Purpose: Aniseikonia is a binocular anomaly in which the two eyes perceive images of different sizes and/or shapes. It is usually assumed to be constant as a function of visual field angle (VFA) (i.e., angular distance from the line of sight). This is correct for optically induced aniseikonia, such as the aniseikonia that is associated with anisometropia and probably also pseudophakia. The purpose of this article is to show that if the aniseikonia is of retinal origin, then the aniseikonia may no longer be constant as a function of VFA (i.e., field-dependent aniseikonia). Design: Case report, with the patient having a unilateral epiretinal membrane. Methods: The aniseikonia was measured in vertical and horizontal directions with a customized version of the Aniseikonia Inspector software. The VFA was made variable by changing the dimensions of the comparison targets in the direct comparison procedure. Main Outcome Measure: Aniseikonia as a function of VFA. Results: The patient exhibited good repeatable aniseikonia, ranging from 23%to 2.5%for VFAs ranging from 0.36°to 5.7°. Higher angles had lower aniseikonia. A control subject did not show this field-dependent aniseikonia. Conclusions: Aniseikonia may vary with VFA due to a retinal cause such as an epiretinal membrane. The problem with field-dependent aniseikonia is that it cannot be corrected fully with conventional optics, which exhibit an approximately constant magnification as a function of VFA. Nevertheless, by correcting 5%to 10%aniseikonia, which showed up in the VFA measurement range at 2°to 3°, our patient had improved visual comfort, especially for reading. Purpose: Aniseikonia is a binocular anomaly in which the two eyes perceive images of different sizes and / or shapes. It is usually assumed to be constant as a function of visual field angle (VFA) (ie, angular distance from the line of sight) This is correct for optically induced aniseikonia, such as the aniseikonia that is associated with anisometropia and probably also pseudophakia. The purpose of this article is to show that if the aniseikonia is of retinal origin, then the aniseikonia may no longer be constant as a function of VFA (ie, field-dependent aniseikonia). Design: Case report, with the patient having an unilateral epiretinal membrane. Methods: The aniseikonia was measured in vertical and horizontal directions with a customized version of the Aniseikonia Inspector software. The VFA was made variable by changing the dimensions of the comparison targets in the direct comparison procedure. Main Outcome Measure: Aniseikonia as a function of VFA. A control subject did not show this field-dependent aniseikonia. Conclusions: Aniseikonia may vary with VFA due to a retinal The problem with field-dependent aniseikonia is that it can not be corrected fully with conventional optics, which exhibit an approximately constant magnification as a function of VFA. Nevertheless, by correcting 5% to 10% aniseikonia, which showed up in the VFA measurement range at 2 ° to 3 °, our patient had improved visual comfort, especially for reading.