An optical calibration system for refractometers was designed and built using a singlet lens and a translatable detector that induces spherical error, making it a simple and easy-to-use system. The system was calibrated using spectacles and then validated by correcting blurred images created by a series of ophthalmic test lenses. A linear regression model of the correction of the ophthalmic test lenses showed nearly direct correlation with an offset of 0.12 D and an R² value of 0.9998. An updated version of the system was designed in Zemax OpticStudio with a singlet lens, which could induce spherical error accurate to that of the human eye within ≤ 0.05 D. In future work, this system will be manufactured and tested in the lab.