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Smart Exposure Calculator

The exposure needed to correctly expose a scene at ISO 100 is governed by the daylight constant K=12.5, which relates illuminance (lux), aperture (f-number) and shutter speed in seconds: t₁₀₀ = K·N²⁄L. For other ISO settings the effective exposure time scales inversely with ISO, so t = t₁₀₀ × (100÷ISO).

Enter the measured illuminance of the scene in lux. Use a light meter or a smartphone app that reports lux.
Select the lens aperture you plan to use (e.g., 5.6, 8.0). A lower number means a wider opening.
Choose the ISO sensitivity of your camera. Common levels are listed; higher ISO lets you use faster shutter speeds but increases noise.

What it is

The Smart Exposure Calculator computes the shutter speed required to correctly expose a photographic scene based on the measured illuminance (lux), chosen aperture f-number, and selected ISO sensitivity. Using the standard daylight constant K=12.5 for ISO 100, it translates the relationship L = K·N²⁄t into an explicit time value and then scales the result for any ISO setting by a factor of 100 divided by the ISO number. This tool eliminates guesswork, especially in variable field lighting, enabling photographers to quickly select shutter speeds that yield proper exposure while balancing depth‑of‑field and noise considerations.

How to use it

First measure or estimate the scene’s illuminance in lux using a light meter or smartphone app. Choose the lens aperture you intend to use by entering its f-number value. Select your camera‘s ISO setting from the dropdown; higher ISO values allow faster shutter speeds but introduce more image noise. Click calculate and read the recommended shutter speed displayed in seconds – this is the precise exposure time needed for accurate highlights, shadows, and overall color fidelity.

Worked example

1. Measure scene illuminance = 500 lux. 2. Select aperture f/5.6 and ISO 200. 3. Compute base shutter for ISO 100: K·N² / L = 12.5 × (5.6)² ÷ 500 = 0.784 s. 4. Adjust for ISO 200: multiply by (100 ÷ 200) = 0.5, giving 0.392 s. 5. Round to three decimals → 0.392 seconds, which will correctly expose the image at the chosen settings.

Inputs

  • Scene Illuminance (lux): 500
  • Aperture f-number: 5.6
  • ISO Setting: 200

Result

  • Recommended Shutter Speed: 0.392

Frequently asked questions

What does the daylight constant K of 12.5 represent?

K is a calibrated factor that connects illuminance in lux with shutter speed for ISO 100 under typical daylight conditions. It emerges from the CIE exposure formula and standard camera sensor response; using it ensures accurate speed calculations across common lenses.

Why does ISO affect shutter speed linearly?

ISO measures the sensor’s sensitivity to light. A doubling of ISO halves the amount of light needed for a correct exposure, so you can halve the shutter time. Thus shutter speed scales inversely with ISO and is reflected in the formula t = t₁₀₀ × (100 ÷ ISO).

Can I input lux values below 0.001?

The calculator imposes a minimum of 0.001 lux to avoid division by zero and to keep computational results practical. Extremely low light levels are still handled mathematically but will produce extraordinarily long shutter times.

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