Film Agitation

I’d like to stress that consistency in development is very important. Without consistency, you will never process a film the same way twice - causing obvious problems. Agitation is one of the key variables, together with time and temperature, that we must keep very tight control of.

Let me explain.

When we process the film it must be agitated enough for the developer to act evenly on the image.  On the other hand we don’t want to stand there agitating continuously or we will lose some of our edge effects and possibly cause uneven development through the solution surging across the negatives.  A problem sometimes experienced using JOBO’s and the like. 

It is perceived wisdom that the minimum agitation the film should have, to ensure even development, is two inversions of the tank every 30 seconds.  This is based on full strength  developers with short developing times (5 minutes or so). However, with more dilute solutions and longer development times I have quite happily left a film for two or three minutes without agitation to improve acutance.  Sometimes, I have even employed stand development where I leave the film for hours with no agitation at all! 

Over Agitation

We must be able to identify problems with our agitation technique. Over agitation will be seen as either both edges being developed differently than the centre (with twist agitation) or as even, highly developed lines running across the negatives (with inversion agitation). 

Under Agitation

Insufficient agitation of the film (often with highly active developers at normal strength) also causes streaks and uneven development. This is usually most noticeable at the lower film edge where development may be considerably less than the upper part of the film and as streaks below highlight areas. 
Negatives effected in these ways are ruined.  
For all these reasons, I agitate constantly for the first 30 seconds of development, thoroughly soaking the emulsion, and subsequently 10 seconds on each further minute.  It’s a reasonable compromise between undesirable uneven development and desirable compensation and edge effects. When you have found the agitation you are happy with do not change it.