SDR — Standard Dynamic Range — is the quiet default everything has used since color TV and early digital cameras: a reference white around 100 nits, a gamma-ish transfer curve (sRGB), and a color gamut roughly matching what a 1990s CRT could show. It works everywhere, which is exactly why it's still the baseline HDR has to stay compatible with.
Three things HDR actually changes
Peak brightness. SDR's reference white sits at 100 nits. HDR formats can specify highlights far above that — a bright sky, a light bulb, a reflection — without needing to compress them down to fit.
Transfer function. SDR uses the sRGB curve, tuned for a fixed, modest brightness range. HDR typically uses PQ (absolute, mapped directly to real nit values) or HLG (relative, scene-based, broadcast-friendly) instead — both distribute code values very differently across the brightness range.
Often, gamut too. HDR content is frequently paired with a wider color gamut like Rec.2020 or Display P3. This is a separate axis from dynamic range — you can have wide-gamut SDR and (in principle) narrow-gamut HDR — but in practice they tend to travel together.
What doesn't change
Resolution, file format compatibility (mostly), and — for a well-built HDR file — how it looks on a screen that doesn't support HDR at all. A gain-map HDR JPEG's base image is a normal SDR JPEG; an SDR-only viewer just shows that and never knows there was more available.
Seeing it for yourself
The honest complication: whether you actually SEE a difference depends on your specific screen and browser reporting real HDR support, not just on the file. Our Preview tool shows you the real file (labeled "Actual HDR preview") next to a tone-mapped SDR approximation (labeled "Simulated HDR preview") side by side, and tells you plainly which one your current display reports it can actually do — no guessing.