How is ISO calculated from exposure settings?

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).

What ISO should I use to avoid noise?

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.

What is EV (exposure value)?

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.

Does ISO affect image quality?

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.

PhotoCalcs

ISO Calculator — Required ISO for Target Exposure

Calculate the ISO needed for correct exposure given your aperture, shutter speed and lighting conditions.

Aperture (f-number)

e.g. 5.6

Shutter Speed (1/x)

Enter denominator, e.g. 125 for 1/125s

Lighting ConditionsScene brightness in EV

How We Calculate This

This calculator uses the standard photographic exposure equation to find the required ISO:

Exposure Equation

EV_camera = log2(N² / t)

Where N = f-number and t = shutter speed in seconds (1/125 = 0.008s).

ISO Calculation

ISO = 100 × 2^(EV_camera - EV_scene)

The result is then matched to the nearest standard ISO value (100, 200, 400, 800, etc.).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.