Certified Ophthalmic Medical Technologist (COMT) Practice Exam

What optical phenomenon causes discomfort in spectacles with near add power that lacks slab off?

1. Double vision

2. Image blending

3. Image jump

4. Distorted vision

The correct answer is: Image jump

The optical phenomenon that causes discomfort in spectacles with near add power that lacks slab off is image jump. This occurs at the boundary between the distance and near zones of a bifocal or multifocal lens. When there is a significant difference in power between these zones, as is common in lenses with a near add, the transition can result in a noticeable shift of the image when the line of demarcation is crossed. This abrupt change can lead to what patients often describe as a jumping sensation of the image, which can be quite disorienting and uncomfortable. In the absence of a slab off, which is a technique used to create a gradual change in the lens thickness and thereby smooth out the transition, this unpleasant visual experience is exacerbated. Slab off involves modifying the lower portion of the lens to help with the compensatory effects of prismatic displacement that occur at the add power, thereby reducing the image jump effect and improving visual comfort for the wearer. This aspect of lens design highlights the importance of customized fitting and adjustment of eyewear, especially for patients requiring significant near vision correction alongside distance vision correction.