Findings of the Film-Developers-Study

Grain structure

The grain structure of analogue images (in this case, black and white images in particular) is an important characteristic in which analogue and digital images differ significantly. The grain structure is by no means just a limitation of analogue technology but a design tool.

The characteristic grain structure of analogue images results from the fact that the image consists of irregularly distributed silver crystals that also differ in size and structure. These are formed during exposure and subsequent development from the silver bromide crystals embedded in a gelatin layer (the emulsion) by reducing the silver bromide to metallic silver. The most important properties in summary:

  • The individual silver crystals (the silver grain) have a typical size in the range $0.2 - 1 \mu m$ and are not directly visible in a ‘normal’ optical image of the negative. Example: a very good enlarging lens such as the Apo-Rodagon 2.8/50mm has an MTF of 0.88 (manufacturer's specification) at an optimum aperture of 5.6 for 20 lp/mm, which corresponds to a point spread function with $\sigma = 4 \mu m$.

  • The graininess visible in the image of an evenly illuminated surface is the result of the blurred image of the irregular grain structure (granularity) caused by the limited resolution.

  • One of the quantitative measures developed by KODAK is RMS granularity (1) , which, according to studies such as (2), characterises the visually perceived graininess of the image. How RMS granularity is defined and how it was measured in this study can be found here –> details.

  • In contrast to the classical method developed by KODAK, my method also allows the determination of the visible structure size of the grain, i.e. the typical scale of light-dark fluctuations that results from the underlying grain size together with grain agglomerations and optical resolution.

    1) eg. KODAK Technical Publication E-58
    2) Smith, Jack C., "The measurement of RMS granularity of black and white photographic materials using a visual comparison technique" (1980). Thesis. Rochester Institute of Technology.

RMS Granularity

Fig 1: RMS granularity for different films and developers.

Figure 1 shows the measured RMS granularity for the 36 film-developer combinations examined in detail. The documentary film SPUR Ultra R 800 is also added for reference. According to KODAK, the RMS values for normal films should be between 5 and 50.

As you would expect, the film type is the first thing to consider when deciding on the granularity. High-speed films typically have a higher granularity than low-speed films.

Of the low-speed films, TMX (KODAK Tmax100) exhibits less granularity on average than the comparable Ilford Delta 100 film and is also finer-grained than the considerably less sensitive (classic) Ilford Pan F.

EiThe picture is similar for ISO 400 films: TMY (KODAK Tmax400) is slightly finer-grained on average than Delta400, which, although a flat-crystal film, has a similar granularity to the classic TriX.

Developer: Especially with the ISO 400 class films, the influence of the developer is significantly greater than the difference between the film types. Depending on the developer, the granularity can be changed by a factor of about 1.5 - 2, similarly for the Delta100. As expected, Rodinal produces pronounced granularity in all films, while XTOL is consistently fine-grained. It is striking that SPUR SD2525 with Ilford Delta100 produces very little granularity, whereas with all other films it tends to be at the higher end. The classic metol hydroquinone developer KODAK D76 produces very acceptable to good granularity with all films.

Visual structure size

In addition to the RMS granularity, the evaluation of the microscopic images also allows the determination of the apparent structure size of the grain, which results from the convolution of the microscopic grain structure with the resolution of the optical image. Simulations show that this is about $3\mu m$ for optimal imaging in the case of very fine grains. Figure 2 shows the relationship between RMS granularity and visual structure size for all 6 films.

Fig. 2: RMS granularity and apparent structure size of the grain structure for all films and developer combinations.

For TriX and especially Delta400, the correlation between RMS value and structure size is clearly visible. The grain structures become larger and more accentuated, depending on the developer, which is expressed in a higher RMS value. For Tmax400 (TMY), the RMS value varies similarly to the other two films in the ISO 400 class, but the structure size is almost constant.

For the low-sensitivity films, Delta100 reacts most noticeably to different developers, while Tmax100 (TMX) shows very low grain values, both in RMS granularity and in structure size, with all developers tested.

Figure 3 shows the grain images (area with optical density D=1) for the four flat-crytal films in Rodinal an XTOL respectively.

XTOL always provides a softer grain, while Rodinal accentuates the grain significantly more (higher RMS values) and, in particular with the Ilford films Delta100 and Delta400, also increases the structure size.

All 36 grain patterns can be found, together with the images of resolution and edge sharpness, on —-> details all films.

The relationship between RMS granularity, resolution and edge sharpness is shown on ——> this page.