Findings of the Film-Developers-Study

resolution, edge sharpness and halo

The ability to resolve structures in an image has no sharp limit as a function of the fineness of the structures. Rather, the significance, i.e. the recognisability of structures, continuously decreases with increasing fineness. It is therefore necessary to define an objective criterion for this. This definition is to a certain degree arbitrary, but it allows for the objective comparability of different results. How the resolution power was defined in this study is described in detail here. The procedure in a nutshell:

  • A high-quality Siemens star (metal on glass) is exposed onto the film using the contact method..

  • A microscopic image is digitised at 100x magnification from the developed film image.

  • The digital information of the image is Fourier transformed along lines of fixed line density into the space of the spatial frequencies.

  • The limit of significance, the maximum resolution, is determined from the decrease in the signal strength of the fundamental harmonic of the pattern with increasing line density..

  • The relative strength of the 3rd harmonic at 8 LP/mm characterises the edge sharpness.

  • The presence or absence of the 2nd harmonic characterises the susceptibility of dark structures to glare (developmental halo).

What is important is that no subjective impression is applied; instead, the same algorithm is used for all film-developer combinations. This ensures that the results can be compared objectively.

Figure 1 shows the results for the resolution of all film-developer combinations tested. The resolution is given as line-pairs per millimetre (LP/mm). As described in the measurement method, the ‘reference film’ Ultra R800 was measured in the same way. This is an extremely low-sensitivity ‘document film’ with a high steepness that was developed in its special developer (Nanotec UR). Its resolution can only be estimated here, but it is in any case well beyond 250 LP/mm. More information on this can be found here.

Fig. 1: Measured resolution in line pairs per millimetre (LP/mm) for six different films in six different developers

Interpretation:

  • The influence of the developer on the resolution is considerable and exceeds the influence of the film type..

  • TMX (Tmax100) and Delta 100 achieve resolution values comparable to those of PanF, although they are 2-4 times more sensitive.

  • TMY (Tmax400) clearly outperforms the comparable Ilford film Delta 400 in resolution with all developers (except FX39). With the right developers, especially Wehner/Alpha, it achieves values that correspond to those of ISO 100 films and are in the range of 80 LP/mm and above..

  • The classic TriX shows comparable resolution values to the flat crystal Delta 400.

  • It is noticeable that Wehner/Alpha are among the top developers for all films except PanF.

  • FX39 apparently only harmonises very well with Delta100, it drops significantly with the ISO 400 films in particular..

Figure 2 shows the parameter of the edge sharpness, which was also obtained from the microscope images of the siemens stars. It varies from 0 to 1, where 1 represents optimal edge sharpness. You can read about how this is determined here. .

Fig. 2: Edge sharpness parameters for the 36 film-developer combinations. A value of 1 indicates the maximum possible edge sharpness.

Interpretation:

  • Similar to the resolution, the differences are significant, although the PanF can benefit from its low sensitivity and almost matches the value of the ‘reference film’.

  • TMY (Tmax400) also performs excellently in terms of edge sharpness in the right developer, achieving comparable and even better values than films in the ISO 100 class.

  • As with resolution, Delta400 is less sharp than Tmax400, but slightly outperforms the classic TriX..

  • The ‘ranking’ of the developers is similar to that for resolution, with the poor performance of FX39 with ISO 400 films being even more noticeable.

Figure 3 shows examples of microscope images of 3 film-developer combinations with low, medium and good edge sharpness.

Fig. 3: Microscope images of the Siemens star in the range of 8 LP/mm (grey line) for three different film-developer combinations. The respective analytically determined sharpness parameters are given in the image headings.

The third parameter determined for the Siemens stars is the over-shining (development halo) of dark stripes in the area of unexposed stripes. How this is determined is described here. In contrast to resolution and edge sharpness, the lowest possible over-shining is desirable here.

Fig. 4: Width of the transmission zone (halo) of darker (exposed stripes) measured at 8 LP/mm.

Interpretation:

  • of the image-based films, from PanF onwards, with all developers, a very low over-exposure in the range of under 2 µm. With the ‘reference film’, the transmission is not detectable.

  • TMX (Tmax100), Delta100 and TMY (Tmax400) show typical halos of 2-4 µm in width, depending on the developer.

  • Of all the films, TriX exhibits the greatest dependency of the halo width on the developer; it varies from 2.5 µm in Wehner/Alpha to 8 µm in FX39.

  • FX399 produces a very distinct halo, especially in films of the ISO 400 class, which also causes the poor resolution (see Fig. 1).

  • Wehner/Alpha provides a small amount of halo in the range of 2-2.5 µm in all films (except for Delta400).

The halo is one of the limiting factors for the resolution. Since it occurs on both sides of the dark structures, a halo width of $b [\mu m]$ limits the resolution to $\frac{1000}{4 b}$ LP/mm. Example: TriX in FX39 with $b=8\mu m$ cannot resolve more than about 30 LP/mm because at this linewidth the bright structures are completely obscured. Halo widths of $b=2.5 \mu m$ only reach this limit at 100 LP/mm, where other factors (grain structure) usually limit the resolution.

Figure 5 shows as an example the microscope images of 3 film-developer combinations with weak, medium and strong halo formation. The grey circle lines mark 8 LP/mm. Again, the note: all siemens stars are exposed so that the exposed (dark) structures in the outer area of the star (2 LP/mm) have an optical density of D = 1. With TriX in FX39, the transmission of

Fig. 5: Micrographs of the siemens star at 8 LP/mm (grey line) for three different film-developer combinations. The analytically determined widths of the transmission zone (halo) are given in the figure headings.

  • In addition to the data for resolution, edge sharpness and halo, the granularity (RMS granularity) of all combinations was also determined. The results are presented here: (grain structure).

  • Possible relationships between the parameters are presented on these pages: (-> Correlations).

  • The density curves, development parameters and all microscope images of the central area (80 LP/mm) and the 8 LP/mm area of the siemens stars, as well as the grain images at 250x magnification, can be found on the individual pages of the films: (-> individual pages of films).