Final answer:
For single vision lenses without prism correction, the Optical Center (OC) should be aligned with the center of the patient's pupils in primary gaze to avoid unwanted prismatic effects and ensure visual comfort.
Step-by-step explanation:
When the patient does not have any prescribed prism in their eyeglasses, the Optical Center (OC) of their lenses is usually aligned with the patient's pupil center when they are in primary gaze. This is the case for single vision lenses without any prism correction.
The OC should be placed such that it is in line with the patient's visual axis to avoid unwanted prismatic effects, which can cause visual discomfort and eyestrain. When fitting glasses, it is important to ensure the alignment is accurate, especially for higher prescription lenses where incorrect placement can have more noticeable effects.