Category: <span>Darkroom</span>

Category: Darkroom

Rollei RPX 25 Reciprocity Failure

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I am on a lookout for a new film for my long exposure photography. When doing research on available slow films I came across the Rollei (Maco) RPX 25. I liked the specs of the film, and I was greatly impressed by its close cousin, the Rollei Infrared.

For some time I have used Foma 100 as my go-to long exposure film. Firstly for its low, low price, and secondly for its very high reciprocity failure. Usually a high reciprocity failure is thought of as an weakness. Films showing less of a failure – such as Fuji Acros – are preferred. But in the special world of long exposure photography a bad reciprocity failure is actually an asset. You get two extra stops of light loss for free! However, I have recently experienced quality issues with the Foma film and started looking for other options.

One issue I found with the RPX 25 film is that the published reciprocity failure is quoted only for 4 times: 2, 10, 20 and 50 seconds. Of these the 10, 20 and 50 second times are outside the usual sequence of exposure times. This was a problem to me, as with such a slow film I expected to run into reciprocity failure areas quite often. Both on purpose, shooting with heavy ND filters, and unintentionally in low light.

To make sense of it I tried modelling the exposure times in my trusty RStudio statistical package. I tried several different regression algorithms and settled on a power regression. This seemed to fit the data better than an exponential. I was able to model a function that matched the reported times closely. The biggest difference is about 4% off at the 50 second data point, and I can live with that.

Rollei RPX 25 Reciprocity Table (modeled)

measured: 0:01 0:02 0:02 0:03 0:04 0:06 0:08 0:12 0:15 0:20 0:30 0:45 0:50 1:00 1:30 2:00
adjusted: 0:01 0:02 0:03 0:04 0:06 0:11 0:16 0:27 0:36 0:52 1:28 2:30 2:52 3:37 6:08 8:54

RPX 25 Reciprocity Failure Chart

Ansco 70 Lith Print Developer

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When I first came across the lith process I was intrigued. All the pictures I found on the internet looked different (in a good way) from the usual black and white prints. The colors and sense of unpredictability inherent to the process appealed to me. But in the Czech Republic, where I live, no commercial lith developer was (or is) easily available. If I wanted to try the process I had to mix my own developer. After some research I settled on the Ansco 70.

The formula calls for common and inexpensive chemicals and does not contain formaldehyde. It is capable of deep blacks, rich midtones, and when used with the Foma Fomatone paper also of warm tones for which the lith process is justly famous.

Prague Bridges - a lith print using the Ansco 70 developer

At first I tried the developer in recommended dilution 1 part A + 1 part B + 17 parts water. After some practice I found the warm colors of slow development easier to obtain when using a more diluted developer.

Now I use a nearly homeopathic dilution of 1 part A and B to some 50 or 60 parts warm water. When printing on size 18×24 cm paper this means 15 ml A + 15 ml B + 800 ml water and 50 ml of Old Brown. Such heavily diluted developer needs to be replenished often – after processing two prints or so I add 10 ml of A and B.

Daisies - a lith print using the Ansco 70 developer

Ansco 70 recipe:

The published formula of Ansco 70 is usually calculated to mix 1 liter of the solutions A and B. The solutions have working life of several months, but considering the high dilution I work with one liter of stock solution is too much for me to use up before it goes bad. The chemicals are not expensive, but I don’t like the idea of throwing away good developer. I found it better policy to prepare only 500 ml at a time.

Solution A:

  1. Water 375 ml
  2. Hydroquinone 12,5 g
  3. Potassium Metabisulfite 12,5 g
  4. Potassium Bromide 12,5 g
  5. Cold water to make 500 ml

Solution B:

  1. Cold Water 375 ml
  2. Sodium Hydroxide 18 g
  3. Cold water to make 500 ml

Dissolving Sodium Hydroxide is strongly exothermic reaction, best performed with caution and wearing protective glasses.

A selection of my lithprints made with the Ansco 70 developer can be found in my lithprint gallery.