Final answer:
The image discrepancy with light and dark sides is due to uneven lighting, not a grid error. It results in a perceptual illusion and can be affected by the direction of light and object shadows.
Step-by-step explanation:
When an image appears with one side lighter and the other side darker, the error that has likely occurred is related to uneven lighting during the photographing process. This isn't a 'grid error' per se, but rather a common photographic issue. In scenarios where light is coming from one direction, such as from the left, the objects on the right may cast shadows, leading to a perceptual illusion of depth or a 3D effect. This phenomenon can also occur with artwork or posters that have been lit unevenly, resulting in two contrasting fields where one side of the image is brightly lit and the other is in shadow.
Similar effects can be observed in images containing optical illusions, where parallel lines may not seem parallel unless the image is viewed from a certain angle. Subtle patterns on wallpapers may only be visible under specific lighting conditions, further compelling the challenge of accurate perception. This exemplifies the importance of proper and even lighting to maintain the integrity of an image's appearance.