# Exposure Value Calculator

> Computes the exposure value EV = log2(N²/t) of a camera setting, the ISO-100 normalised light value EV100, an illuminance estimate in lux (incident-meter calibration C = 250) and a description of the lighting the settings correspond to.

- Calculator id: `exposure-value` · Category: Photography & Optics (`photography`) · Tool name: `calculate_exposure_value`
- Canonical page: https://tttkmbb.com/photography/exposure-value · This document: https://tttkmbb.com/photography/exposure-value.md · JSON definition: https://tttkmbb.com/photography/exposure-value.json

## Purpose

Computes the exposure value EV = log2(N²/t) of a camera setting, the ISO-100 normalised light value EV100, an illuminance estimate in lux (incident-meter calibration C = 250) and a description of the lighting the settings correspond to.

**Use when:** You want to express aperture, shutter and ISO as a single exposure number, estimate how bright a scene is, or check settings against the Sunny 16 rule.

**Do not use when:** You want to translate one setting into another with the same exposure (use equivalent-exposure) or to work out a long exposure with an ND filter (use nd-filter-exposure).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `aperture_f` | number |  | required | Lens f-number, e.g. 2.8 for f/2.8. (min 0.5, max 128) |
| `shutter_seconds` | number | s | required | Exposure time in seconds. Enter fractions as decimals: 1/125 s = 0.008, 1/60 s = 0.016667, 1/1000 s = 0.001. (> 0, max 86400) |
| `iso` | number |  | optional, default 100 | ISO sensitivity setting (100, 200, 400, ...). (min 1, max 10000000) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `ev` | number | EV | log2(aperture_f² / shutter_seconds), independent of ISO. |
| `ev100` | number | EV | ev − log2(iso / 100): the scene brightness these settings expose correctly. |
| `illuminance_lux_estimate` | number | lx | 2.5 × 2^ev100 lux, the incident-light meter relation with calibration constant C = 250 (average subject reflectance assumed). |
| `lighting_description` | string |  | Scene type that corresponds to ev100 (EV 15 bright sun, EV 12–13 overcast, EV 7–8 bright indoor, EV 4–6 dim indoor, EV ≤ 0 night). |
| `shutter_text` | string |  | The shutter speed as a conventional fraction, e.g. '1/125 s'. |

## Formula

`ev = log2(aperture_f² / shutter_seconds); ev100 = ev − log2(iso / 100); illuminance_lux = 250 × 2^ev / iso = 2.5 × 2^ev100`

EV definition per the APEX system; the lux relation follows the ISO 2720 incident-meter equation EV = log2(E·S / C) with C = 250 lx·s, so it assumes a correctly exposed scene of average reflectance and is only an estimate (±1 EV).

## Data Sources

- Wikipedia – Exposure value — https://en.wikipedia.org/wiki/Exposure_value (reference, retrieved 2026-09-24)
- Wikipedia – Light meter — https://en.wikipedia.org/wiki/Light_meter (reference, retrieved 2026-09-24)
- Wikipedia – Sunny 16 rule — https://en.wikipedia.org/wiki/Sunny_16_rule (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/exposure-value?aperture_f=…&shutter_seconds=…`
- `POST https://tttkmbb.com/api/v1/calculate/exposure-value` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/exposure-value · OpenAPI operationId `calculate_exposure_value` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "exposure-value", "inputs": {…}}`

## Example

- Sunny 16: f/16, 1/125 s, ISO 100: inputs `{"aperture_f":16,"shutter_seconds":0.008,"iso":100}` → `{"ev":15,"ev100":15,"illuminance_lux_estimate":80000,"lighting_description":"Bright or hazy sun with distinct shadows (EV 15, Sunny 16 rule)"}`
- Indoor: f/2.8, 1/60 s, ISO 400: inputs `{"aperture_f":2.8,"shutter_seconds":0.016667,"iso":400}` → `{"ev":8.9,"ev100":6.9,"lighting_description":"Bright indoor lighting: offices, shops, well-lit rooms (EV 7–8)"}`

```
GET https://tttkmbb.com/api/v1/calculate/exposure-value?aperture_f=16&shutter_seconds=0.008&iso=100
```

## Limitations

You want to translate one setting into another with the same exposure (use equivalent-exposure) or to work out a long exposure with an ND filter (use nd-filter-exposure). EV definition per the APEX system; the lux relation follows the ISO 2720 incident-meter equation EV = log2(E·S / C) with C = 250 lx·s, so it assumes a correctly exposed scene of average reflectance and is only an estimate (±1 EV). All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**What is the difference between EV and EV100?**

EV only combines aperture and shutter. EV100 also removes the ISO, so it describes the light in the scene: f/2.8 at 1/60 s is EV 8.9 at any ISO, but at ISO 400 it exposes an EV100 6.9 scene correctly.

**Why is the lux value only an estimate?**

Camera meters read reflected light and are calibrated for mid-grey subjects; the conversion assumes that and uses C = 250, while meter makers use constants between 240 and 340, so real illuminance can differ by about ±1 EV.

**How do I enter 1/250 s?**

As a decimal: 1/250 = 0.004 s. Common values: 1/1000 = 0.001, 1/500 = 0.002, 1/125 = 0.008, 1/60 = 0.016667, 1/30 = 0.033333.

## Related

- [Equivalent Exposure Calculator](https://tttkmbb.com/photography/equivalent-exposure.md) — Find other aperture/shutter/ISO combinations with the same exposure.
- [ND Filter Exposure Calculator](https://tttkmbb.com/photography/nd-filter-exposure.md) — Shutter speed after adding a neutral-density filter.
- [Flash Guide Number Calculator](https://tttkmbb.com/photography/flash-guide-number.md) — Aperture or distance for a flash from its guide number.