"The painter paints a farmer in the picture."
Translation:Pictor agricolam in pictura pingit.
Yes, the macron does distinguish the nominative & ablative cases from each other. The macron also lengthens the vowel, so it is important for the sake of speaking and listening (in classical pronunciation).
As far as I know, ecclesiastical pronunciation does not observe vowel length, so if you plan to only speak or read church latin then learning macrons is not crucial.
Are you thinking of the plural -a ending (in nominative, accusative and vocative) on neuter nouns, like aedificia, itinera, cubicula (buildings, journeys/routes, bedrooms)?
Those are neuter nouns of either the 2nd or 3rd declension (there are a few neuters in 4th decl., but I haven't seen any here on Duolingo).
The neuter -a is always short.
(In the 1st decl., there is no plural ending in -a, long or short.)
Don't you think that we'd better use 'in picturam' rather than the ablative case? I guess this sentence tells a process with direction - a painter makes a farmer's figure 'comes' into the picture, which doesn't mean it already stands there. English is not my mother language, hopefully I have made myself understood.
Could it also (grammatically) mean that the picture itself is a visual depiction of a painter standing in the field and painting a farmer?
Technically it should. (though the very idea of depicting a painter inside a painting is a modern-art motif which wasn't prevailing at all in classical paintings [neither as an idea nor as a feasibile situation, since the instruments for painting were not meant for outdoor use] - and if it were to be, then you should have said "in pictura, pictor agricolam pingit").
Ye-es, I think it is a possible interpretation. Only, it it not attested in the antiquity. Incidentally, are you sure that the painting instruments could only be used indoor? I wonder. Outdoor statues were certainly painted, and I think outdoor wall paintings existed too.
Well, I do know that one of the catalysts for modern movements in art (which was characterized, also, in abundance of nature landscape paintings) in 19th century was the mere fact that the painter could finally leave his studio and go out to nature and paint the object first-hand, whereas painter in older times could not simply achieve that. It's quite similar to what happened with video cameras in 20th century - cinema verità documentary ["fly on the wall"] was made possible due to the fact that one-person could finally carry a camera over his shoulder and (no longer in need to rely on heavy optical machinery,) the filmmakers could leave the studio locations and merge into reality in order to take shots.