Stop

A stop is a doubling or a halving of exposure. One stop more means twice as much light reaches the sensor (or, with ISO, that the picture is made twice as bright); one stop less means half. It matters because every exposure control on your camera, and most of the numbers photographers use to talk about light, are counted in this one unit, so once you can count stops you can trade aperture, shutter speed and ISO against each other in your head.

The photo above is an example. Thirty seconds at f/14 is far more exposure than a daylight scene needs. A neutral density filter, rated in stops, almost certainly took the extra light away (filters are not recorded in the file, but nothing else explains 30 seconds in daylight), and the long shutter it allowed smoothed the water and stretched the clouds. Every part of that decision was arithmetic in stops.

Want to feel a stop under your fingers? In the camera simulator every click of aperture, shutter or ISO is a third of a stop, and the meter needle moves with it.

Where the word “stop” comes from

Early lenses had no adjustable diaphragm. Photographers slid thin metal plates, each with a hole of a fixed size, into a slot in the lens barrel. The plate stopped part of the light, so the plates were called stops, and swapping to a smaller hole was “stopping down”. The plates are long gone, but the words stayed: stopping down still means choosing a smaller aperture, and “opening up” a bigger one. Over time “a stop” stopped meaning a piece of metal and came to mean the amount of light that changes between one standard aperture and the next, which is exactly a factor of two.

One unit for every control

The useful way to picture a stop is as a step on a ladder where each rung holds twice the light of the rung below it. It does not matter whether you climb by opening the aperture, leaving the shutter open longer, or raising ISO: one rung is one rung. That is what makes the exposure triangle work. Three controls with different physical units (an opening size, a length of time, a gain setting) all move the picture’s brightness in the same currency.

Doublings also match how you see. Your eyes judge brightness roughly by ratio, not by amount, so going from 1 unit of light to 2 looks like about the same step as going from 100 to 200. Each stop looks like a similar change whether it happens in the shadows or the highlights. That evenness is why photographers count in stops instead of in raw amounts of light.

The stop table: every control in one unit

Each row below lets one stop less light into the picture than the row above it. Read across a row and every cell is the same change, said in a different control’s language. The aperture and shutter columns start from f/2 and 1/30 only as an example; the steps are what matter.

Three things in the table are worth noticing. The aperture and flash distance columns grow by about 1.4 each row, not by 2, because both work through area: light spreads over the area of the opening, and over the area of a sphere as it travels from a flash. The ISO column runs the other way, because ISO adds no light; it is the setting you would have to raise to get back the brightness the other columns took away. And an ND filter’s density goes up by 0.3 per stop, because 0.3 is the base 10 logarithm of 2.

Counting stops on each control

Aperture: why the numbers step by 1.4

An f-number is the focal length divided by the diameter of the opening. The light the lens passes depends on the opening’s area, and area goes with the diameter squared. To double the area you multiply the diameter by the square root of 2, about 1.414, so each full stop on the f-number scale is a factor of 1.4: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Bigger numbers mean smaller openings and less light.

Two counting rules fall out of that. Every second number on the scale doubles, so halving the f-number is two stops: f/8 to f/4 is two stops more light, four times as much, not twice. And the printed numbers are rounded. The true values are 1.41, 2.83, 5.66, 11.3 and 22.6; f/5.6, f/11 and f/22 are labels, close enough that the rounding never matters when you shoot.

Shutter speed: plain doublings

Shutter speed is the easiest control to count, because time is linear: twice as long is twice the light. The standard full stops are 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second, then 2, 4, 8, 15 and 30 seconds. Here too the labels are rounded for convenience. The true halvings from 1 second are 1/128 and 1/256, written as 1/125 and 1/250, and 1/60 stands in for 1/64.

ISO: brighter pictures without more light

ISO doubles in full stops: 100, 200, 400, 800, 1600, 3200, 6400. Each step makes the recorded image twice as bright, but it does that by amplifying the signal, not by gathering more light. The sensor collected the same light at ISO 1600 as it would have at ISO 100 with the same aperture and shutter. That is why a stop of ISO is not quite the same as a stop of aperture or shutter: the picture gets brighter, but the noise, which comes mostly from how little light was captured, does not improve.

Thirds and halves: the clicks between

The exposure scale in your viewfinder or on the rear screen shows stops and thirds together.

Most cameras move in third stops, so three clicks make one stop. Some can be set to half stops, where two clicks make one. The third-stop values are worth recognizing, because you will see them on every screen:

• Aperture: f/2.8, f/3.2, f/3.5, f/4, f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16
• Shutter: 1/250, 1/200, 1/160, 1/125, 1/100, 1/80, 1/60
• ISO: 100, 125, 160, 200, 250, 320, 400

So f/14, the aperture of the harbor photo at the top, is two thirds of a stop smaller than f/11 and one third of a stop wider than f/16. A lens sold as f/1.8 instead of f/1.4 is two thirds of a stop slower, which is why fast lens comparisons are usually about fractions of a stop.

Still wondering why bigger f-numbers mean less light? What is aperture explains the f-stop scale and why the numbers go that way.

Trading stops between controls

Because every control counts in the same unit, you can take a stop from one and give it to another and the brightness stays put. Some worked trades:

• You are at f/5.6 and 1/500, and you want more depth, so you stop down to f/11. That is two stops less light, so the shutter must give two stops back: 1/500 to 1/250 to 1/125.
• You are at 1/60 and need to freeze a child running, which wants about two stops faster. 1/60 to 1/125 to 1/250. Pay for it with ISO 400 to 1600, or open from f/8 to f/4.
• You are at 1/250 and want a soft pan blur at 1/8. Count: 1/125, 1/60, 1/30, 1/15, 1/8 is five stops more light, which five stops of smaller aperture, lower ISO or ND filter must cancel.

Stops also add up across controls, which is how you compare two exposures that share nothing. These two frames were made at Silver Lake:

Count from the first to the second. The shutter went from 1 second to 60 seconds: 60 is between 32 (five stops) and 64 (six stops), and the exact figure is 5.9 stops. The aperture went from f/14 to f/8: f/14, f/13, f/11, f/10, f/9, f/8 is five clicks, a stop and two thirds (1.6 stops). ISO went from 100 to 400, two stops. That totals about 9.5 stops, more than 700 times the exposure. Since the two frames look about equally bright, roughly nine and a half stops had to be taken away somewhere else: by an ND filter, by dimmer light, or by both. Nothing else in photography lets you compare a 1 second and a 60 second picture in one sentence.

Want to try a trade and see what it costs? The exposure triangle page has a tool to trade one setting for another: it locks the brightness, lets you move one setting and asks which other one pays.

Equal stops, different pictures

Trading stops keeps the brightness the same. It does not keep the picture the same. Every stop you move between controls changes something else:

• Stops taken from the aperture change depth of field. f/2 and f/16 at the right shutter speeds give the same brightness, one with a soft background and one sharp from front to back.
• Stops taken from the shutter change motion. Shutter time is what turned the harbor water at the top into a flat sheet: 30 seconds is about 14 stops longer than 1/500, which would have shown every ripple, and at the slow end you get motion blur from your hands as well as the subject.
• Stops taken from ISO change noise. Two pictures at ISO 100 and ISO 6400 can be equally bright, but the second was made from 64 times less light, and it shows in the shadows.

So the question when you trade is never “how many stops?” alone. It is “which of these looks am I willing to pay with?”

Try it: lock the exposure, move one setting a few stops, and choose which other setting pays for it. The brightness never changes; the depth of field, the motion and the noise do.

Stops and exposure value

Exposure value (EV) is a single number that names a pair of aperture and shutter settings, and one EV is one stop. The formula is EV = log₂(N²/t), where N is the f-number and t the shutter time in seconds. f/1 at 1 second is EV 0. f/16 at 1/125 is EV 15, the setting the sunny 16 rule gives for bright sun at ISO 100.

The catch: a higher EV lets in less light. EV 15 is a small aperture and a fast shutter, right for bright sun. The harbor photo at the top, at f/14 and 30 seconds, works out to about EV 2.7, the kind of setting you would expect at night. In daylight that difference had to come from the filter. When a meter or a chart says a scene “is EV 12”, it means the scene is bright enough that the EV 12 settings (at ISO 100) expose it correctly.

The plus and minus scale on your camera’s exposure compensation dial runs the other way. There, +1 means one stop brighter than the meter suggests. Both scales count stops; they just point in opposite directions.

Want the EV numbers for real scenes, from candlelight to snow? Exposure value explained has the scale and how it ties to ISO.

Stops beyond the exposure triangle

Neutral density filters

An ND filter is rated in stops, or in a name that hides them. ND8 cuts the light to 1/8 (three stops); ND64, six stops; ND1000 is very close to ten stops, because 2¹⁰ is 1,024. Optical density adds 0.3 per stop, so a 0.9 filter is three stops and a 3.0 filter is ten.

To use one, multiply the shutter time by 2 for every stop. If your scene meters at 1/125, a six-stop filter needs 64 times as long: 64/125 is about 1/2 second. A ten-stop filter needs 1,024/125, about 8 seconds. That is how the harbor picture at the top got from a normal daylight exposure to 30 seconds.

Working out a ten or fifteen stop exposure in the field? The ND filter exposure calculator does the multiplication for you.

Flash power and distance

Flash power settings are stops too: full, 1/2, 1/4, 1/8 and so on, each one stop less. Distance counts in stops through the inverse square law. Light from a small source spreads over an area that grows with the square of the distance, so doubling the distance from flash to subject spreads it over four times the area: two stops less, not one. Moving the flash 1.4 times farther costs one stop. The guide number wraps these up into one figure, and the flash guide number calculator does it for a given distance.

Dynamic range

Dynamic range is the span between the darkest and brightest tones a sensor or a scene holds, counted in stops. Each stop doubles the ratio: 10 stops is 1,024 to 1, and 14 stops is 16,384 to 1. When a scene spans more stops than the sensor can hold, something has to go. In the sunset photo above the exposure held the color of the sky, and the city’s buildings, many stops darker, went to black. That is a silhouette, and it is a choice about where to spend your stops. You can see the same choice on a histogram, which shows how many stops of the scene are crowding against either edge.

When no single exposure fits, bracketing shoots the same scene at set stop intervals, often two stops under, normal and two stops over (at 1/125 base: 1/500, 1/125 and 1/30), so the frames can be blended.

Is your scene wider than your sensor? What is dynamic range shows how to tell if a scene is beyond your camera, and what to do when the scene has more range than the sensor.

Stabilization ratings

Image stabilization is rated in stops of shutter time. A “five stop” rating means you can, in principle, hold the camera at a shutter time 32 times longer than without it: if you would normally need 1/125, five stops later is about 1/4 second. Real results vary with your hands and the lens, and stabilization does nothing for a moving subject.

Stops are not percentages

Because each stop is a doubling, stops multiply. They do not add like percentages:

Three stops down is not “three times 25 percent”. And a stop up followed by a stop down lands exactly where you started, which percentages do not (50% more then 50% less leaves you at 75%). Thinking in stops avoids that trap.

Common mistakes with stops

• Treating “stop” and “f-stop” as the same thing. A stop is a unit, like an inch. An f-stop is one of the aperture settings that happens to be a stop apart from its neighbors. Shutter speeds, ISO and filters all have stops too.
• Thinking f/4 to f/8 is one stop. The number doubled, so it is two stops: a quarter of the light. One stop from f/4 is f/5.6.
• Counting clicks as stops. On most cameras one click is a third of a stop. Three clicks is one stop.
• Believing ISO adds light. It brightens what was captured. If you can get the stop from the aperture or shutter without ruining the picture, the result is cleaner.
• Expecting equal exposures to look equal. They match in brightness only. Depth, motion and noise all change with the trade.
• Reading a higher EV as brighter. On the exposure value scale, higher means less light let in. Only the compensation dial counts plus as brighter.
• Moving a flash twice as far and adjusting by one stop. Twice the distance is two stops.

Try this: count the stops in one scene

Ten minutes, any camera that lets you choose the aperture or set exposure compensation (on a phone, use the exposure slider and a pro or manual mode if it has one).

1. Put the camera in aperture priority at ISO 100 and point it at a scene that does not change, such as a window-lit corner of a room. Set f/4 and write down the shutter speed the camera picks.
2. Click the aperture to f/5.6, counting clicks. Note the new shutter speed. Then go to f/8 and note it again.
3. Go back to f/4. Using exposure compensation, take five frames at −2, −1, 0, +1 and +2.
4. Put the five frames side by side on a screen, and look at the histogram of each.

How to judge it: from f/4 to f/5.6 should be three clicks on most cameras, and the shutter time should roughly double; at f/8 it should be about four times the f/4 time. If it is, you have watched a stop being paid for. In the five frames, notice that each step looks about the same size, and how far the histogram moves with each one. That spacing is what “one stop” looks like in your own work, and the next time someone says “give it a stop” you will know what to expect.

Want to check you can count and trade stops without the camera in your hand? Try the exposure quiz on stops, ETTR and manual mode decisions.

Questions about stops

Is a stop the same as an f-stop?

No. A stop is a doubling or halving of exposure, from any control. An f-stop is an aperture setting, such as f/5.6; neighboring full f-stops happen to be one stop apart, which is where the shared name comes from.

How many stops is f/2.8 to f/8?

Three: f/2.8, f/4, f/5.6, f/8. That is one eighth of the light, so you would need a shutter time eight times as long, or three stops more ISO, to keep the same brightness.

Is one EV the same as one stop?

Yes, a change of one EV is a change of one stop. The difference is direction: on the exposure value scale a higher number lets in less light, while on the exposure compensation dial +1 means brighter.

Why does my camera move in thirds of a stop?

A full stop is a big change, often too big to fine-tune a bright sky or a skin tone. Thirds give finer control while keeping the arithmetic easy: three clicks is always one stop, whichever control you turn.

Is a stop of ISO as good as a stop of aperture or shutter?

It gives the same brightness but not the same image. Aperture and shutter change how much light the sensor collects, and more light means cleaner pictures. ISO brightens whatever light was collected, so a stop of ISO usually brings a little more visible noise.

What is a T-stop?

A T-stop is an aperture rating measured by how much light the lens actually transmits, instead of calculated from its geometry. Two lenses at the same f-number can differ by a fraction of a stop in brightness; at the same T-stop they match, which is why cinema lenses use them.