ISO

Sensitivity to light. More precisely, on digital cameras ISO is how much the captured signal gets amplified.

• Low ISO (100, 200): little amplification → clean, but needs lots of light
• High ISO (3200, 12800): heavy amplification → shoots in the dark, but noise/grain gets amplified too

The trap of the word "sensitivity"

In the film era, ISO really was the chemical sensitivity of the film. On digital, the sensor's physical sensitivity is fixed; the question is how much gain to apply during signal processing.

Which is why shooting at high ISO ≈ shooting at low ISO and pushing exposure in post (on ISO-invariant sensors).

Why noise appears

Amplifying a signal also amplifies noise. In dark scenes the original signal is small, so the SNR (signal-to-noise ratio) gets noticeably worse when boosted.

Base ISO

The native sensitivity at which the sensor performs cleanest. Usually ISO 100 or 200.

• Dynamic range is widest at base ISO.
• Anything below it (e.g., ISO 50) is often "extended ISO" → tends to clip highlights instead of helping.

Dual Native ISO

Some cameras have two native sensitivities built into the sensor circuit. Above a threshold, the second circuit kicks in and noise drops again.

→ On these cameras you get the counterintuitive result that ISO 12800 can be cleaner than ISO 6400. If your camera has dual native, lean into the second base.

Practical heuristics

• Bright scenes: lock to base ISO.
• Low-light stills: push shutter to handheld limit, open aperture, and only then raise ISO.
• Low-light video: shutter is locked, so → open aperture → jump ISO to native 2.

Related

• exposure-triangle — Parent note
• aperture
• shutter-speed