When I attach a OM 50mm be it my 1.4, 1.8 or 3.5 the view through my viewfinder is clean and sharp as is my OM 28mm 2.8 & 3.5 But when I attach a Tokina 17mm 3.5 the view through my viewfinder appears vague and speckled with spots/specks for want of better words, also it is not the easiest to focus. My focusing screen is a 1-3. Is this normal, what does it indicate, is the 17mm lens dirty?, is focusing screen dirty?, I have cleaned all lens thoroughly. The same goes for other non OM lens, they are in the longer range eg Vivitar 80-200mm etc also less than stellar clarity. My thanks in advance for any advice offered.