Impressionism is actually rooted in very scientific investigations. Famous impressionists like Monet were focused on conveying landscape in a direct, optically correct way. They were obsessed with painting color exactly how the eye understood it. They wanted to record scientifically the shifting of light and color in an environment. In that way, it is not subjective at all.
Impressionism could have some subjective themes in the way they represent a fleeting moment. In recording light and color in a single moment as they try to do, the painting becomes very expressive. Paintbrush strokes are clear and exaggerated, the texture and color of the paint is rich and vibrant. The label "Impressionism" was originally an insult--critics claimed that Monet was merely creating impressions of classical landscapes. Impressionist landscapes are becoming more and more abstract, and in that sense, it could be read as addressing subjective associations of landscapes and observation.