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Photography Exposure Value & Reciprocity Calculator

Calculate Exposure Value (EV) and EV100, and compensate for film reciprocity failure during very long exposures in photography.

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Aperture (f-number)

Metered shutter speed (seconds)

ISO speed

Reciprocity factor (p exponent)

Results

Equivalent EV100

EV 5.0

Exposure Value (Current settings)EV 5.0

Adjusted Shutter Speed (Film)2.46 seconds

Estimated Light ConditionNight street scenes / City lights

Tip

Save this tool to quickly calculate light levels, select camera settings, and adjust exposure times for film photography.

Formula

Exposure and reciprocity formulas

EV = log₂(Aperture² / Shutter). EV100 = EV - log₂(ISO / 100). Schwarzschild Time = Shutter^p

Calculates the logarithmic light values and scales the required shutter time to compensate for analog emulsion sensitivity dropoff.

Step by step

How the calculation works

1Enter your lens aperture (f-number) and metered shutter speed (in seconds).
2Provide the film or sensor ISO speed rating.
3Set the reciprocity factor exponent (p) of your film (use 1.0 for digital cameras or films with no reciprocity failure).

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How to understand and use this calculator

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 ( N^2/t ), 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 ( ISO/100 ). 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_(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) ≈ 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) ≈ 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.

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.

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.

How to understand and use this calculator

Exposure Value (EV) and Light Measurement Science

Film Physics and Reciprocity Failure Corrections

How to Use This Calculator

Worked Example: Reading a Metered f/8, 2-Second Exposure

Related Calculators

For depth-of-field planning at your chosen aperture, see the Depth of Field Hyperfocal Distance Calculator.

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.

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?+

Photography Exposure Value & Reciprocity Calculator

Inputs

Results

Exposure and reciprocity formulas

How the calculation works

Learn more about this calculation

Understanding Calculator Formulas: How DTC Turns Inputs into Results

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Photography Exposure Value & Reciprocity Calculator

Inputs

Results

Key output metrics

Exposure Time Compensation

How to understand and use this calculator

Exposure Value (EV) and Light Measurement Science

Film Physics and Reciprocity Failure Corrections

How to Use This Calculator

Worked Example: Reading a Metered f/8, 2-Second Exposure

Related Calculators

Related concepts

Frequently Asked Questions

Exposure and reciprocity formulas

How the calculation works

Learn more about this calculation

Understanding Calculator Formulas: How DTC Turns Inputs into Results

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Key output metrics

Exposure Time Compensation

How to understand and use this calculator

Exposure Value (EV) and Light Measurement Science

Film Physics and Reciprocity Failure Corrections

How to Use This Calculator

Worked Example: Reading a Metered f/8, 2-Second Exposure

Related Calculators

Related concepts

Frequently Asked Questions