I'm not familiar with the story, but this is what I take from the comment above:

* captain arrives in FTL ship looking for evidence of a thing

* captain isn't satisfied with a painting offered as evidence, because it could be fake

* aide says that photography is unknown to the natives, implying that the production of a photograph would be taken by the captain as having more evidentiary value

=> captain knows about FTL but is unfamiliar with the technology required to produce false but convincing photographic images