「電影感」取決於對比曲線，而非加上一層噪點：風格預設實際改變了甚麼

"Make it look like film" sounds like a texture request. It is really four separate changes to how your pixels behave, and the difference between applying them and just stamping noise on top is the difference between a photo that reads as scanned film and a photo that reads as a filtered phone shot.

Here is what actually changes, and how to tell what a tool did.

The four components of a film look

Grain sits in the midtones, not everywhere. Organic film grain is strongest in the middle values and nearly invisible in deep shadows and blown highlights, because it is an artifact of dye clouds, not an overlay. A noise layer applied uniformly at a fixed opacity is the single most recognizable tell of a fake: grain over jet-black areas does not happen on real film.

Highlights roll off instead of clipping. Film negative handles overexposure gracefully — bright areas compress into a soft shoulder rather than snapping to white. Digital sensors clip hard. A film grade pulls that shoulder back, which is why overexposed windows in a graded photo keep some detail.

Halation adds a faint glow around bright sources. Light passing through the emulsion reflects off the film base and back into the layers, producing a red-orange halo around highlights — most visible on small bright points (street lights, jewelry, backlit hair). It is subtle; heavy halation filters read as costume.

The contrast curve is a color-negative curve, not a contrast slider. Analog color negative has a long, gentle S-curve with lifted blacks and compressed saturation in the extremes. This is the biggest single contributor to "the film feel" and the one a noise stamp completely skips: you can add perfect grain to an untouched digital curve and it still looks digital.

How to check what a tool actually did

Four checks, no expertise needed:

1. Zoom to 100% on a dark area. Grain there means a uniform noise stamp; clean shadows with grain only in midtones suggests a real grade.
2. Find a bright light source. Is there a soft glow around it (halation) or a hard white blob (clipped)? Was a faint warm rim added or nothing?
3. Compare highlights before and after. Did detail appear in previously blown areas (a shoulder was applied) or is it the same clipping with noise on top?
4. Look at the blacks. Lifted, slightly faded blacks are film; pure zeros with grain are a stamp.

If a tool fails all four, it sold you a texture, not a look.

Where style presets get more aggressive

Grading presets (film look, Polaroid fades, cross-process) stay in tonality: your edges, faces and composition are untouched. Render presets (watercolor, pencil sketch, oil painting) are a different operation entirely — the image is re-interpreted, hatching and paper texture replace pixels, and fine details like eyelashes can be simplified. Neither is wrong, but they answer different needs: grade when you want the same photo with a different mood; render when you want a different medium. Problems start when a "sketch" tool quietly re-renders faces — that is when likeness drifts, and it is worth checking at 100% before you print.

I build aiphotoeditor.best, where the film-grain preset is an AI re-grade of the whole image — grain, highlight roll-off, halation and the color-negative curve together, with your scene and faces kept intact — alongside watercolor, pencil sketch and six other style presets. The AI path is three free edits a day, no account; simple local filters, including a mono film look, are unlimited and never upload your photo.

Honest limits: a re-grade can slightly soften faces (that is the film curve doing its job), and no preset fixes an out-of-focus source — grading changes mood, not optics.