Originally Posted by jho288
Take the simple example of focusing on an eye. Perceptively, 'eye sharpness' is often (and most easily) judged by how easy it is to distinguish the eyelashes from one another. If the subject is closer (or you're using a longer focal length), the eye takes a larger portion of the image. That means there's more physical separation of the eyelashes based on where they fall on the sensor, and thus it will appear sharper.