Exposure Calculator

The calculation of photographic exposure relies on the fundamental relationship between Exposure Value (EV), aperture, ISO, and shutter speed (time). The core mathematical formula is expressed as:

EV = log₂( (100 × Aperture²) / (ISO × Time) )

In this logarithmic scale, every whole integer change in EV represents a doubling or halving of the total light reaching the camera sensor. By inputting known variables like your ISO and desired aperture, the calculator resolves the remaining parameters needed for a balanced exposure.

Let us walk through a practical scenario where you are shooting outdoors during golden hour. You decide to set your ISO to 400 and your camera aperture to f/4.0, while your shutter speed is set to 1/250 of a second.

First, square the aperture value: 4² = 16. Next, multiply this by 100 (the base ISO): 16 × 100 = 1600. Then, multiply your ISO (400) by the time value in seconds (1/250 or 0.004): 400 × 0.004 = 1.6. Divide the numerator by the denominator: 1600 / 1.6 = 1000. Finally, take the base-2 logarithm of 1000, which yields an Exposure Value (EV) of approximately 9.97. This tells you precisely how bright your scene is and helps you cross-reference reciprocity adjustments if lighting conditions shift.

Always verify your base ISO before calculating, as digital sensors react differently to native versus boosted ISO ranges. Remember that changing one pillar of the exposure triangle requires an equal and opposite adjustment in another to maintain the same EV. Finally, use histogram checks on your camera screen alongside this calculator to prevent clipped highlights or crushed shadows in high-contrast scenes.