Why do you agitate the developing tank while developing film?

Jim Mowreader
When you develop film, the chemical that's touching the film wears out after a while--the real term is "reaches exhaustion," but that just means it wore out. When you agitate the tank, the worn-out developer is exchanged for fresh. Over-agitation can cause development streaking because the solution travels through the sprocket holes too much. Too little agitation can result in lowered contrast and un-even development (mottling).Comment on overagitationYou "overagitate" by moving the tank too rapidly. You can overagitate if you turn the tank over once every five minutes, if you flip it really, really hard when you do. There are rotary processor systems such as Jobo that continuously agitate the tank, but it's a very slow, gentle agitation so you don't get streaks around your sprocket holes. I also remember being into H&W Control film, which was a film-developer system that promised nearly grainless, super long tonal scale images. It actually did it too, if you didn't mind that the film was ISO 25. You had to continuously agitate it. Wouldn't work any other way, or so the instruction sheet said. I figured if they went to the good time and trouble to tell you to do it they had a reason, so I always did it and it worked like the book said it would.