ISO Calculator — Required ISO for Target Exposure

ISO is derived from the exposure equation: EV = log2(N^2 / t) at ISO 100. If the camera EV (from your aperture and shutter speed) is higher than the scene EV, you need a higher ISO to compensate. ISO = 100 x 2^(EV_camera - EV_scene).

Use the lowest ISO that gives correct exposure with your chosen aperture and shutter speed. Modern cameras handle ISO 800-3200 well. For best quality, stay at base ISO (100 or 200) and use a tripod for slow shutter speeds. Higher ISOs introduce more noise but may be necessary in low light.

EV is a number representing a combination of aperture, shutter speed and ISO that produces the same exposure. EV 0 is f/1.0 at 1 second at ISO 100. Each increase of 1 EV halves the light. Bright sunlight is about EV 15, a dim interior is EV 7, and night scenes are EV 3 or lower.

Yes. Higher ISO amplifies the sensor signal, which also amplifies noise (grain). This reduces dynamic range and colour accuracy. Modern full-frame sensors produce usable images up to ISO 6400-12800, while APS-C and Micro Four Thirds sensors typically show more noise at the same ISO.