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PRINT & MEDIA CALCULATOR

Exposure Calculator

Find an equivalent shutter duration when aperture and ISO change for the same scene brightness.

Calculator

s
YOUR RESULTS
Equivalent target shutter duration0.04 s
Reciprocal shutter denominator
25 1/s
Shutter-duration change
2 stops
Scene-equivalent EV at ISO 100
10.644

Page guide

Equivalent shutter duration and stop adjustment

Positive stops mean a longer shutter duration; negative stops mean a shorter one. The reciprocal denominator expresses a conventional fractional shutter speed when appropriate. Equivalent settings match nominal brightness, while motion blur, depth of field, and noise can differ.

For the separate task of sizing a finished photograph for print, the Print Size Calculator relates pixel dimensions to printing resolution.

The formula

Target time = original time × (target f-number/original f-number)² × (original ISO/target ISO). Duration change in stops = log₂(target time/original time).

Equivalent brightness uses shutter time divided by the squared f-number, with the ISO setting applied proportionally. A smaller physical opening needs more exposure time, while higher ISO needs less time for the same nominal brightness.

Worked example

An exposure of f/4, 1/100 s, ISO 100 becomes f/8, 1/25 s, ISO 100 for equivalent brightness. The shutter duration increases from 0.01 to 0.04 s, a two-stop change. Raising target ISO to 400 restores the 0.01 s duration.

How to use this calculator

  1. Enter original aperture f-number, original shutter duration using the units shown.
  2. Set original iso, target aperture f-number, target iso.
  3. Calculate and review equivalent target shutter duration alongside the supporting quantities.

Enter shutter speed as a duration

The shutter field accepts seconds. For 1/250 s, enter 0.004 rather than 250. Long exposures can be entered directly, such as 2 seconds. Camera settings are often limited to stepped values, so use the nearest available setting and check the meter or histogram rather than expecting every calculated duration to appear on the camera.

Brightness is only one part of the photograph

Changing aperture changes depth of field and can affect lens behavior. Changing shutter duration changes motion rendering. Changing ISO changes amplification and noise characteristics without increasing the light collected at fixed aperture and duration. Scene luminance, filters, lens transmission, flash timing, and long-exposure reciprocity effects are outside the simple equivalent-setting relation.

Assumptions & limitations

What this calculation assumes

  • Scene illumination and lens transmission remain unchanged between exposures.

What to keep in mind

  • Excludes flash, neutral-density filters, reciprocity failure, metering offsets, and camera setting increments.

Common questions

Does raising ISO collect more light?

Not at fixed shutter time and aperture. It changes the nominal signal response and permits different exposure settings.

Why is aperture squared?

For a fixed focal length, opening area varies inversely with the square of the f-number, so equivalent time follows the squared ratio.

Does equivalent brightness mean an identical photograph?

No. Changing shutter time, aperture, or ISO can change blur, depth of field, and noise even when nominal brightness matches.

Sources & further reading

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