For a flat subject, the distance that gives the most captured detail is essentially the closest distance at which the camera can still focus reliably — its minimum focus distance.

Why: the closer the subject is, the larger its details are projected onto the sensor, so they use more pixels. As you move farther away, the same details cover fewer pixels, and eventually the finest details may become too small for the lens/sensor to resolve well.

So in practice, to maximize recorded detail:

• Move as close as possible while maintaining accurate focus.
• Keep the phone parallel to the flat subject.
• Use good light and steady support to preserve sharpness.

The lens’s focal length by itself doesn’t change this basic rule: at a given distance, what matters is how large the subject is rendered on the sensor. For document-style shooting, the practical “optimal distance” is therefore the minimum focus distance, adjusted slightly if needed for better edge sharpness or autofocus consistency.