A stop is the unit photographers use to measure changes in light: one stop up doubles the light and one stop down halves it. Counting stops of light works the same way for aperture, shutter speed, ISO, flash power, filters and exposure compensation, so one simple idea covers nearly every exposure decision you make.
Why It Matters
Every exposure control on your camera changes brightness in steps of different sizes and with different numbers. Without a common unit, it is hard to know how much to change when a photo is too dark. With stops, a single rule applies: if the picture needs to be twice as bright, open up one stop, in whatever control you choose.
Stops also let you compare things that look unrelated, such as a flash that is two stops weaker, a filter that cuts light by three stops, or a sensor that records twelve stops of contrast.
Beginner Counting Stops Of Light
A stop is a factor of two. That is the whole idea. Everything else is learning how each control moves in factors of two.
- One stop brighter means twice the light.
- Two stops brighter means four times the light (2 x 2).
- Three stops brighter means eight times the light (2 x 2 x 2).
- One stop darker means half the light, two stops darker means one quarter, and so on.
You never need to calculate the multiplier. Just count the steps on the dial. Each full step on any of these scales is one stop:
| Control | One Stop Brighter | Example Steps |
|---|---|---|
| Aperture | Smaller f-number | f/8, f/5.6, f/4, f/2.8, f/2 |
| Shutter speed | Longer time | 1/500, 1/250, 1/125, 1/60, 1/30 |
| ISO | Higher number | 100, 200, 400, 800, 1600 |
| Exposure compensation | Plus direction | 0, +1, +2 |
| Flash power | Next higher fraction | 1/16, 1/8, 1/4, 1/2, full |
Notice that f-numbers do not double. They multiply by about 1.4 each step because the amount of light depends on the area of the opening, not its width. The lesson what is aperture explains why.
Using Stops On Your Camera
- Take a photo in a mode of your choice and look at the result on the screen.
- Decide how much brighter or darker you want it. A small change is about 1/3 stop, a clear change is 1 stop, and a big change is 2 stops.
- Press the exposure compensation button (marked with a plus and minus square) and turn the dial. Each full number, such as +1, is one stop.
- Take the photo again and compare. The scene should look the amount you expected: twice as bright at +1, four times at +2.
The camera achieves this by changing the setting you did not fix. See exposure compensation guide for when to use it.
Trading Stops Between Settings
Say your correct exposure is f/4, 1/250, ISO 400, and you want a deeper focus with f/8. That is two stops less light. To restore it, you need two stops back from somewhere: slow the shutter two stops to 1/60, or raise ISO two stops to 1600, or split them, one stop each: 1/125 and ISO 800. Any combination that adds back two stops gives the same brightness. This is the exposure triangle at work, covered in exposure triangle explained.
Common Mistakes
- Thinking two stops means twice the change. It means four times the light.
- Counting the f-numbers in the list instead of the steps between them. f/8 to f/16 is two stops, not eight.
- Mixing up directions: a smaller f-number is brighter, but a smaller shutter fraction (1/1000) is darker.
- Forgetting that Auto ISO can change brightness behind your back in Manual mode, so your stop counts no longer add up.
Advanced Stops As A Base-Two Logarithm
The Math
A stop is a unit on a base-two logarithmic scale. The number of stops between two light quantities A and B is log2(A / B). If A is 8 times B, that is log2(8) = 3 stops. If A is 1000 times B, it is log2(1000), about 10 stops, since 2 to the power 10 is 1024. Because logarithms turn multiplication into addition, you can add stops from independent controls: a 2 stop aperture change plus a 1 stop shutter change is 3 stops in total.
Absolute Exposure Value
Exposure value (EV) fixes a reference: EV 0 is f/1 at 1 second at ISO 100. Each stop up is one EV. The formula is EV = log2(N squared / t), where N is the f-number and t is the shutter time in seconds. Bright sun with f/16 at 1/100 second gives log2(256 x 100) = log2(25,600), about EV 14.6, which is why the sunny 16 rule works near EV 15.
Filters And Optical Density
Neutral density filters are specified three ways. A filter marked ND8 passes one-eighth of the light, which is 3 stops. Density is the base-ten logarithm of attenuation, so ND 0.9 is also 3 stops, because 10 to the 0.9 is about 8. Each 0.3 of density is about one stop. A ND1000 filter is 3.0 density, about 10 stops. Several brands use different naming, so check whether a number is a stop count, a density or a factor. See camera filters guide.
Flash, Distance And Dynamic Range
- Flash. Manual power settings run in halves: 1/1, 1/2, 1/4 and so on. Each is one stop. Flash power at a distance follows the inverse square law, so doubling the distance reduces the light by two stops, as detailed in inverse square law.
- Dynamic range. The span between the brightest tone a sensor can hold and its noise floor is measured in stops. A ratio of 4,000 to 1 is about 12 stops. A camera quoted at 14 stops can record a contrast of roughly 16,000 to 1 with usable detail, depending on how the manufacturer defines usable.
- Bit depth. A linear raw file with 14 bits per channel has 16,384 levels, so it can encode at most 14 stops from the darkest nonzero value to the brightest. The brightest stop uses half of all the levels, the next uses a quarter, and the darkest stops use very few, which is why noise is worst in shadows.
Why It Is Logarithmic
Human vision judges brightness roughly in ratios rather than differences, so equal ratios of light look like equal steps in brightness. Using stops matches that response, which is why exposure, histograms and tone curves are all built in stops. For finer steps between whole stops, see third stop increments.
Try It
- Set the camera to Manual and photograph a plain wall in steady light. Adjust until the meter reads zero, and note aperture, speed and ISO.
- Close the aperture two full stops, such as f/4 to f/8.
- Restore the exposure by slowing the shutter, for example 1/250 to 1/60. Take the photo.
- Repeat but restore with ISO instead. Compare the three images: brightness should match, and only blur and noise may change.
Common Questions
How much light is one stop?
One stop is a factor of two. Going up one stop doubles the light reaching the sensor, and going down one stop halves it. Two stops is four times, three stops is eight times, and ten stops is about one thousand times.
Is a stop the same as an EV?
A change of one EV equals one stop, so the terms are interchangeable when describing differences. EV also names an absolute brightness level, with EV 0 set at f/1, one second and ISO 100.
How many stops is f/2.8 to f/8?
Three stops. The sequence is f/2.8, f/4, f/5.6 and f/8, and each move to the next number halves the light. Three moves means one eighth of the light, so you would need three stops of shutter or ISO to make up for it.
Why do cameras show thirds of a stop?
Thirds give finer control than whole stops. Many cameras step in thirds by default, so three clicks make one stop. You can usually switch to half or full stops in the menu.
Related Lessons
- Third Stop Increments to understand the finer steps on your dials.
- Exposure Compensation Guide to apply stops when the camera misjudges a scene.
- Equivalent Exposures to swap stops between settings with a plan.
- Dynamic Range Explained to see how stops measure what a sensor can capture.
- Inverse Square Law to calculate how light falls off with distance.