Put an ISO 100 frame and an ISO 3200 frame of the same scene side by side, and the 3200 frame can look sharper at a glance, before you look closer and realize it's actually resolving less real detail. That's not a contradiction. It's two different things both called "sharpness," and film speed pulls them in opposite directions.
The crystal-size story is the familiar one: faster film needs bigger silver halide crystals to reach a latent image with fewer photons, and bigger crystals develop into bigger, more visible clusters of density. Why ISO 400 is coarser than ISO 100 covers that mechanism directly, and ISO 3200 just continues the same trend further — crystals large enough to be individually distinct even at a normal viewing distance, not just under a loupe.
What that post doesn't get into is what those bigger crystals do to an edge once you have them.
Two Different Kinds of "Sharp"
Resolution is how much fine detail a system can separate — two closely spaced lines, still visible as two lines rather than one blur. Acutance is how abrupt an edge looks — how quickly a transition from dark to light appears to happen, regardless of how much fine detail survives around it. A system can lose resolution and gain acutance at the same time, and that's exactly what happens as grain coarsens.
A large silver crystal sitting near a real edge in the scene creates a local overshoot: density that's slightly higher or lower than the smooth transition the optics alone would have produced, concentrated right at the boundary. Your visual system reads that overshoot as extra edge contrast. The edge looks more defined, more "punchy" — even though the crystal responsible for it is a random artifact of grain structure, not new information about the scene.
Fine grain doesn't do this as strongly. Small crystals average out over an edge more smoothly, tracking the true optical transition more faithfully, but without the exaggerated contrast the large-crystal overshoot provides.
Three Points, Not Two
ISO 100 sits at one end: crystals small enough that grain mostly stays invisible at normal viewing sizes, edges rendered close to what the lens actually delivered, acutance neither boosted nor suppressed by the emulsion.
ISO 400 is the point where the trade starts becoming visible rather than theoretical. Crystals are large enough to contribute a real acutance boost at edges, without yet being so large that the boost overwhelms genuine fine detail. This is a large part of why push-processed or native 400-speed stocks read as "gritty but sharp" rather than simply "soft and grainy" — the grain is adding apparent edge contrast at the same time it's adding visible texture.
ISO 3200 crosses into the range where the crystals are no longer small relative to the fine detail in the frame. At that scale, the "overshoot" isn't a subtle nudge on top of a resolved edge anymore — the grain itself is comparable in size to the detail you're trying to resolve, so grain and information start competing for the same space in the image rather than grain merely riding on top of it. Apparent sharpness can still read as high, because acutance from grain overshoot is doing a lot of the perceptual work, but the underlying resolution — how much of the scene's actual fine structure survived — has measurably dropped.
Why This Trips People Up
It's tempting to treat "grainy" and "soft" as synonyms, or their opposites as automatically paired: fine grain should mean sharp, coarse grain should mean soft. The acutance effect breaks that pairing. A coarse-grained ISO 3200 frame can out-punch a technically higher-resolution ISO 100 frame in a quick look, because punch and resolution aren't the same axis.
This is also why grain structure and lens sharpness interact rather than simply stack. A very sharp lens paired with a coarse-grained stock hands you a genuinely well-resolved edge for the grain to add its overshoot on top of — often the most "bitingly sharp" combination in practice. A soft lens paired with the same stock has less real edge for the grain to punch up, so the grain reads as texture sitting on top of a soft image instead of texture reinforcing a sharp one.
None of this changes the underlying trade from the crystal-size story: bigger crystals still mean less recoverable fine detail, full stop. What changes is how that loss actually presents itself to your eye — not as an even, obvious softening, but as a texture that can disguise its own cost by making the image look more assertive at the exact edges where detail is being lost. You can compare the two effects directly by adjusting grain and sharpening separately in the Cineon studio and watching where apparent sharpness and real resolved detail start to diverge.