Kayvan Karimi, 30, was convicted by a revolutionary court of "insulting the sacred" for his film "Writing on the City", about political slogans daubed on the walls of Tehran's streets.
He remains free as the case is under appeal but if the next hearing on December 23 goes against him the sentence could be carried out.
Speaking of their "shock and disbelief" the filmmakers urged the sentence to be rescinded by "applying justice in the appeals court where misunderstandings can be settled".
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"We Iranian documentary makers, like other artistic and cultural guilds of this land, know their right is to create work in a safe and stress-free environment far from accusations," they wrote.
Karimi, an Iranian Kurd, earlier received support from French, Italian and Spanish filmmakers.
"This film has nothing to do with sacred issues" and the prosecution did not say "what scenes were an insult" to Islam, his lawyer, Amir Raeesian, has said.
Artists and several reformist journalists were recently arrested in Iran and accused of being part of a network financed by US intelligence, aided by European countries including Britain, the Netherlands and Sweden.
Iran's supreme leader, Ayatollah Ali Khamenei, has warned against economic and cultural infiltration from the West since a nuclear deal with six leading states, including the US, was struck with Tehran in July.