Involved in the biosynthesis of C30 carotenoids. Catalyzes the oxidation of the terminal methyl side groups of 4,4'-diapolycopene to yield 4,4'-diapolycopen-4,4'-dial via the aldehyde intermediate 4,4'-diapolycopen-al. Also able to catalyze the oxidation of the terminal methyl side group of 4,4'-diaponeurosporene to form 4,4'-diaponeurosporen-4-al. It has moderate to low activity on the C40 substrates neurosporene and lycopene, and has no detectable activity on zeta-carotene or beta-carotene. 4 AH2 + all-trans-4,4'-diapolycopene + 4 O2 = 4 A + all-trans-4,4'-diapolycopene-4,4'-dial + 6 H2O 2 AH2 + all-trans-4,4'-diaponeurosporene + 2 O2 = 4,4'-diaponeurosporenal + 2 A + 3 H2O Carotenoid biosynthesis. Cells lacking this gene accumulate the fully unsaturated C30 carotenoid backbone 4,4'-diapolycopene with some additional less unsaturated intermediates. CrtNb is not a carotenoid desaturase, in spite of its apparent sequence homology. Belongs to the carotenoid/retinoid oxidoreductase family.