Foma’s Fomapan line is among the least expensive panchromatic films still in production, which makes it a common entry point into traditional black-and-white work. Two characteristics complicate its use: the effective speed that yields full shadow detail often sits below the rated value, and the emulsions lose sensitivity sharply in long exposures. The three films are not built alike. Only Fomapan 200 Creative is described by Foma as containing “T-crystals” — a core/shell tabular silver-halide grain — while Fomapan 100 Classic and Fomapan 400 Action are traditional panchromatic emulsions, the 400 widely characterised as a cubic-grain film. Note the irony: the tabular 200 shows the steepest short-time reciprocity correction of the three, so emulsion geometry alone does not predict the behaviour.

A note on terminology: “T-grain” is Kodak’s T-GRAIN trademark, used for the tabular emulsion in T-MAX. Foma’s own wording for the 200 is “T-crystals,” a related but distinct construction, and that is the language used here.

Box speed versus metered speed

Foma rates the three emulsions at ISO 100/21°, ISO 200/24°, and ISO 400/27°. Those figures follow ISO 6:1993, which fixes the black-and-white negative speed point at 0.10 density above base-plus-fog, measured under a defined contrast: the film must be developed so that a point 1.30 in log exposure above the speed point reaches 0.80 density above it. The standard places the speed point near the start of the usable curve; it does not promise that every deep-shadow value is separated, and with these emulsions the lowest shadows frequently fall onto the toe of the characteristic curve, where they record with little tonal separation.

The reason these films appear to “meter slow” is structural, not a loose rating. Zone System film-speed testing places a metered shadow on Zone I at roughly the same 0.10 over base-plus-fog, but as a metered placement rather than a sensitometric speed point. That criterion typically yields an effective speed about 2/3 stop below the ISO figure. So rating Fomapan 100 at EI 50 to 64, or Fomapan 200 at EI 100 to 160, lifts the shadow off the toe and onto a part of the curve with usable separation. Fomapan 400 benefits from the same logic, downrated to roughly EI 200 to 250.

These ratings are not abuse of the film. Foma states that each emulsion “gives good results even when overexposed by 1 EV or underexposed by 2 EV without any change in processing.” Shooting the 100 at EI 50 is exactly one stop of overexposure — squarely inside Foma’s own quoted latitude, no compensating development required.

Anchoring the rating to development

An exposure index means little without a development regime behind it, because the speed and contrast curves Foma publishes are themselves referenced to a specific developer. Foma’s curves and MTF data for Fomapan 100 are measured in Ilford Microphen at 20 °C, developed to gamma 0.6. For everyday work at EI 50 to 64, a standard fine-grain regime holds shadow placement without crushing highlights: Ilford ID-11 or Kodak D-76 at stock dilution, 6 to 7 minutes at 20 °C. Other published times for the 100 at 20 °C include Fomadon R09 at 1+50 for 8 to 9 minutes, Fomadon LQN at 1+10 for 7 to 8 minutes, Microphen at 5 to 7 minutes, Perceptol at 8 minutes, and Xtol or Fomadon Excel at 5 to 6 minutes. Foma’s agitation scheme is continuous for the first 30 seconds, then the first 10 seconds of every following minute.

Reciprocity failure in long exposures

Reciprocity failure is the breakdown of the assumption that halving illumination and doubling time yields equal density. It appears once exposures grow long and light levels low. All silver-halide films show it, but Foma’s datasheets describe a steep correction and publish it as three discrete anchor points — 1 s, 10 s and 100 s metered — not a continuous formula:

Metered time	Fomapan 100	Fomapan 200	Fomapan 400
1/1000–1/2 s	1× (0)	1× (0)	1× (0)
1 s	2× (−1 stop)	3× (−1.5 stops)	1.5× (−1 stop)
10 s	8× (−3 stops)	9× (−3 stops)	6× (−2.5 stops)
100 s	16× (−4 stops)	18× (−4 stops)	8× (−3 stops)

The 200 is steepest early — 3× at one second against the 100’s 2× — and the 400 is mildest late, asking only 8× at one hundred seconds where the others demand 16× to 18×.

To use the table, find the row at or above your metered time and multiply. A metered 10 s on Fomapan 100 falls exactly on the 10 s row: 8× lengthening, so the actual exposure is 80 seconds (equivalently, three stops added). For an off-table value you must interpolate between the published points and round up, because the table is three points rather than a smooth curve. A metered 4 s on the 100 sits between the 1 s row (2×) and the 10 s row (8×); there is no exact published factor, so take the next anchor up — treat it conservatively nearer the 8× end and round the result upward rather than trusting a linear guess.

The folk wisdom that Foma’s figures overcorrect beyond a few seconds is just that — anecdotal — unless tied to a named tester with a measured counter-table. Treat the published factors as Foma’s stated values and, if you want to refine them, run a step-wedge test on your own film and developer rather than splitting the difference by feel.

Why correction grows non-linearly

The mechanism is latent-image formation, described by Gurney-Mott theory. A developable latent-image speck on a grain needs a stable cluster of roughly four or more silver atoms. At normal intensities photons arrive close enough together that the cluster builds before it can decay. At low intensity the photons arrive sparsely, and the unstable single- or double-atom sub-image left by the first photons decays — the trapped electron and silver atom are lost — before later photons arrive to complete a stable cluster. The grain therefore needs a larger total exposure to register the same density, and the shortfall worsens the longer and dimmer the exposure runs.

Schwarzschild quantified the departure from linearity in 1899 with E = I · t^p, where p is the Schwarzschild coefficient. Ideal reciprocity is p = 1; low-intensity reciprocity failure means p < 1 (Schwarzschild’s own plates gave roughly p ≈ 0.86). Because the exponent sits below one, the extra light required scales faster than the metered time — which is precisely why a fixed stop of compensation will not do, and why Foma’s correction climbs from one stop at a second to four stops at a hundred.

Characterising the films

Grain and resolution set the trade-off against the slower working speeds. Foma quotes a resolving power of 110 lines/mm for both Fomapan 100 and Fomapan 200. RMS granularity, measured in Microphen at 20 °C developed to gamma 0.6 and read at density 1.0, is 13.5 for the 100, 14.0 for the 200, and 17.5 for the 400. The 100 and 200 sit close on both axes, so the choice between them is less about sharpness than about speed and that steeper early reciprocity curve on the 200; the 400 trades visibly coarser grain for its extra speed and its milder long-exposure correction.

Sources: FOMAPAN 100 Classic, 200 Creative and 400 Action datasheets (FOMA BOHEMIA); ISO 6:1993, Determination of ISO speed; Karl Schwarzschild (1899) on the I·tᵖ reciprocity law.