Hong Life CalculatorsUnderstanding the exposure value calculator — concept and principle
Exposure value (EV) is a single number that sums up a combination of aperture and shutter speed. This calculator gives the EV of your settings, converts it to the scene brightness at ISO 100 (EV₁₀₀) and compares it with the "Sunny 16" rule. Photographers use EV to swap settings without changing exposure, to read light meter tables and to judge how bright a scene really is.
How it is calculated
- EV = log₂(N² / t), where N is the f-number and t is the shutter time in seconds.
- Scene EV₁₀₀ = EV − log₂(ISO / 100).
- Shutter is displayed as 1/(1/t) seconds.
- Sunny 16 check: if EV₁₀₀ is within 1 stop of 15 the result reads "≈ bright sun"; above that "brighter than sun", below "darker than sun".
Results are rounded to two decimals.
The principle
The light reaching the sensor is proportional to t and to the aperture area, which scales with 1/N². N²/t is therefore inversely proportional to the light admitted, and taking log base 2 turns it into stops: each +1 EV means half as much light reaches the sensor. Different pairs with the same EV give the same exposure, for example f/8 at 1/125 s and f/5.6 at 1/250 s. In the APEX system this is EV = Av + Tv, with Av = log₂N² and Tv = log₂(1/t). Raising ISO by one stop makes the sensor need half the light, so the same settings correspond to a scene one stop darker; hence the ISO correction.
Worked example
Defaults: f/8, 0.008 s (1/125 s), ISO 100.
- N² / t = 64 / 0.008 = 8,000.
- EV = log₂(8,000) ≈ 12.97 (Av = 6, Tv = log₂125 ≈ 6.97).
- ISO 100, so EV₁₀₀ = 12.97 − log₂(1) = 12.97.
- Shutter: 1 / 0.008 = 125, shown as 1/125 s.
- 12.97 is about 2 stops below 15, so the check reads "darker than sun".
These match the calculator's output.
How to read the result
Typical EV₁₀₀ values: bright sun on sand or snow about 16, bright sun about 15, hazy sun about 14, overcast about 12–13; indoor room lighting is far lower, often single digits. EV 13 at ISO 100 suits a cloudy-bright or open-shade day. A "darker than sun" verdict is not an error, it only says the scene is dimmer than direct sunlight.
Common mistakes
- Entering 125 instead of 0.008 for the shutter speed; the field takes seconds.
- Assuming higher EV means a brighter photo. Higher EV settings let in less light.
- Using marked f-numbers (f/5.6, f/11) as exact values; they are rounded, so EV results can differ by a few hundredths.
Frequently asked questions
What is the EV of f/8 at 1/125 s?
About 12.97, usually called EV 13.
What is the Sunny 16 rule?
In bright sun, f/16 with a shutter speed of about 1/ISO seconds gives a correct exposure, which corresponds to EV₁₀₀ ≈ 15.
Does ISO change the EV of my settings?
No. ISO changes the scene brightness those settings are right for, which is EV₁₀₀.
Sources: ISO 2720, General purpose photographic exposure meters (photoelectric type) — Guide to product specification · S. F. Ray, Applied Photographic Optics, Focal Press