If two identical films are immersed in developer for the same period of time and one is agitated intermittently while the other is agitated continuously, the one that is agitated continuously will be denser and of higher contrast. In short, if the ‘standard’ time is used the film will be over-developed. The photographer starting with a Rondinax tank will realise that agitation is continuous compared with the recommended, intermittent, agitation for vertical tanks. Therefore, when using a Rondinax the recommended development time at a particular temperature has to be decreased. But by how much?
Agfa, in their instruction manuals, were of little help since they just gave recommended times for their own films and developers in their (i.e. Rondinax) tanks. The early 1950s instructions for the Essex suggest a reduction of 25% (i.e. for 35 mm film) while those for the Kent suggest 10% (i.e. for 120 film). I used to reduce the standard development time at a particular temperature in the Rondinax 60 by about 10% using D76 and, I think, by less than this for the now discontinued Unitol (a softer, i.e. less contrasty developer).
Ilford recommend a 15% reduction for continuous agitation on a modern pack of ID-11 (D76). In processing films to illustrate this website, I have used a 15% reduction for both ID-11 (D76) and Rodinal. I also looked at the popular Massive Dev Chart app for the iPhone and the assumptions on which its recommended development times are based†. Their ‘standard’ (i.e. intermittent) agitation is 30-60 seconds of continuous agitation followed by 5-10 seconds (3 inversions of a tank) per minute. For continuous agitation in a Jobo rotary processor, they recommend a 15% reduction in development time.
So, a 15% reduction seems a good starting point for present day films and developers. I say a good starting point because it has been said many times before that the density and contrast of the negative should be matched to the requirements of the next processing stage. Therefore, one can only suggest a starting point from which the user might deviate in order to obtain negatives of a particular characteristic. In the old days (and the present day for those to stay with traditional printing) the light source used in the enlarger had a marked effect on the contrast of the print. There was endless discussion on the merits of different forms of lighting. As time went on there was a tendency for less contrasty negatives to be favoured in order to provide maximum gradation from black to white. In the 2010s, the negative is often, perhaps more usually, scanned. Here again, scanners are reckoned to be able to cope better with a negative of lower contrast than one with high.
The unit commonly used to indicate contrast in a negative is gamma and I do apologise to any readers who feel I am instructing their maternal grandparent to aspirate ova. Gamma is the slope of the main part of the curve that relates density (i.e. blackness) to log exposure. Therefore, a high gamma denotes high contrast (density increases quickly as the exposure is increased). The old standard Kodak adopted for ‘normal’ contrast was 0.8 and, for lower contrast 0.65. It became very fashionable in to aim one’s development at a particular gamma value, 0.7 for example, by shortening the ‘standard’ development time for a particular type of film.
Some films and developer combinations may behave differently. Different developers increase contrast at different rates as development proceeds. Roger Hicks, the thinking man and thinking woman’s photographer*, illustrates this point very well using old graphs from Ilford. Thus between 6 and 8 minutes of development, contrast increased by 1 gamma in the developer Microdol, by 1.5 gamma in ID-11 (D76) and by only 0.5 gamma in Perceptol. So, reducing the development time by a constant percentage would have a different effect on contrast in these three old developers.
Roger Hicks also shows an old Ilford graph that shows the change in contrast with development time in tanks that are agitated continuously (the film and developer are not stated). In order to decrease the contrast to that which would have been obtained with intermittent agitation, the development time has to be decreased by about 25%. My guess is was these sorts of data which led Johnsons of Hendon to recommend a reduction of 25% to users of the Essex 35 in the instruction manual.
One bonus of buying old books is that interesting papers have sometimes been left between the pages. Thus an old instruction leaflet for Kodak Microdol dated 1956 came to be on my desk. That leaflet provides recommended development times to achieve 0.65 and 0.8 gamma for both intermittent and continuous agitation. It covered all the Kodak film of the time. For the panchromatic roll films, I calculated that the recommended reduction for continuous agitation was 13-22% for a gamma of 0.65 and 20-23% for a gamma of 0.8. For 35 mm films, the reduction was 13-21% for 0.65 and 18-22% for a gamma of 0.8.
The Microdol leaflet also defined intermittent agitation as ‘brief but thorough one a minute’. My guess is that modern practice, entailing an initial 30-60 seconds of continuous agitation, lessens the time difference between intermittent and continuous. The emulsion of modern films is also thinner. That property could also affect the difference between intermittent and continuous agitation since the distance for developer to diffuse within the emulsion must be shorter.
Having said that a 15% reduction seems as good a starting point as any, I must take this opportunity to raise another question. Continuous agitation can be taken to mean that the film is completely immersed in the developer and that mixing of the developer is continuous. But that is not really what happens in a Rondinax. Approximately half the film is not immersed in developer at any one time, regardless of the rate at which the winding knob is turned. If the knob is turned slowly, is there time for exhaustion of developer in the dense areas of the film as the film passes through the air? In other words, is development slower in a Rondinax being in rotated continuously than in a conventional tank being agitated continuously I would like a chemical engineer to work out the kinetics but I suspect that agitation in a Rondinax is not quite the same as continuous agitation in a vertical tank.
Adding successful combinations of time, film and developer to a comment below and indicating how the time deviates from that recommended for intermittent development will help others as they begin using Rondinax tanks.