How does an out-of-focal grid error manifest on an image?

An out-of-focal grid error typically manifests as underexposure towards the edges of an image. This occurs due to the misalignment of the grid, which is designed to absorb scatter radiation and improve image contrast by allowing primary radiation to pass through more evenly. When the grid is not positioned correctly in relation to the x-ray tube and the film, the primary x-rays may not reach the edges of the image receptor effectively, leading to a noticeable decrease in exposure at those areas. As a result, the edges of the image appear darker compared to the center, where the exposure remains more consistent.

The other options do not accurately represent the typical manifestation of out-of-focal grid errors. For instance, even exposure throughout the image would suggest proper functioning of the grid. Overexposure towards the edges does not align with the mechanics of how a grid is supposed to work, and overexposure at the center of the image would indicate a different type of issue, such as improper technique or equipment malfunction. Thus, the characteristic of underexposure at the edges is what makes this answer correct.