In Section 26.1, we showed that the angular magnification of a magnifying glass is equal to the ratio of the near-point distance to the focal length of the lens (m_θ = s_N /f, Eq. 26.14). Hence, by making the focal length sufficiently short, a very large magnification can be achieved in principle. However, other practical considerations limit the maximum useable value of m_θ. Explain why it is hard to find magnifying glasses with magnifications greater than about 20.