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 panchromatic films built on conventional, non-tabular silver-halide emulsions, which is the first thing worth getting right: neither datasheet calls the grain “cubic.” Kodak’s F-4017 (May 2007) lists Tri-X 400’s features as fine grain, wide exposure latitude, high sharpness and high resolving power, and Ilford’s HP5 Plus sheet (November 2018) describes a traditional medium-speed emulsion. What they are not is a T-grain film: that distinction belongs to T-Max 400 and Delta 400, whose flattened tabular crystals are a separate engineering choice. The differences between HP5 Plus and Tri-X are real but narrow, and they show up in tonal shape, push behaviour, and reciprocity rather than in any headline specification. Kodak’s sheet warns that its published curves are representative of production coatings and do not apply directly to a particular box or roll, so cite the version you are working from.

Rated Speed and the Characteristic Curve

Both films meter as ISO 400. Ilford rates HP5 Plus at ISO 400/27 and recommends an exposure-index range of EI 400/27 to EI 3200/36, noting that this range “is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard.” Kodak rates Tri-X 400 (400TX) at ISO 400, and its processing recommendations are built to land on a contrast index of 0.56. That single number is the anchor the comparison needs: “normal contrast” for Tri-X is not a feeling, it is CI 0.56, and development time is the lever that sets it.

That matters because contrast is development-controlled. Gamma rises with time in the developer, so any claim that “HP5 is lower contrast than Tri-X” is only meaningful at matched CI. Each maker publishes its characteristic curve under different conditions: Ilford’s HP5 Plus curve is plotted for Ilfotec HC 1+31, 6 1/2 min at 20C with intermittent agitation; Kodak’s Tri-X curves come from D-76 and T-MAX at the times listed on the sheet. To put both films on the same CI you read the contrast-index-versus-time curve for each developer, pick the time that gives 0.56 (or whatever target you want), and develop to that. Comparing tonal “shape” without stating developer, time and agitation is comparing two undefined processes.

A Corrected Development-Time Table

The numbers most often misquoted for these films are the normal-development times. Here are the datasheet figures for the two developers the films share, at 20C, box speed, small tank:

Developer	HP5 Plus	Tri-X 400
ID-11 / D-76 stock	7 1/2 min (ID-11)	6 3/4 min (D-76)
HC-110 dilution B	5 min	3 3/4 min

For reference, Tri-X in D-76 1:1 is 9 3/4 min small tank (11 min large), and Kodak notes that tank times below 5 minutes risk uneven development. HP5 Plus also runs 9 min in Ilfotec DD-X 1+4 and 6 1/2 min in Microphen stock. These are close enough to confirm the two films occupy nearly the same operating envelope, with Tri-X needing slightly less time to reach a working contrast in the same soup.

Pushing: A Worked Example

Both films are formulated to push, which is much of why they remain available-light standards. Kodak tiers it explicitly: underexpose by one stop and use normal times (slight shadow loss); by two stops to EI 1600 with increased development (more contrast, more grain, lost shadow detail); by three stops to EI 3200 with further increased development, acceptable for some applications, and shoot a test roll first.

Take a two-stop push to EI 1600, 20C, small tank, 30-second agitation, as a reproducible side-by-side. Tri-X 400 in D-76 stock is 9 1/2 min; HP5 Plus in Microphen stock is 11 min, or in Ilfotec DD-X 1+4 is 13 min. Push to the EI 3200 tier and Tri-X in D-76 is 11 min, while HP5 Plus needs 20 min in DD-X 1+4 or 16 min in Microphen stock; Kodak lists HC-110(B) as not recommended at EI 3200. Ilford pairs EI 1600 and 3200 specifically with DD-X or Microphen for the best image quality and maximum film speed. Both makers name the same penalties at every tier: rising contrast, coarser grain, and progressively crushed shadows.

Grain, and Why One Number Cannot Rank It

Kodak publishes a diffuse RMS granularity of 17 for Tri-X 400, classified “fine,” read at net diffuse density 1.0 with a 48-micrometre aperture at 12x magnification. Two caveats make that figure nearly useless as a head-to-head ranking. First, the reading is based on development in HC-110(B) and was generated on the older versions of these films, with Kodak noting only that granularity testing in several developers suggests it should carry over to the current film. Second, RMS depends entirely on developer, density and aperture, so the same film returns a different number under different conditions. Tri-X 320 (320TXP), a genuinely finer and separate film, reads 16 under identical conditions, which shows how little a single point of RMS means.

Ilford publishes no RMS value for HP5 Plus, so any claim that one film is “more open” or “tighter” than the other is a perception, not a measurement. In practice the perceived difference is dominated by developer choice and degree of enlargement, not emulsion identity: both films tighten in fine-grain developers like Perceptol and coarsen visibly when pushed.

Reciprocity and Long Exposures

This is the cleanest handling divergence between the two. HP5 Plus needs no reciprocity correction at all between 1/2 s and 1/10000 s; beyond 1/2 s, the adjusted time is Ta = Tm^1.31, where Tm is the metered time. A metered 10 s becomes roughly 20 s; a metered 50 s becomes about 170 s. Tri-X instead uses a stepped table: at a metered 1 s, open up one stop and cut development 10%; at 10 s, two stops and cut 20%; at 100 s, three stops and cut 30%. At short exposures Tri-X also wants help, adding 1/2 stop at 1/10000 s.

The consequence for night and long-exposure work is concrete. For a metered 10-second exposure, HP5 Plus simply runs about 20 seconds with no development change, while Tri-X wants a two-stop exposure increase and a 20% cut in development time to hold contrast. Knowing which film is on the spool changes the calculation at the moment of exposure, not just in the darkroom.

Handling and Workflow

Some of the “short fixing and washing” reputation HP5 Plus carries is documented rather than folklore. Ilford publishes Ilfostop 1+19 for 10 s at 20C, Ilford Rapid Fixer or Hypam 1+4 for 2-5 min at 20C, and an economical wash sequence after fixing: fill the tank and invert five times, drain and refill and invert ten times, then drain, refill and invert twenty times, finishing with Ilfotol 1+200 as a wetting agent. Its agitation regime is four inversions in the first 10 s, then four inversions during the first 10 s of each subsequent minute, reducing times by up to 15% for continuous tray agitation. Kodak’s Tri-X small-tank routine is initial agitation for the first 30 seconds then 5 seconds every 30 seconds, with total darkness for loading and, if a safelight is unavoidable, a Kodak No.3 dark green filter behind a 15-watt bulb at 1.2 m or more, used only after development is half complete.

The base materials differ too. HP5 Plus is coated on 0.125 mm (5-mil) acetate in 35mm, 0.110 mm (4-mil) clear acetate edge-numbered 1-19 in 120, and 0.180 mm (7-mil) polyester in sheet film; Kodak’s F-4017 quotes a base only for the roll-film Tri-X 320, which sits on a 3.9-mil acetate base, and does not state a base thickness for Tri-X 400. Both are panchromatic, with HP5 Plus characterised to tungsten light at 2850K and Tri-X sensitive out to roughly 650nm, so filter factors differ: a Wratten No.25 red costs Tri-X eight stops’ worth of light in daylight (a factor of 8). None of this changes the conclusion that the two films share an operating envelope, but it is the difference between knowing why your negatives look as they do and guessing.