# Photography Exposure Value & Reciprocity Calculator Calculate Exposure Value (EV) and EV100, and compensate for film reciprocity failure during very long exposures in photography. --- - **Canonical URL:** https://dothecalculation.com/calculators/photography-exposure-value-calculator - **Category:** Hobbies & Leisure Utilities - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## Calculate Exposure Value and Reciprocity Failure Compute Exposure Value (EV), ISO-adjusted EV100, and long exposure corrections for film reciprocity failure. - Logarithmic light sensitivity scales - Film reciprocity failure adjustments - Sunny 16 rule scene descriptions ## Exposure Value (EV) and Light Measurement Science In photography, Exposure Value (EV) is a logarithmic scale representing combinations of camera shutter speed and lens aperture that yield equivalent exposures. The formula for EV at a given setting is: \(EV = \log_2 \left( \frac{N^2}{t} \right)\), where \(N\) is the f-number (aperture) and \(t\) is the shutter speed in seconds. To compare lighting conditions independent of ISO, the values are normalized to ISO 100, known as EV100: \(EV_{100} = EV - \log_2 \left( \frac{\text{ISO}}{100} \right)\). For example, bright direct sunlight corresponds to EV100 = 15 (which aligns with the "Sunny 16" rule: f/16 aperture with a shutter speed equal to 1/ISO). Indoor lighting is typically around EV100 = 5 to 7. ## Film Physics and Reciprocity Failure Corrections For ordinary exposure times (between 1/1000s and 1s), the relationship between light intensity and exposure time is reciprocal: doubling the time is equivalent to doubling the light. However, at long exposure times (typically over 1 second for analog film), this relationship breaks down. This phenomenon is called reciprocity failure, or the Schwarzschild effect. To compensate, the adjusted exposure time is calculated using Schwarzschild's law: \(t_{\text{adjusted}} = t^p\), where \(t\) is the metered exposure time and \(p\) is the reciprocity exponent of the film (typically between 1.1 and 1.4). If a film has a reciprocity factor of 1.3, a metered 10-second exposure requires an actual exposure of \(10^{1.3} \approx 20\) seconds to avoid underexposure. ## How to Use This Calculator Enter your aperture (f-stop), metered shutter speed in seconds, and ISO, then enter your film's reciprocity exponent if you are shooting long-exposure film (leave the default for digital or short exposures). The calculator returns EV, ISO-normalized EV100, the scene lighting description, and — for exposures over 1 second with a reciprocity exponent above 1.0 — the corrected exposure time. ## Worked Example: Reading a Metered f/8, 2-Second Exposure With the calculator's defaults — f/8 aperture, a 2-second metered shutter speed, ISO 100, and a reciprocity exponent of 1.3 — the Exposure Value comes to EV 5, which at ISO 100 is also EV100 5, matching the "Night street scenes / City lights" lighting category. Because the metered time (2 seconds) exceeds 1 second and the reciprocity exponent is above 1.0, the corrected exposure time is \(2^{1.3} \approx 2.46\) seconds — about 23% longer than the meter reading, which is exactly the kind of adjustment reciprocity failure requires for long film exposures. ## Related Calculators For depth-of-field planning at your chosen aperture, see the [Depth of Field & Hyperfocal Distance Calculator](/calculators/photography-dof-hyperfocal-calculator). ## Frequently asked questions ### What is Exposure Value (EV)? A logarithmic scale that combines shutter speed and aperture to represent exposure levels. ### What does EV100 mean? The exposure value standardized to an ISO of 100, allowing photographers to measure absolute ambient light levels. ### What is reciprocity failure? The loss of sensitivity in photosensitive film during extremely long or short exposures, requiring extra exposure time. ### What is the Sunny 16 rule? A method for estimating correct exposure in bright sunlight without a light meter: set aperture to f/16 and shutter speed to 1/ISO. ### Do digital sensors suffer from reciprocity failure? No. Digital sensors maintain a linear response to light even during long exposures, though sensor noise increases. ### What is the Schwarzschild exponent? A coefficient (p) representing a film emulsion's sensitivity drop during long exposures. ### How does EV relate to lux? At ISO 100, an EV of 0 corresponds to approximately 2.5 lux of illuminance. ### Why does EV decrease as f-number increases? Increasing the f-number reduces the aperture size, which reduces light transmission, requiring a longer shutter speed. ### What is a typical EV for night street photography? Night city street scenes are typically around EV 7 to EV 8 at ISO 100. ### How does doubling ISO affect EV? Doubling ISO increases the exposure sensitivity by 1 stop, equivalent to an increase of 1 EV. ### Why is EV useful if I already use aperture and shutter speed directly? EV lets you compare or communicate exposure across different aperture/shutter combinations that produce the same result — useful for matching a light meter reading to any equivalent camera setting. ### What reciprocity exponent should I use for common films? Typical values range from about 1.1 for modern films with minimal reciprocity failure to 1.3–1.4 for older or specialty emulsions — check your specific film's data sheet for the manufacturer-published exponent. ## Related concepts - **The logarithmic scale of light stops** — Aperture and shutter progression in powers of two. - **Schwarzschild compensation** — Mathematical corrections for silver halide chemical responses. - **Sunny 16 exposure model** — Solar irradiance baselines for camera sensor calibration. ## Related guides - [Understanding Calculator Formulas: How DTC Turns Inputs into Results](https://dothecalculation.com/blog/site-guides/understanding-calculator-formulas) — Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision. ## Related calculators - [Crochet Blanket Size & Yardage Calculator](https://dothecalculation.com/calculators/crochet-blanket-dimensions-calculator) — Calculate the yarn yardage, skein count, and finished blanket weight based on your stitch density, gauge, and yarn class. - [Geocaching Coordinates Distance & Bearing Solver](https://dothecalculation.com/calculators/geocaching-distance-bearing-calculator) — Solve the exact distance and compass bearing between two GPS coordinates, or project a new waypoint using the Haversine formula. - [Model Rocket Aerodynamic Drag & Apogee Calculator](https://dothecalculation.com/calculators/model-rocket-drag-apogee-calculator) — Simulate rocket flight to calculate peak altitude, apogee, drag coefficient, and maximum velocity using numerical integration. - [RC Battery C-Rating & Continuous Current Calculator](https://dothecalculation.com/calculators/rc-battery-c-rating-calculator) — Calculate safe continuous discharge limits, average throttle current, and expected flight runtime for your LiPo battery. - [Telescope Dawes' Limit & Resolving Power Calculator](https://dothecalculation.com/calculators/telescope-resolving-power-calculator) — Calculate telescope magnification, Dawes limit, Rayleigh criterion, and field of view based on your aperture and eyepiece. - [Wood Shrinkage & Seasonal Movement Calculator](https://dothecalculation.com/calculators/wood-shrinkage-radial-calculator) — Model exact wood shrinkage and seasonal dimensional changes based on relative humidity, temperature, and wood species type. --- _This calculator is for educational and planning purposes. 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