The level of melanin in the eye changes.

Usually, pictures are hung at eye level for viewing, but if hanging in a group some may necessarily be higher than eye level while others may be lower than eye level.

eye

eye

Holding the camera at the level of the adult human eye. It's the most common angle used. Try "toddler level" once in a while to remind you of what things used to look like (in my case) decades ago! Try almost ground level sometimes (especially for uncluttered landscapes).