About the F-Stop Calculator
This f-stop calculator tells you exactly how many stops of light separate two camera settings. Pick aperture, shutter speed or ISO, enter a starting and an ending value, and it returns the difference in stops, whether the change brightens or darkens the exposure, the light ratio, and the nearest third-stop count you will see on your camera’s dials. A chart of every marked full and third stop between the two values is listed below the result.
It is handy when balancing the exposure triangle — for example, if you open up from f/8 to f/4 you need to shorten the shutter or lower the ISO by the same number of stops — and when working out ND filter compensation, flash power changes or bracketing sequences. Students learning exposure can also use the chart to memorise the standard f-number sequence.
Marked camera values are rounded: f/5.6 is really f/5.657 and 1/125 s is really 1/128 s, so the exact stop count between two marked values can differ from a whole number by a few hundredths. The third-stop result shows the step count your camera would display.
With the default inputs, the difference is 3.03 stops. Change any value above to recalculate instantly.
How to use the f-stop calculator
- 1Choose whether you are comparing apertures, shutter speeds or ISO values.
- 2Enter the starting value (shutter speeds can be typed as 1/250 or 2).
- 3Enter the ending value.
- 4Read the difference in stops and whether the exposure gets brighter or darker.
- 5Use the table of marked full and third stops to find intermediate settings.
Formula and method
One stop is a doubling or halving of light. Shutter time and ISO are linear in light, so the number of stops between two values is simply the base-2 logarithm of their ratio. The light passing through a lens is proportional to the area of the aperture, which scales with 1 ÷ N², so the stop count between two f-numbers is twice the base-2 logarithm of their ratio — each full stop multiplies the f-number by √2 ≈ 1.414.
The light ratio is 2 raised to the number of stops. The “on camera dials” figure rounds the exact difference to the nearest third of a stop, because camera markings such as f/5.6, f/11 and 1/125 s are rounded versions of the exact √2 and power-of-two sequences.
- N₁, N₂
- Starting and ending f-numbers
- t₁, t₂
- Starting and ending shutter times (seconds)
- S₁, S₂
- Starting and ending ISO values
Worked examples
f/2.8 to f/8
2 × log₂(8 ÷ 2.8) = 3.03 stops, so f/8 lets in about 8.16 times less light than f/2.8. On the camera that is three full clicks of stops (f/2.8 → f/4 → f/5.6 → f/8); the extra 0.03 comes from rounding of the marked numbers.
1/250 s to 1/30 s
The exposure time grows from 0.004 s to 0.0333 s, a ratio of 8.33, which is log₂(8.33) = 3.06 stops more light. On the dial it is three full stops: 1/250 → 1/125 → 1/60 → 1/30.
ISO 100 to ISO 1600
ISO 1600 is 16 times ISO 100, and log₂(16) = 4, so raising the ISO brightens the image by exactly four stops — enough to go from 1/15 s to 1/250 s at the same aperture.
f/1.4 to f/1.8
2 × log₂(1.8 ÷ 1.4) = 0.73 stops, which cameras show as two third-stop clicks (f/1.4 → f/1.6 → f/1.8). The image receives about 1.65 times less light.
Frequently asked questions
What is the f-stop sequence?+
The standard full-stop sequence is f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Each number is the previous one multiplied by √2 (about 1.414), and each step halves the light reaching the sensor.
How many stops is f/4 to f/8?+
Two stops. f/4 → f/5.6 is one stop and f/5.6 → f/8 is another, so f/8 lets in a quarter of the light of f/4. To keep the same exposure you would need a shutter speed four times longer or an ISO four times higher.
Why does a bigger f-number mean a smaller aperture?+
The f-number is the focal length divided by the diameter of the aperture. At 50 mm, f/2 means a 25 mm opening and f/8 a 6.25 mm opening, so higher f-numbers are smaller holes that let in less light.
What is a third stop?+
Most cameras adjust aperture, shutter speed and ISO in one-third-stop clicks, so three clicks make one full stop. A third stop changes the light by a factor of about 1.26, a subtle but visible difference in exposure.
Is a stop the same for aperture, shutter speed and ISO?+
Yes. A stop always means doubling or halving the light (or sensitivity). Opening one stop of aperture, doubling the shutter time, or doubling the ISO all brighten the image by the same amount, which is why you can trade them against each other.