Hello, I recently bought a second hand Praktica MTL3. With it I got several lenses (28mm f2.8, 50mm f1.8, 135mm f2.8), a flash and a precision filter. The man who sold me the camera + accessories told me he used the filter to protect the lens from scratches. I was wondering what the intended use is of a precision filter. Just as a side note: I'm quite new to photography so I don't have much knowledge about the subject. Thank you for your help in advance.