Experimental investigation of focal position in regular astigmatic eyes with zero spherical equivalent

Astigmatic eyes are commonly believed to focus on distant targets with a circle of least confusion on the retina when the spherical equivalent (SE) is 0. We tested this hypothesis in young adults by measuring ocular focus under induced astigmatism. Twenty healthy volunteers (20.2 ± 0.8 years) first received full refractive correction. We then applied three regular astigmatic states (1, 2, and 3 D; axis 180°), each with an SE of 0 D. Over-refraction was recorded for 3 s at 5 Hz using an open-field autorefractor. The objective SEs were significantly myopic under all conditions (- 0.33 ± 0.09 D, - 0.49 ± 0.22 D, - 0.77 ± 0.27 D; all p < 0.001). It increased with the astigmatic magnitude (p < 0.001). Despite a theoretical SE of 0 D, accommodation led to a shift in the retinal image plane between the circle of least confusion and the second focal line, resulting in a mild myopic shift. In conclusion, in young individuals with astigmatism and an SE of 0, accommodation intervention did not place the circle of least confusion on the retina, resulting in a slight myopic shift. This outcome should be considered when performing refractive corrections for astigmatism.