Exposure Calculator
Find the new shutter speed needed to maintain correct exposure after changing aperture. Ideal for photographers adjusting depth of field without altering brightness.
About this calculator
Exposure is controlled by three variables: aperture (f-stop), shutter speed, and ISO. Aperture affects exposure as the square of the f-number ratio because it controls the area of the lens opening. When you change only the aperture, the new shutter speed that preserves the same exposure is: newShutter = currentShutter × (targetAperture / currentAperture)². This follows directly from the inverse-square relationship: doubling the f-number reduces light by a factor of four, requiring a four-times-longer shutter speed. This calculator focuses on the aperture-to-shutter relationship, isolating the two-stop arithmetic photographers need most when pulling focus or swapping to a prime lens mid-shoot.
How to use
You are shooting at f/2.8 with a shutter speed of 1/500 s. You want to stop down to f/5.6 for greater depth of field. newShutter = (1/500) × (5.6 / 2.8)² = (1/500) × 4 = 4/500 = 1/125 s. So you must slow the shutter to 1/125 s to keep the same exposure. Enter currentAperture = 2.8, currentShutter = 0.002 (1/500), and targetAperture = 5.6 to confirm this result instantly.
Frequently asked questions
How do I calculate a new shutter speed when I change aperture?
Multiply your current shutter speed by the square of the ratio of the new aperture to the old aperture: newShutter = currentShutter × (targetAperture / currentAperture)². Each full f-stop doubles or halves the light, so the shutter must change by the same factor to compensate. For example, going from f/4 to f/8 is two stops darker, so the shutter time must quadruple. This calculator does that arithmetic automatically for any aperture pair.
What is a stop in photography and why does the square law apply?
A stop is a doubling or halving of light. Aperture f-stops are defined so that each step changes the diameter of the opening by a factor of √2, which changes the area — and therefore the light passing through — by exactly 2×. Because area scales with the square of diameter, the exposure formula uses (aperture ratio)² rather than a simple linear ratio. This means a one-stop change in f-number corresponds to a two-times change in shutter speed, not a proportional one.
When should I use this calculator instead of relying on the camera's auto-exposure?
Auto-exposure is convenient but unpredictable when shooting in manual mode for consistency — for example, in a studio where you want identical exposure across hundreds of shots with varying depth of field. It is also essential when using older manual lenses that have no electronic aperture communication. Cinematographers use equivalent-exposure math when swapping lenses mid-scene to maintain exposure continuity. Entering values here gives you the exact shutter speed to dial in before you even raise the camera.