Why are my pictures coming out dark?

I own a pentax k50 along with a 18-55mm lens and a 50-200mm lens. When using the 18-55 lens, pictures are fine.However, when using the 50-200 lens, the pictures are coming out very dark. Its strange because the aperture hole seems to be opening when taking the picture so i cant figure out why the pictures are coming out like this. Does it make sense that the problem is with my camera rather than with the lens, even though i am having no trouble with the other lens?Anybody?