Exposure Calculator
Find the equivalent shutter speed when you change aperture, keeping overall exposure constant. Essential when switching f-stops mid-shoot without altering brightness.
About this calculator
The exposure triangle links aperture, shutter speed, and ISO to control how much light hits the sensor. When you change your aperture, you must adjust shutter speed (or ISO) proportionally to maintain the same exposure value. The relationship between aperture and shutter speed follows an inverse-square law: newShutter = currentShutter × (targetAperture / currentAperture)². For example, doubling the f-number (e.g. f/4 to f/8) reduces light by four times, so you must slow the shutter by 4× to compensate. This calculator isolates the shutter speed adjustment needed when only the aperture changes, assuming ISO stays fixed. Understanding this relationship lets photographers confidently re-expose a scene after swapping lenses or adjusting depth of field.
How to use
Suppose you are shooting at f/4, 1/200 s, ISO 400, and want to switch to f/8 for greater depth of field. Enter currentAperture = 4, currentShutter = 1/200 (0.005 s), and targetAperture = 8. The formula gives: newShutter = 0.005 × (8 / 4)² = 0.005 × 4 = 0.02 s, or 1/50 s. Set your camera to 1/50 s at f/8 and ISO 400 to maintain identical exposure. Always verify with a histogram to confirm no highlights are clipped.
Frequently asked questions
How does changing aperture affect shutter speed in equivalent exposure?
Aperture controls the size of the lens opening, and light transmission scales with the square of the f-number ratio. Closing down by one full stop (e.g. f/4 to f/5.6) halves the incoming light, so you must double the shutter duration to compensate. The formula newShutter = currentShutter × (targetAperture / currentAperture)² captures this precisely. Getting this relationship right prevents under- or over-exposed frames when you adjust depth of field on the fly.
What is the exposure triangle and why does it matter for photography?
The exposure triangle describes the three interdependent settings that determine a photo's brightness: aperture (f-stop), shutter speed, and ISO. Each side of the triangle affects not just exposure but also creative qualities — aperture controls depth of field, shutter speed controls motion blur, and ISO affects image noise. Changing one setting without adjusting another will either over- or under-expose the image. Mastering the triangle lets you prioritize the creative look you want while still hitting the correct exposure.
When should I use an exposure compensation calculator instead of auto exposure?
Auto exposure modes (Av, Tv, P) work well in evenly lit scenes, but they can be fooled by high-contrast situations like backlit subjects or snow. An exposure compensation calculator is especially useful in manual mode when you know your desired aperture and need to derive the exact shutter speed mathematically. It is also valuable on a tripod where precise, repeatable settings matter, such as product photography or astrophotography. Using calculated values removes guesswork and reduces the number of test shots needed to nail exposure.