Extreme macro closeup of a massive dragon's head filling the entire frame. The focus is on the eye which is huge with a vertical slit pupil that reflects like polished obsidian. The iris displays vibrant rainbow colors - deep crimson at the inner ring transitioning through orange, golden yellow, emerald green, sapphire blue, and violet at the outer edge, with intricate crystalline patterns and ancient wisdom gleaming within. The eye's surface has a subtle reptilian texture with fine scales around the edges. The lighting should be dramatic with the iris colors casting prismatic reflections, and the overall mood should convey the awe-inspiring, terrifying scale of the dragon. Ultra-detailed, photorealistic rendering with shallow depth of field emphasizing the eye's immense size.