Final answer:
True, semi-visible markings are etched on the lens to indicate the datum line for both single vision and progressive lenses, facilitating correct lens orientation and optimal visual performance.
Step-by-step explanation:
The statement that semi-visible markings are etched on the back of the lens to indicate the datum line on all single vision and progressive lenses is True. These markings are crucial for proper lens orientation, ensuring that the lens matches the prescription exactly for optimal visual performance. It helps in aligning progressive lenses with the pupil and designating the orientation point for single vision lenses.
Semi-visible markings allow opticians to verify that the lenses have been cut and set into the frame correctly. This is essential because the alignment affects how the wearer sees through different parts of the lens, which is especially important in progressive lenses that have different focal points for distance, intermediate, and near vision.
The precise control over the shape of the cornea highlighted in your reference material is achievable in over 90% of patients, ensuring normal distant vision through exact lens crafting. Similarly, understanding that to obtain clear vision the lens-to-retina distance (di) must be correct, as a normal vision is achievable for objects at distances from 25 cm to infinity.