Fixed ISO Exposure Calculator

What is a Fixed ISO Exposure Calculator?

A Fixed ISO Exposure Calculator is a shooting helper for situations where the scene stays the same, ISO stays fixed, and only shutter speed and aperture need to be redistributed. If you already have one reference exposure that looks correct, you can enter either a target aperture or a target shutter speed and solve for the matching value that keeps brightness unchanged.

That makes it especially useful when you do not need to solve the entire exposure triangle every time. If your goal is to keep ISO where it is while changing depth of field or motion rendering, the calculator gives you a faster path from the reference exposure to a workable new setting.

When this tool is useful

During repeated shooting in the same location, photographers often leave ISO alone and simply rebalance shutter speed and aperture. This tool is built for that workflow so you can see immediately how far you need to move away from the baseline exposure.

• Adjusting background blur – When you want to open up or stop down the aperture without changing ISO and need the matching shutter speed right away
• Securing motion control – When you lock in a faster shutter speed first and need the matching aperture that keeps brightness in place
• Repeated shooting in one location – When you are changing lenses or framing but the light itself has not changed much
• Checking lens limits – When you want to know in advance whether your lens can open wide enough for the shutter speed you want
• Practising manual exposure – When you want to rehearse how one-stop and two-stop changes affect shutter speed and aperture at a fixed ISO

Key features

This tool keeps the “fixed ISO” assumption explicit, then organizes the result around the reference exposure and the target value you want to change. Instead of returning a single number without context, it also shows stop differences, a scene-brightness reference, and the nearest standard camera values.

• Two calculation modes – Switch between shutter-for-target-aperture and aperture-for-target-shutter calculations
• Reference-based math – Start from one fixed ISO and one reference aperture/shutter pair, then solve the matching value instantly
• Stop-difference guidance – Read how many stops wider, faster, narrower, or slower the new setting becomes
• Same-brightness combination table – Compare other aperture/shutter layouts along standard-stop steps while holding brightness
• Scene-brightness reference – See the approximate EV implied by the reference exposure and fixed ISO

How to use it

Enter the reference exposure that already looks right, keep ISO fixed, and choose which value you want to change. As soon as you enter a target aperture or target shutter speed, the result updates instantly and the same-brightness comparison table helps you compare practical alternatives on the spot.

1. Enter the fixed ISO – Choose the ISO you plan to keep throughout this part of the shoot.
2. Enter the reference exposure – Use the aperture and shutter speed from the scene where brightness already looks correct.
3. Choose a mode – Decide whether you want to solve for the shutter speed or for the aperture.
4. Enter the target value – Type the aperture or shutter speed you want to change to.
5. Read the top result first – Check the solved value, stop difference, scene-brightness reference, and comparison table together.

Formula and interpretation notes

At the same ISO and in the same scene, brightness is preserved through the ratio N² / t, where N is the aperture value (f-number) and t is the shutter speed in seconds. If the reference exposure is N₁, t₁ and the target exposure is N₂, t₂, you can keep the same brightness with the relationship N₁² / t₁ = N₂² / t₂.

When you choose the target aperture first, use t₂ = t₁ × (N₂² / N₁²). When you choose the target shutter first, use N₂ = N₁ × √(t₂ / t₁). For example, if the reference exposure at ISO 100 is f/5.6 and 1/125 second, opening the aperture to f/2.8 means the shutter has to accelerate to about 1/500 second to preserve the same brightness.

Camera settings such as f/5.6 and 1/125 second are rounded working steps rather than perfect logarithmic values, so the exact math result and the nearest standard camera value can differ slightly. This tool shows both so it is easier to move from the calculation to a real dial setting.

• Open the aperture by one stop – At the same ISO, the shutter must become one stop faster to keep the same brightness
• Speed up the shutter by one stop – At the same ISO, the aperture must open by one stop to make up the missing light
• What fixed ISO means – Sensor sensitivity stays where it is while you redistribute depth of field and motion rendering
• Scene-change caution – If the light changes, the reference exposure itself has to be refreshed before the result stays meaningful

Frequently asked questions