When processing black and white film in a paterson tank, does agitation build up contrast in just the high values or does it also build up contrast in the mid-tones.
When processing black and white film in a paterson tank, does agitation build up contrast in just the high values or does it also build up contrast in the mid-tones.
I would say more the higher up you get on the curve, as the effect should be most noticeable where the developer exhausts the quickest. However, with normal agitation schemes of a few turns every minute or every 30 seconds, I think not a lot of depletion actually occurs anywhere, so I'd guess that the issue is mostly relevant for reduced agitation schemes.
related question, if some one has curves comparing agitation schemes (with all else equal) Id be curious to see.
The general idea is that agitation preferentially builds density in the highlights. Lack of agitation lessens this, and the idea is that shadows and mid tones receive preferential development. Or it can be considered that, with little or no agitation, highlights receive less development than they otherwise would. Lots available to read online on this topic. I didn't run across and testing or data. I changed to semi-stand development with a more dilute developer a few years ago and it was, for me, a significant step forward in my darkroom results.
An undesirable phenomenon of greatly reduced agitation is bromine drag. These are visual marks indicating uneven development due to insufficient agitation. Not to be confused with surge marks. There are examples on this site showing bromine drag. The Queensboro Bridge image on my Flickr site has some bromine drag effects. They can be interesting but generally not desired.
The purpose of agitation is to replace exhausted developer and bring fresh developer to the developing silver halide particles. Since developer exhaustion is a function of the amount of development (which is itself a function of exposure), exhaustion happens more quickly in the highlights. So, if you reduce development, development in the highlights can reduce while development in the shadows and midtones continue. Reduce agitation even further and maybe development in the highlights and midtones reduce while the shadows continue to develop. So you can get a shouldering of the characteristic curve at various points on the curve depending on how much you reduce agitation.
As already mentioned, there are other side effects of reduced agitation. You can get bromide drag (usually bad) and adjacency effects (usually good as it increases the perception of sharpness). Adjacency effects happen at the border of two tones when the fresher developer from the less exposed side and the more used developer for the more exposed side diffuse through the emulsion across these areas. It results in the edge of the high exposure area getting a little more development than the rest of the high exposure area and the edge of the low exposure area developing a little less than the rest of the low exposure area. So at the edge of these two areas, you get this added contrast.
Having said all of this, it feels like the available modern films respond more grudgingly to this development technique. You often seem to need fairly extreme techniques (such as stand agitation or minimal agitation). I also have an individual preference for getting my negative as close to a straight line as possible because I prefer manipulations at the print stage. In general, I think losing contrast in some area of the curve is easy to do but putting in more contrast is more difficult.
Cheers, DJ
DJ, does this mean you have a standard development? or did you have to adjust your agitation to straighten your curve? if so how?Originally Posted by N Dhananjay
"All else being equal" is bloody difficult to do. Requires a well equipped laboratory. Like they had at Kodak. I'm thinking your best bet for this would be Grant Haist's (Kodak researcher) two volume tome Modern Photographic Processing. He covered the effects of agitation rather extensively IIRC.
Most university research libraries will have a copy.
Bruce Watson
Am I right in thinking that in a situation where you have strong light, reducing agitation with a dilute developer may control those areas whilst at the same time help to increase shadow areas.
Ian, yes. Additionally the old adage still applies - expose for the shadows. Infrequent agitation and dilute developer will not improve shadow density, if it is not there in the first place.
Last edited by chassis; 19-Mar-2017 at 11:19.
I don't think agitation is the best tool for contrast control. Too little agitation increases the risk of uneven development or bromide drag, so you always run the risk of solving one problem only to introduce another one. Much better to use exposure to make sure you have decent shadow areas and just cut development time if you have highlights that are too dense. Times are always consistent between users too whereas agitation isn't. Two inversions might be gentle or strong, or 10 seconds inversion might mean invert twice or five times.
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