The lens
The eye of the camera - has the most impact on the look of images you capture.
The optics
composed of various lens elements and element groups, they are the vital part of the lens. They shape the entering light into an image on the focal plane of your camera, hopefully without any optical aberrations. They must be clean, so the resolution is not hindered.
The aperture
is the light-regulating organ of the lens that also heavily influences the depth of field. In lenses with automatic apertures, the mechanism must be swift and responsive, which requires the aperture blades be spotless. In lenses in need of service they are often covered in oil that comes from deteriorating grease found on the...
Focusing helicoid.
It's the wonderfully simple mechanical device that translates the rotation of the focusing ring into back and forward translation of the lens barrel, allowing the lens to focus on objects that are differently distanced from the focal plane. The helicoids are lubricated with a special grease that prevents wear and allows for a smooth focusing throw. When a lens ages, the grease begins to break down and pollute the internals of the lens with oil buildup and haze.
Lens shutter
When it comes to medium format cameras, it is likely the shutter is housed in the lens, which makes it prone to all the problems that come with being inside a lens, mostly oil buildup. Using a shutter with oil on the blades can be dangerous, so make sure to get it sorted as soon as possible.