I just received a Jupiter 9 85mm f2 in the mail and am evaluating whether it is a good example and I should keep it. I've been noticing that if I photograph the same scene at different f stops, the color temperature in the photos shifts dramatically. I'm using an Olympus E-PM1. I set the camera's WB to Auto and mode to P. Wide open, the images are a bit cool. Once I stop down to about f4.5 (an estimate, as the aperture ring doesn't click at each f stop), the images get into a neutral zone. Then, starting at between f8 and f11, the color goes quite warm (yellowish). What causes this shift? Is it considered normal and I should live with it, or is something wrong with the lens?