I forgot and left one on while taking some pics of my grandson's third birthday. I caught a nice tight close-up of him blowing out the candles on his cake. As soon as the film was developed, a problem was obvious. There were three distinct bright spots perfectly aligned on his forehead. They matched the candle flame spacing perfectly. Apparently, the candle flame image reflected off the front lens surface and bounced off the back of the filter glass. Now if it had been digital, I guess I could have PS'd them out.