When Natalia gets her "contact sheets" she views them on a lightbox. The contact sheets are positive proofs printed on paper. A lightbox, used for viewing negatives or transparencies, would be useless with a print.
When John first looks at Olivia's charcoal drawings, the light reflecting off the paper shows it to be a smooth, semi-gloss surface. Neither the drawing paper for charcoal nor the medium itself reflects light in this way; these appear to be photographs of charcoal drawings.