Originally posted by MarkJerling Dirty rear elements have very little affect on photo quality
Mark, I hesitate to directly contradict you, but contamination or scratches on the rear element do effect image quality and are one of the key things to look for when purchasing a used lens. OTOH, a front element can be pretty ugly and still take a decent photo.
To the OP...You can clean your rear element using lens cleaner and a clean soft cotton cloth. Apply the cleaner to a corner of the cloth, gently swab the contaminated area, and finish with a dry portion of the cloth. Take care to not apply direct pressure or any sort of scrubbing and you will be fine. My practice is to use regular glass cleaner (Windex) and either a white cotton rag or shred of high quality paper towel. Purists might use a Kim Wipe or Tec Pad.
Steve