Looking through his spyglass, Sir William can see Jose Delores up close. Later, when he hands the glass to a British officer the view is much more distant. Spyglasses of that era (1850s) would have had 3X-6x magnification. The extreme close up view would be impossible.
The action takes place on a fictional island in the Caribbean, which would belong to Spain (in the film it points to Portugal, because Spain was going through a military regime and the director was afraid that the film would be censored). The plot is based, in part, on the history of Haiti. The original script referred to a Spanish island, and this can be confirmed by the fact that most of the characters keep their Spanish names, like José Dolores instead of the correct José das Dores. Therefore, the film can be seen as an example of ethnic and linguistic stereotyping in relation to Portugal.
During several scenes shot while Marlon Brando's character is speaking in the conference room after his return to the island, closeups reveal that the spectacles he is wearing have clear plastic earpieces - plastic is a 20th century material.
Portugal never had any colonies in the Caribbean. Its only American colony, Brazil, has no coast in the Caribbean.