Ilford HP5 Plus and Kodak Professional Tri-X 400 are the two black and white emulsions most often treated as interchangeable defaults at ISO 400. Both are traditional cubic-grain, panchromatic films that have remained in production for decades, and both tolerate a wide range of handling. The differences between them are real but narrow, and they show up in tonal shape, grain structure, and how each responds to pushing rather than in any single headline specification. Understanding where the two diverge is more useful than ranking one above the other.

Rated Speed and Tonal Response

Both films are nominally ISO 400. Ilford’s HP5 Plus datasheet rates it at ISO 400/27 and notes that this exposure index is based on a practical evaluation of film speed rather than strictly on the foot-speed point defined by the ISO standard. Kodak likewise rates Tri-X 400 (400TX) at ISO 400 for standard processing.

The practical distinction lies in the shape of the characteristic curve. HP5 Plus is generally described as the longer, gentler curve of the two, holding a smooth gradient through the shadows and into the upper midtones, which tends to render lower-contrast scenes with an even, slightly muted tonality. Tri-X carries marginally more inherent contrast and a more pronounced shoulder, giving highlights a characteristic roll-off and midtones a little more local separation. Neither is a high-contrast film; the gap is a matter of degree, and developer choice can move either curve enough to overlap with the other.

Grain Structure

Grain is where published data is least symmetrical, because the two manufacturers report it differently. Kodak’s Tri-X datasheet specifies a diffuse RMS granularity of 17, classified as “fine,” read at a net diffuse density of 1.0 using a 48-micrometre aperture at 12x magnification, with the figure based on development in HC-110. Ilford does not publish an RMS granularity value for HP5 Plus, so a direct numerical comparison is not possible from the datasheets alone.

In practice the two are close, with HP5 Plus usually perceived as having a slightly more open, pronounced grain pattern and Tri-X a marginally tighter one at box speed. Both pull their grain structure tighter in fine-grain developers and coarsen visibly when pushed.

Development Latitude and Pushing

Both films are formulated for push-processing, which is much of why they remain working standards for available-light shooting. Kodak states that Tri-X 400 can be underexposed by up to three stops, to roughly EI 3200, when development time is increased, while warning of rising contrast, increased graininess, and loss of shadow detail. For normal processing, the Tri-X datasheet lists 7 minutes 45 seconds in D-76 and 4 minutes 30 seconds in HC-110 dilution B at 68 degrees Fahrenheit (20 degrees Celsius) for large-tank processing.

Ilford’s datasheet gives HP5 Plus a recommended exposure index range from EI 400/27 up to EI 3200/36 with extended development, citing developers such as Ilfotec DD-X and Microphen for the highest indices. Its published time for normal processing is 7.5 minutes in ID-11 stock and 6.5 minutes in Ilfotec HC at 1+31, both at 20 degrees Celsius. The broad agreement in these figures underlines the central point: the two films occupy nearly the same operating envelope, and the choice between them rests on the subtle tonal and grain preferences above rather than any decisive technical advantage.