Exposure Value (EV)
Exposure & Optics · 6 specs
Direct Answer
Exposure Value (EV, standardized under ISO 2720) is a logarithmic base-2 scale quantifying combinations of shutter speed and relative aperture that yield identical photographic exposures (EV = log_2(N² / t) at ISO 100). Governs light metering, auto-exposure compensation, dynamic range benchmarks, and the classic 'Sunny 16' outdoor rule.
Source: ISO 2720:1974 (Photography — General purpose photographic exposure meters — Guide to product specification) · Last reviewed Aug 28, 2026
Specs
| Mathematical Formula | EV = log₂(N² / t) — where N is f-number and t is shutter speed in seconds (at ISO 100) |
| EV 0 Baseline | Aperture f/1.0 with a 1.0-second shutter speed at ISO 100 |
| Sunny 16 Rule (EV 15) | Bright direct outdoor sunlight = EV 15 (Set aperture to f/16, shutter speed = 1 / ISO speed, e.g. 1/100s at ISO 100) |
| Typical Real-World EV Benchmarks | EV -4: Full moon night sky | EV 4: Candlelight interior | EV 7: Typical home interior | EV 12: Overcast day | EV 15: Sunny beach/snow |
| Auto-Focus Low-Light Rating | Modern mirrorless AF sensitivity is rated down to EV -6 to EV -9 (operational in near pitch-black darkness) |
| Dynamic Range Measurement | Expressed in EV stops (e.g. 14 EV of sensor dynamic range = a 16,384:1 contrast ratio) |
The Reciprocity Principle
Because EV is a logarithmic measure of total light energy, any combination of aperture and shutter speed that satisfies the equation yields the exact same exposure. For example, at EV 12 (overcast daylight):
- f/4.0 at 1/250 s
- f/5.6 at 1/125 s
- f/8.0 at 1/60 s
- f/11.0 at 1/30 s
All four settings expose the scene with identical image brightness, allowing the photographer to choose between fast action-freezing shutter speeds or deep landscape depth-of-field.