What this helper works out
This helper takes a change in exposure measured in stops and shows the three equivalent ways to achieve it — through ISO, shutter speed or aperture.
All three give the same brightness. What differs is what each one costs you: noise, motion blur or depth of field.
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Before you use it
- A stop is a doubling or halving of light. Positive values brighten; negative values darken.
- Shutter is entered as the denominator — 125 for 1/125 second.
- Aperture is the f-number. Smaller numbers are wider apertures letting in more light.
- The three controls are interchangeable for brightness only. They are not interchangeable for how the picture looks.
Aperture area is what admits light, and area scales with the square of the diameter. To double the area you multiply the diameter by √2, which is why the standard f-stop sequence runs f/1.4, f/2, f/2.8, f/4, f/5.6, f/8 — each step is a factor of √2 and halves the light.
How this calculator works
One stop change, applied three ways:
ISO × 2stops
Shutter denominator ÷ 2stops
f-number ÷ (√2)stopsAll three exponents describe the same doubling. ISO and shutter move by factors of two per stop because they act on light directly; aperture moves by √2 because the f-number describes a diameter and light depends on area.
Worked example: one stop brighter
Using the default figures — ISO 200, shutter 1/125, f/8, one stop brighter:
- ISO: 200 to 400
- Shutter: 1/125 to 1/63
- Aperture: f/8 to f/5.7
Any one of those gives the same exposure. Doubling the ISO adds noise; halving the shutter speed risks motion blur and camera shake; opening the aperture narrows depth of field. Which to choose depends entirely on the subject — a moving subject rules out the shutter, a landscape needing front-to-back sharpness rules out the aperture, and a low-light scene may leave only the ISO.
Common mistakes
- Treating the three as equivalent in every respect. They match on brightness only.
- Entering the shutter as a fraction. Use the denominator.
- Forgetting that a smaller f-number is a wider aperture. The numbering is inverted.
- Opening the aperture on a group shot. Depth of field will not cover everyone.
- Raising ISO first by habit. It is often the right answer and not always.
Frequently asked questions
What is the exposure triangle?
The relationship between ISO, shutter speed and aperture, each of which can be traded against the others to keep the same exposure. It is a useful teaching model, though it slightly misleads — ISO does not gather light the way the other two do, it amplifies what was gathered, which is why it adds noise rather than changing what the sensor received.
Which should I change first?
Whichever matters least for the picture you want. For a moving subject, fix the shutter first and let the others follow. For a portrait with a soft background, fix the aperture. For a tripod-mounted landscape in good light, neither constrains and the ISO stays at its base. The subject decides, not a rule.
What is base ISO?
The sensor's native sensitivity, typically ISO 100 or 200, at which it produces the cleanest image with the greatest dynamic range. Raising the ISO amplifies the signal and the noise together. Lowering it below base on cameras that allow it is usually a processing trick and can reduce dynamic range.
How slow can I hand-hold?
The traditional rule is one over the focal length — 1/50 second for a 50 mm lens — which predates image stabilisation and assumes a steady technique. Modern stabilisation commonly buys three to five stops beyond that. It does nothing for subject movement, which is a separate constraint.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — photography in public places and consumer rights on equipment
- MoneyHelper — budgeting for equipment and hobbies
Sources are checked at publication and can change — how I choose and check references.