Rodinal is a non-solvent liquid-concentrate developer. It results in very strong grain (for lack of solvent action) and gives poorer film-speed compared to most other developers. However, it has a distinctively sharp look that is highly sought-after in certain circles.
Rodinal is a very flexible developer because it allows the user to vary the shape of the HD curve (characteristic curve) as well as obtain both compensation and adjacency effects by dilution and variations in agitation.
Rodinal concentrate will keep for decades in its original bottle and still works after going dark brown and crystallising somewhat. Once diluted, it must be used within hours and used only once (one-shot).
Rodinal makes an honest representation of the film grain without any softening, therefore it can look more-grainy in comparison to other developers. It is worth noting that Rodinal does not make the grain any physically larger (the resolution of the film is not compromised), but the RMS variation in image brightness due to grain is larger.
In other words, Rodinal produces grain with high spatial frequency but also high amplitude.
Use with fast films will yield the largest and most prominent grains, while use with very fine films (typically slower emulsions) can yield smooth-looking and very high resolution results.
This developer is particularly excellent with Efke 25, Ilford Pan-F, Acros and TMAX 100, all of which are very fine-grained.
Use at 16C instead of 20C can produce finer looking grain with some films, though the development time must be extended accordingly.
A favourite application of Rodinal is semi-stand development, which achieves compensation, adjacency and increased film-speed. A drawback to this approach is dull highlights due to the compensation effects.
A particular danger of stand development with Rodinal and some films is bromide drag, which can cause streaks in the image originating at highlights.
The quoted minimum quantity of Rodinal required to develop a single roll completely is 10mL of concentrate, however many report that it works fine with as little as 6mL. However, it is possible that very dense (high-key) rolls will have poorer highlight rendition due to exhaustion when less then 10mL is used.
This minimum quantity in conjunction with higher dilutions means that large quantities of developer are often required, e.g. one litre per roll at 1+100. Those who disbelieve the 10mL-minimum recommendation will assert here that the large volumes at high dilution are not required for semi-stand processing; the little agitation means that much of the volume would never come in contact with the film and therefore it's sufficient to use about 500mL per roll for 1+100.
1+50 is a more-commonly used dilution; it usually gives less-apparent grain at the cost of reduced adjacency and compensation effects.
Rodinal as most know it was produced and sold by Agfa and the formula changed slightly in the 1960s. Some old-stock is still around in 2011 but Agfa no longer produce it. The following products are apparently identical though:
Calbe has been making the pre-1960s version, sold as:
The active ingredient in Rodinal is p-aminophenol, which can be synthesized from acetaminophen (paracetamol); this home-made version is generally referred to as parodinal.
"Rodinal Special" is an entirely different developer, despite the name.
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