While not the slowest film available (compared to ISO 6 of KONO! Donau), the Rollei RPX 25 is slowest panchromatic black and white emulsion one can buy after the discontinuation of Efke 25. While Rollei Ortho 25 and ATO 2.1 are ISO 25, they are orthochromatic emulsions which mean they are sensitive only to blue and green lights. Adox CMS 20 II (ISO 20) is slower than RPX 25 but it is an orthopanchromatic (i.e. reduced red sensitivity) reversible film which requires a dedicated developer to achieve best results. In terms of availability, the Rollei RPX 25 can be bought in following formats: 135 (36 exposures), 135 (bulk), 120 roll, and 4×5 (25 sheets).
The contact sheet above, from the “last frame on the second row” to the “first frame on the 4th row” shows the same scene being exposed at ISO 12, 16, 20, 25, 32, 40, 50, 100 (all metered by the 75/25 centre-weighted meter of the Nikon F-801). To find out how much of the highlights can be recovered digitally from a film scan, the frame which was exposed at ISO 12 would be the best candidate for testing. The results is shown as below:
It seems that RPX 25 can be exposed at ISO 12 if needed. However, due to the flatness of the result, I would simply stay at ISO 25 unless capturing shadow details is a priority. Despite that, the latitude of RPX 25 is surprisingly wide for a “special purpose” emulsion. This enables it to capture large amount of details in situations with extremely contrasting lightings, such as the scene below. With scan and processing carefully done, the result can be quite satisfactory.
The next characteristic to look at for an emulsion with such a slow speed is of course resolution and grains. The results, I have to say, is amazing. The two sets of images below are two 2400 dpi film scans and their respective 100% crop. The upper set was taken with a Tamron Adaptall-2 28mm f2.5 and the lower set was taken with a Nikon Series E 50mm f1.8.
Well, how about contrast and tonal range? Simply speaking, it is just amazing and the following sample image says it all: