What is a stop in photography?

A stop is a doubling or halving of the amount of light reaching the sensor. Going up one stop doubles exposure; going down one stop halves it. Aperture, shutter speed, and ISO all share this same stop scale, which is why they can be traded against each other.

How is the aperture stop difference calculated?

Aperture controls light by area, so the stop difference is two times the base-2 logarithm of the f-number ratio. The formula is stops = 2 x log2(f2 / f1). For example f/2.8 to f/5.6 is a ratio of 2, which equals exactly 2 stops.

Why is shutter speed different from aperture?

Shutter speed and ISO scale light linearly, so one stop is a simple doubling. The formula is stops = log2(t2 / t1) for time and log2(iso2 / iso1) for sensitivity. A move from 1/125 s to 1/30 s is roughly 2 stops brighter.

What are half and third stops?

Modern cameras let you adjust exposure in fractions of a stop. A third of a stop is the finest common increment (0.333 EV), a half stop is 0.5 EV. The tool rounds your result to the nearest of these so you know exactly which click to use.

Does the order of the two values matter?

Yes. The first value is the reference. If the second value lets in more light the result is positive (brighter); if it lets in less the result is negative (darker). Swapping the two flips the sign but keeps the magnitude.

Photography Stop Calculator

The stop is the universal currency of exposure. Whether you change aperture, shutter speed, or ISO, every adjustment can be measured in stops — and one stop always means double or half the light. This calculator tells you precisely how many stops separate any two settings, then snaps that figure to the nearest full, half, and third-stop click so you can reproduce it on a manual camera.

How it works

The three exposure parameters reach the same stop scale through slightly different maths because they control light in different ways.

Aperture controls light by the area of the lens opening, and area grows with the square of the diameter. Because the f-number is inversely related to diameter, the stop difference between two f-numbers is:

stops = 2 x log2(f2 / f1)

So f/2.8 to f/4 is 2 x log2(4 / 2.8) = about 1 stop, and f/2.8 to f/5.6 is exactly 2 stops.

Shutter speed and ISO are linear: doubling the exposure time or the sensitivity is exactly one stop.

stops = log2(t2 / t1)          (shutter, in seconds)
stops = log2(iso2 / iso1)      (ISO)

A positive result means the second setting is brighter; a negative result means it is darker.

Worked example

Suppose you are at f/4, 1/250 s, ISO 200 and you want to switch the aperture to f/8 while keeping exposure constant. The aperture change is 2 x log2(8 / 4) = 2 stops darker. To compensate you need 2 stops more light elsewhere — for example halving the shutter twice from 1/250 to 1/60 s, or raising ISO from 200 to 800.

Notes and tips

The standard full-stop f-number sequence is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 — each value is the previous multiplied by the square root of 2.
The standard full-stop shutter sequence is 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000 s (the camera rounds 1/16 to 1/15, etc.).
ISO full stops double: 100, 200, 400, 800, 1600, 3200, 6400.
For shutter speeds, enter the value in seconds. Use 0.004 for 1/250 s, or type the fraction the tool accepts as a decimal.

Everything runs locally in your browser — no images or values are uploaded.