I exposed two APX 100 (135) at 80 ASA. On each film I exposed test frames at Zone I and Zone VIII. The first film I developed at 17°C in 1+50 Rodinal for 21 minutes (rotary development) and achieved d 0.03 at Zone I and d 1.42 at Zone VIII. To get better results I developed the second film as the first but increased development time to 23 minutes.This time I achieved d 0.2 at Zone I but d 1.39 at Zone VIII where I expected a higher density compared to the first film too. Can anybody explain this to me? Did I miss the basics of densitometry?