Which of the following is NOT a reason for insufficient visibility in a visual examination?

Insufficient visibility in a visual examination can severely hinder the quality of the inspection, potentially allowing defects to go unnoticed. In this context, the correct choice, too much contrast, does not typically contribute to insufficient visibility. In fact, high contrast between the surface of the material and the defects can enhance visibility, making it easier for inspectors to identify issues.

On the other hand, inadequate lighting, improper positioning, and worn eyeglasses can all lead to decreased visibility. Inadequate lighting reduces the ability to adequately see and evaluate the inspected area, while improper positioning can obstruct the inspector's view of the weld or material being examined. Worn eyeglasses can impair corrective vision, further compromising the ability to see defects clearly. Thus, while these factors can lead to challenges in visual examinations, too much contrast tends to facilitate, rather than impede, the detection of defects.