between distance horizontal strabismus measurements taken with the synoptophore and PCT. Fifty-four measurements (n = 42: 28 with ET, 14 with XT) were analysed. The PCT was repeated by a second examiner and the measurements (n = 17) were compared to those of the first examiner's prism measurements.

The main finding was a high agreement between the measurements obtained by the two methods (ICC: R = 0.91). There was an eso-shift of the deviation at the synoptophore (compared with the PCT) in 26% and an exo-shift in 28%. Therefore, over-convergence was just as likely to occur as un-der-convergence. The findings cannot support the popular notion that the synoptophore exaggerates the angle in ET and understates it in XT. Furthermore, it was shown that the variability between the two measurement methods was similar to the inter-examiner variability for the PCT. It was concluded that the synoptophore measurements are valid and that the phenomenon of over-convergence with this device (‘instrument convergence’) is not a relevant clinical entity.