Answer
What is ISO in photography?
ISO controls the sensor's sensitivity to light. Low ISO (100) for bright scenes and clean images; high ISO (3200-12800) for dim scenes at the cost of noise. Modern cameras handle high ISO far better than they did a decade ago.
The exposure triangle leg
ISO is one of the three controls (with shutter speed and aperture) that determine exposure. Doubling ISO doubles brightness — and adds noise.
When to push ISO
- Indoor / low-light handheld photography
- Action where you cannot lengthen shutter speed
- Astrophotography (3200-6400 is normal)
ISO ceilings to know
- Modern full frame: clean to ISO 6400, usable to 25,600
- Modern APS-C: clean to ISO 3200, usable to 12,800
- Phones: aggressive noise reduction makes ISO numbers misleading
The noise reduction trade-off
High ISO noise can be cleaned in post (Lightroom, DxO PureRAW, Topaz). For social media output, ISO 12,800 is now perfectly publishable on full frame.
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