I have tested HP-5+ processed in my Jobo CPP-2 with and without a prewet, and I am opting to exclude the prewet as I have noticed that prewetting reduces film speed. First I developed HP-5+ at 8 1/2 min., 75 degrees in Microphen 1:1 with a prewet. I got a Zone I density (.09) at EI 500. I did the same, but excluded the prewet, and I got a Zone I density (.10 ) at EI 640. My Zone IX densities also increased, but not by much: 1.17 with prewet, 1.22 without prewet. I have read in another post that Illford does not reccomend prewetting because it reduces film speed. My testing confirms this. Has anyone else run tests with and without a prewet?

-- William Marderness (wmarderness@hotmail.com), January 09, 2000

Answers

I haven't used a densitometer, but I wouldn't recommend skipping the prewet in a JOBO tank. Going to a prewet eliminated the minor problem of uneven development (on Tech Pan) and led to noticeably more controllable highlights w/TMX. Is the small difference you measured really going to be a problem in the darkroom?

-- John O'Connell (joconnell@adelphia.net), January 09, 2000.

There is much debate concerning a prewet. Even Jobo technicitans disagree (I have read on Jobo's website). Some say a prewet aids even development, while Illford says that prewetting can cause uneven development. Who know? If I get an extra 1/3 stop speed without a prewet and have no other problem, then I see no reason to do it. A shorter wet time is also an advantage.

-- William Marderness (wmarderness@hotmail.com), January 09, 2000.

Here is what Jobo has to say about using a prewet:

Jobos recommendation of a 5 minute prewet of B&W film is probably one of the most disputed suggestions we have made. It causes many people to choose up sides. Certainly many people have chosen to use the prewet, and are more than happy with the continued excellent results they obtain. They have made the other necessary adjustments, and their process is clean and consistent. The original suggestion came from Germany, with the statement that the H&D Curves were closer to the manufacturer's curves when we used a prewet with rotary.

Several technical writers/lab technicians in Germany disagreed with the idea of prewet. Using a Jobo processor they produced a list of films and developers, with processing times, NOT using prewet. We have tried some of these combinations. Every one which we tried performed beautifully. The chart shows what they produced.

More information on the effect of prewet comes to us in an article in the May/June 1994 issue of Darkroom & Creative Camera Techniques. Phil Davis (Beyond the Zone System) has an article entitled "How Prewetting Film Can Affect the Image". While Phil is not dealing with rotary processing in his article, his findings apply. To quote his conclusions, "In most instances, prewetting will influence image formation to some measurable degree. The probable result will be some increase in image density and contrast, and occasionally a slight boost in film speed. It is also apparent that all films are not affected the same way, and an individual film's response to prewetting may not be the same in all developers."

Of course Jobo has a track record regarding the use of the five minute prewet, but it is apparent that this type of information was not available when we established the blanket 5 minute approach. The best advice that can be given is to echo Phil Davis and Paul Schranz. They were writing 7 years apart, but have said the same thing. Ill quote Phil Davis, "...it is a matter for the individual photographer to decide, so I leave it up to you to determine how prewetting affects the materials of your choice." What more can we say?

-- William Marderness (wmarderness@hotmail.com), January 09, 2000.

Some testing is in order....

If you omit the prewet and don't have any problems with unevenness, streaks or airbells, then don't do it any more. If you get airbells but no uneveness, then try using a one-minute prewet.

Ilford incorporates some sort of wetting agent in the emulsion; that's why they don't recommend a prewet.

With HP5+ in several developers I've found no significant differences between a quick one-minute prewet and no prewet.

Just in case you don't find Phil's article, he found that the films tested responded differently to a prewet; no blanket finding could be made.

-- John Hicks (jbh@magicnet.net), January 10, 2000.

The only combination I have tested for prewet effects is not the one you are asking about, but it might provide a cautionary note. I recently decided to switch to Jobo rotary processing for LF and MF film. The first combination I checked throroughly was Agfa APX 100 in POTA. I measured the characteristic curve with and without the prerwet. The effect of prewet was disastrous. It did not affect the lower densities much, but raised the upper densities by a large amount, and produced a very humped characteristic curve. With no prewet, The curve was nice and straight as you please. Before doing any important work, I would suggest you measure the characteristic curves with and without. I am sure that different film/developer combinations will not all produce the same effect.

Al Robinson

-- Al Robinson (alrob@vsta.com), November 22, 2000.