Ilford Ortho Plus 80

Datasheet-derived original, tier: ⚠datasheet-validated, partial-fidelity. Primary source: Ilford Ortho Plus technical information, Oct 2019. Archived at X-T5/_reference-sources/datasheets/ilford/ortho-plus.pdf, 5 pages, read in full. Settings in the notes.

What the datasheet says

Filter factors, and note that they invert between light sources:

Finding 1: the spectral story is told twice in plain numbers

Most datasheets describe spectral sensitivity only as a wedge spectrogram, a chart. Ilford's Ortho Plus sheet encodes it twice more, in figures you can act on.

The speed split: ISO 80 daylight, ISO 40 tungsten, exactly one stop.

Tungsten light is red-rich and blue-poor. A film blind to red therefore loses half its effective sensitivity the moment you move it indoors. That single pair of numbers is a quantitative statement of the film's red blindness, and it is unusually direct. Most emulsions publish one speed and leave the spectral behaviour to a graph.

The filter table inverts, for the same reason. Under tungsten a yellow filter has a factor of 1. It costs nothing, because there is scarcely any blue left for it to remove. A blue filter costs more under tungsten (5 vs 3) because blue is the scarce commodity there. You can read the film's spectral response straight off that table without ever seeing the spectrogram.

This is worth recording as a general technique: where a datasheet gives filter factors under two light sources, the ratio between them is a spectral measurement in disguise.

Finding 2: this is the spectral inverse of SFX 200, and the bank now brackets the spectrum

Two films at opposite ends of the same axis, both derived from Ilford sheets, and. Instructively, both land in the same partial-fidelity tier for the same structural reason.

The reason is worth stating as a principle: an in-camera colour filter subtracts a band from a panchromatic capture. An emulsion's spectral sensitivity is a shape, a curve that can be high in one region and zero in another, independently. Subtraction cannot reproduce an arbitrary shape. That is why:

• SFX needs sensitivity above where the sensor's IR-cut filter stops → unreachable.
• Ortho needs red suppressed and blue lifted → no single filter does both.

The bank's core spectral argument now has two worked examples from opposite directions.

The blue problem, and why this recipe makes an experiment urgent

ACROS+G gets the reds right and the blues wrong. A green filter darkens red and blue; an ortho emulsion darkens red while remaining fully sensitive to blue. On real Ortho Plus a blue sky renders pale to blown, the chalk-white skies of pre-1930s photography. On this recipe the sky goes darker.

But there may be a way out, and it hinges on a question this bank has already flagged as contested.

the notes records that five B&W recipes here carry large, deliberate, differing WB shifts on monochrome bases, including Plus-X 125 at —/—, which its author states he derived directly from the film's published spectral-sensitivity curve and field-tested for two years. The bank's own documentation had claimed WB has "little effect on the grey tones themselves" on monochrome bases; that claim is now marked contested.

If WB does act as a channel mixer before the monochrome conversion, then a strong −Red / +Blue shift on plain ACROS would suppress red and lift blue at the same time, which is precisely orthochromatic response. A filter cannot do that. A channel mixer can.

So the cheap experiment already specified, one scene containing saturated red, green and blue plus a grey scale, shot on Monochrome at —/0, —/—and —/—. Has a concrete payoff: it either hands Ortho Plus a genuinely faithful build, or it confirms ACROS+G as the ceiling and closes the question.

Ortho Plus is now the strongest argument for running that test. Recorded here and cross-linked from the recipe.

Finding 3: best reciprocity characteristic in the bank

Ta = Tm^1.25, with no correction needed out to a full second.

That is the lowest exponent of the three Ilford sheets in this collection that publish a formula:

The differences are modest against FP4 but real, and they compound. Combined with ISO 80, a high-resolution copy design and availability in sheet film, everything about the emulsion points at slow, considered, tripod work. Architecture, still life, copy stands, long exposures.

Changes no digital setting. Recorded because it says what the film was built to do.

Where each setting comes from

• ACROS+G: the only filter that darkens red, which the datasheet names as the film's visible signature. Note it is being used against its marketed purpose: Fujifilm sells +G as the portrait filter for "pleasing skin tones." Here it produces the opposite, which is faithful. Ortho renders lips and blemishes emphatically dark, and that is why pre-panchromatic portraiture looks the way it does. Ilford does list "high quality portrait" among its applications; the portraiture meant is the stark graphic kind.
• Sharpness —: the bank's top tier, shared only with Pan F Plus 50. Earned by "high-resolution" appearing twice in the opening paragraphs, and by the film's original purpose: a copy film's entire job is resolving a flat original.
• — / Shadow —: from Gbar 0.62–0.70. Delta 400's single published Gbar 0.62 earned Highlight —/ Shadow —; Ortho's range tops out firmer, so it sits one notch harder at the highlight end. A directly comparable, datasheet-to-datasheet derivation.
• Grain —, —: ⚠inference, not a published figure. No RMS or grain index appears in the sheet. Placed by speed class (ISO 80) and stated purpose (high-resolution copy film). First thing to revisit if a scan disagrees.
• —, ISO 250—: and an honest note: box speed ISO 80 is not reachable on the X-T5, because — pins the floor at 250. The film is slower than the digital recipe can be.
• —: no edge-contrast or micro-contrast language anywhere in five pages. Same standard as Delta 100, XP2 Super and FP4 Plus.

Where it sits: eleven B&W originals

The Ilford B&W line is now nine deep and spans four emulsion technologies. Conventional (Pan F, FP4, HP5, Ortho), tabular T-grain (Delta 100/400/3200), chromogenic (XP2), and extended-red (SFX), every one datasheet-derived from an archived primary source.

Validation status

Datasheet-validated, partial-fidelity. No scan. Every setting traces to the archived sheet; the blue limitation is named in the recipe, this file and the ledger.

Highest-value evidence to add: a single frame containing both a strong red object and a blue sky. It tests both halves of the fidelity claim at once. Reds should go near-black (expected pass), sky should go pale (expected fail), and would quantify exactly how far the recipe sits from the film.

Derived: 2026-07-26