Exposição Equivalente

ISO equivalent exposure: keeping brightness constant

Double the ISO and you've gained 1 stop of light. To keep the brightness where it was, close the aperture by a stop or cut the shutter time in half. So ISO 100 + 1/100s lands at the same exposure as ISO 200 + 1/200s or ISO 400 + 1/400s, and the formula t₂ = t₁ · (ISO₁/ISO₂) tells you the new shutter time. Shooting at your sensor's base ISO — usually 100 or 64 — gets you the widest dynamic range and the cleanest image. Push the ISO up and you pick up shot noise while losing room in the highlights. A handful of modern sensors are dual native ISO (the Sony A7S III at ISO 640 and ISO 12800; ARRI Alexa, RED, the Canon C-series cinema bodies). These have a second analog gain stage that resets the noise floor once you reach the higher value.

Applications

Fast reciprocity tweaks when the light shifts in the middle of a scene. Handheld work in low light and astrophotography. High-ISO video on broadcast and event jobs. And sizing up how different camera bodies behave at the same sensitivity before you commit to one for a shoot.

FAQ

Does ISO add light to the sensor? No. ISO is gain applied after capture; only aperture and shutter change the actual light reaching the sensor.

Why not always shoot at base ISO? Low ISO requires longer shutter or wider aperture, which can mean motion blur or insufficient depth of field — you trade noise for sharpness.

What is the second native ISO good for? On dual native ISO cameras, jumping to the higher base (e.g. 12800) gives cleaner shadows in dark scenes than gaining up from the lower base.