Every film loses sensitivity as exposure times lengthen. The light-meter assumes that halving the intensity and doubling the time leaves the exposure unchanged, but below a certain light level that bargain breaks, and a negative held open for several seconds records less density than the metered value predicts. This is reciprocity failure. On most emulsions it forces a correction that grows with exposure time, and often a development adjustment on top. Fujifilm Neopan 100 Acros II is notable because that correction stays at zero far longer than almost any comparable film.

Where the Light Goes

The reciprocity law E = I × t holds only across a narrow window, roughly 1/5 second to 1/1000 second. Outside it, exposure follows E = I × t^p with p less than one; Karl Schwarzschild, fitting the effect around 1899, used an exponent of about 0.86. The mechanism is grain-level. A silver-halide crystal becomes developable only once a latent-image speck grows to a stable cluster of roughly four reduced silver atoms. At normal light levels photons arrive fast enough to build that cluster before it falls apart. At low intensity they trickle in, and the unstable one- and two-atom sub-image specks decay between arrivals before the cluster ever reaches the developable threshold. Density is lost, and it is lost most where light is weakest: in the shadows.

That is why a long exposure does not merely darken uniformly. The dim values fail first and worst, the bright values hold, and the negative stretches in contrast. Correcting it means both adding exposure and, on many films, pulling development to flatten the gain back out.

What the Datasheet Specifies

The current Acros II data sheet for 135 format (Fujifilm Ref. AF3-0258E, 2020) states that no exposure compensation is required for shutter speeds shorter than 120 seconds. For exposures from 120 to 1000 seconds it specifies one fixed correction: +1/2 stop, applied as a half-stop increase in lens opening. There is no graduated table and no separate development change. Beyond 1000 seconds, about 16.7 minutes, the film is simply uncharacterised. The single useful rule is therefore: nothing under two minutes, +1/2 stop out to sixteen minutes, and bracket or test past that rather than extrapolate.

This is not new behaviour invented for the reissue. The figures are inherited unchanged from the original Neopan Acros (I), which specified the same 120-second threshold and the same +1/2 stop to 1000 seconds. Acros II is, on this measure, the original emulsion’s discipline carried forward intact.

How Other Films Compare

Set the films against one metered reading of 10 seconds and the gap is plain.

• Acros II: no correction. Expose 10 seconds, develop normally.
• Kodak T-Max 100 (Publication E-31, July 2002, Table 3): +1/2 stop, or extend to about 15 seconds; no development change.
• Ilford HP5 Plus (HARMAN reciprocity sheet, v2, Dec 2023): the correction follows Tc = Tm^P with P = 1.31, so 10^1.31 gives a corrected 20 seconds.
• Kodak Tri-X 400 (E-31, Table 1): +2 stops, or an adjusted 50 seconds, and a 20% development cut to hold the contrast back.
• Fomapan 100 (Foma data sheet): already off the rails at one second, where it needs a full stop, so by ten seconds it is well into multi-stop territory.

Most of these films begin demanding compensation around the one-second mark. T-Max 100 and Ilford’s emulsions are well-behaved, but they still correct from roughly a second up. Acros II carries a metered reading straight through to two minutes before asking for anything at all.

A Worked Example

Meter a quiet night scene and the reading comes back as f/8 at 8 seconds. On Acros II you set exactly that, f/8 for 8 seconds, and develop as normal: D-76 1:1 for 10.5 minutes at 20°C, continuous agitation for the first minute then five seconds each minute after. The negative comes off the reel with the tonal relationships the meter promised.

Shoot the same scene on Tri-X 400 and the 8-second metered value sits where Kodak’s chart calls for roughly +2 stops, so you open up to f/4 or run the shutter to about 35 seconds. Then, because the long exposure has stretched the shadows, you also cut development by around 20% to bring the contrast back. The Acros II frame needed one decision; the Tri-X frame needed three.

The Grain and Sensitivity Behind It

Acros II is a medium-speed orthopanchromatic negative film rated ISO 100/21°, on a 0.134 mm grey triacetate (TAC) base. Orthopanchromatic means it is sensitive across the visible spectrum but with the red response held back relative to a true panchromatic film, which lifts reds slightly and keeps greens and skin tones from going chalky, part of why its tonal scale reads so cleanly.

Fujifilm credits two technologies: Super Fine-Sigma Grain Technology for the grain itself, and P.I.D.C. (Precision Iodine Distribution Control) Technology for processing stability. The data sheet claims the world’s highest standard in grain quality among ISO-100 black-and-white films, and the measured numbers back the sharpness: a diffuse RMS granularity of 7 (Microfine, 1.0 above base-plus-fog) and resolving power of 60 lines/mm at 1.6:1 chart contrast, rising to 200 lines/mm at 1000:1. Development is undemanding across a range of soups, D-76 stock at 7.25 minutes / 20°C, ID-11 at 6.75 minutes, Microfine stock at 10 minutes, or Super Prodol at EI 80 for 4.25 minutes.

That low granularity is what makes the reciprocity behaviour worth having. A frame held open for ninety seconds with no correction also carries no compensation-induced grain growth, no development pull, and none of the shadow compression that would coarsen the result on a film fighting its own reciprocity curve. The clean negative and the flat 120-second response are the same advantage seen from two directions.