Using coins for scale is discouraged as it will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins can also reinforce a geographical bias, and some coins' designs are copyrighted.

Ideally, a photograph should include a ruler with the subject (example) or an added scale marking. SI ("metric") units are the most commonly used worldwide (see meter and centimeter).