In a cylinder lens, what happens if the strongest meridian does not run exactly through the E.D. meridian?

When the strongest meridian of a cylinder lens does not align perfectly with the effective diameter (E.D.) meridian, it can lead to discrepancies in the prediction of lens thickness. Specifically, because the cylindrical power is not optimally aligned, the lens is likely to show an error on the advantageous side.

This occurs because the thickness calculations for the lens rely on accurate meridian alignment. If the strongest meridian, which is where the most cylindrical power is concentrated, is off, it can result in the lens being evaluated as thicker than it actually is. The misalignment causes a change in the optical behavior of the lens, affecting how light is refracted and potentially leading to a situation where the predicted thickness based on the E.D. is not accurate.

Thus, the thickening observed is an overestimation due to the inappropriate alignment, hence the assessment that the lens might appear slightly thicker than what is accurate reflects the nature of the optical calculations involved.