Greetings, I am trying to understand how to determine if a lens has enough coverage for a 12x20 camera. If I look at how to determine focal length, then the diagonal of the negative, which is approx. 23.25 would yield a 590.55 mm focal length lens. However, on this website I read someone's response that a 210mm f/9 would cover a 12x20 neative. What part of this formula am I missing and how can I determine by just looking at a lens if it will provide the coverage I need.

Thanks for any response.