Focusing on the nearest eye is mainly a practical and perceptual rule of thumb.

For a single portrait, focusing on the near eye gives you the best chance of keeping the whole face looking sharp. Since depth of field extends behind the focus point as well as in front, placing focus on the nearest eye helps cover the rest of the face. If the far eye still isn’t sharp enough, the real fix is usually to change aperture, distance, or camera angle—not to focus farther back.

There’s also a visual reason: if the closest eye is sharp, viewers tend to perceive the face more naturally as being “in focus.” If the nearer parts of the face are softer than parts behind them, it often looks wrong or distracting.

The same idea applies to groups. Images usually look more natural when the front person or front row is the sharpest, rather than having nearer faces softer than people behind them.

A final rule of thumb: blur in front of the focus point often feels less pleasing than blur behind it, so focusing nearer tends to keep the out-of-focus area pushed toward the back of the scene.