I'm sure there is a setting like "minimum shutter speed" that I cannot find in the menu, but on my OM-1 right out of the box with default settings, I am unable to get a slower shutter speed than 1/640 in any of the shooting modes. I can get as fast as 1/32000 but no slower than 1/640. Where do I change this so that I can achieve the full range of shutter speeds, either automatically or manually, based on my shooting mode?