Certified Ophthalmic Medical Technologist (COMT) Practice Exam

What is the purpose of slab off in spectacle lenses?

1. To increase lens thickness

2. To alleviate image jump

3. To correct for astigmatism

4. To reduce glare

The correct answer is: To alleviate image jump

The primary purpose of slab off in spectacle lenses is to alleviate image jump, which is particularly important for individuals with significant aniseikonia due to difference in lens powers between the two eyes. When patients have a substantial difference in prescription strength, particularly for bifocals or progressive lenses, the mismatch can cause one eye to see an image that appears higher than the other, resulting in a visual discomfort known as image jump. Slab off involves removing a specific amount of lens material from the lower portion of the lens with the higher prescription power. This technique creates a more gradual transition between different lens powers, thus reducing the disparity in image heights and allowing for smoother visual alignment. As a result, it can significantly improve the patient’s comfort and visual experience by allowing them to see more uniformly with both eyes while avoiding abrupt jumps in image position when they move their gaze between different sections of their lenses. While aspects such as increasing lens thickness, correcting astigmatism, or reducing glare are important considerations in optometry and lens design, they do not directly relate to the specific function of slab off.