Abstract
Multifocal lenses have regions of different optical power, correcting far and near vision for presbyopes with one pair of glasses. Optical distortions are an unavoidable side effect of those lenses. From previous research and reports of spectacle wearers we know that such distortions cause unnatural and often discomforting motion percepts. Depending on design choices and the intended refractive correction, the strength and shape of distortions vary between lenses. For the design of spectacle lenses it is important to understand how perceived distortions depend on different parameters of the spectacle lens. In this work we analyze physical distortions induced by multifocal glasses with varying correction on a subjective, psychophysical scale of distortions. To measure the internal scale we performed a psychophysical experiment in virtual reality (VR). The image in VR was transformed to replicate the distortions of ten simulated lenses. Subjects looked around freely in a virtual indoor environment while three different distortions were presented consecutively in each trial; they were asked for the pair of distortions that appeared more similar: first to second, or second to third. We estimated the scale as a coordinate for every lens with an ordinal embedding algorithm, so that the distances in the scale maximally agree with the subjects' perceived similarities. The fit is best with a single coordinate dimension for the perceived distortions, even though we varied multiple lens parameters, resulting in complex transformations of the images. This perceived distortion is increasing with the optical power in the far vision area of the lens and the additional power in the near vision area of the lens. Distortions for lenses with negative optical power in the far area appear closer to undistorted vision with increasing power in the near area, suggesting a compensation of distortions for negative lenses.