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Speaking Your Camera's Exposure Language:
Aperture, Shutter Speed, and ISO
Jim Doty, Jr.

Apertures, Shutter Speeds, and ISO Settings are Exposure's Big 3. They are the three camera settings that determine the exposure for every photo you take. If you want to speak your camera's language and make it do it's tricks for you, you need to understand the language of exposure.

Thinking in Stops

Before we look at the big 3 individually, you need to think in stops. Aperture, shutter speed, and ISO settings on the camera are all indicated in stops and fractions of stops. Just what is a "STOP"?   A "stop" means the amount of light has been doubled or cut in half.

"One stop more light" means the light has been doubled. Two stops more light means there is four times as much light (the light has been doubled twice). Three stops more light means __________ (fill in the blank)?  The answer is 8 times more light (doubled three times).

"One stop less light" means the amount of light has been cut in half. Two stops less light means there is only 1/4 as much light, three stops less means 1/8 as much light and so on.

If the light has changed, but not in even full stop increments, the increase is measured in fractions of stops. With most recent camera meters, the light can be measured in 1/2 and/or 1/3 stop increments.

Stops can refer to how much light is available, as in "It is four stops brighter in the sun than in the shade of that building." Or to how much light you allow through the camera, as in "I reduced the exposure by one stop of light."

FYI: In some photo books and articles you will find the terms "steps" used instead of stops. Like stops, each "step" refers to the doubling or halving of the amount of light.

The human eye is not a good judge of exactly how much light there is, which is why we have light meters.

This illustration will give you an idea what exposure looks like in one stop increments. You will see this illustration again in a metering article.

Exposure Tonality Chart
Exposure Tonality Chart in One Stop Increments
18% Gray Card (top row), red cloth, clear blue sky,
part of a yellow bowl, and green grass.


Five different subjects were photographed using five different exposure settings on the camera. As you move from left to right in each row of photos, each photo has one stop more exposure than the one to it's immediate left. The photos in the far right column are four stops lighter than the photos in the far left column.

The numbers in the top row of photos indicate whether the photos in that column were exposed using the camera meter's recommendation. The zero over the center column means the photo in that column were exposed according to the camera's meter reading . The photos in the other columns were exposed at one or two stops more or less than what the camera meter recommended. As an example, all the photos in the left hand column were exposed at two stops less than what the camera meter suggested. As you will learn in another article in this series, many subjects look best when they are exposed at a different exposure setting than the one the camera meter recommends. Knowing how and when to vary from the camera meter's recommendations is the difference between average exposures and great exposures!

Now that you have a handle on thinking in terms of stops, and an idea what stops look like in actual photos, let's look at apertures, shutter speeds, and ISO settings.

Lens Apertures

Lens apertures determine how much light can flow through the lens at any one moment in time. Inside most modern lenses are a set of aperture blades that open and close to create an opening of varying sizes that lets light pass through the lens. You can see the aperture blades inside the lens in this photo. The aperture opening is the dark octagonal shape in the center.

Aperture blades inside a lens.
Aperture blades in a 50mm lens set to f/8


The size of the aperture opening is indicated in "f-stops" (or "f stops"). In the case of the photo above, the aperture is set to f/8. "f-stops" are the ratio of the diameter of lens opening to the focal length of the lens. If the diameter of the aperture opening is 25mm and the focal length of the lens is 50mm, the f-stop or "focal ratio" of the lens is 25:50 or 25/50 or 1/2 so the f-stop is written f/2.   As long as you remember that f -stops are actually fractions  (the reason for the "/") with the number one on top, it will be easy to remember that f/2 (1/2) is a very wide lens opening and f/16 (1/16) is a very small lens opening. 1/2 a piece of pie is a lot more than 1/16 of a pie. In the photo below, the f-ratio of the maximum aperture is written as "1:1.4" which is the same as 1/1.4 or f/1.4.

Here is the same lens set to f/2, a very wide aperture.

50mm lens set to f/2

50mm lens at f/2


And here is the same lens set to f/11, a much smaller aperture. At f/11, most of the light entering the lens is blocked by the aperture blades so it doesn't reach the film/sensor.

50mm lens set to f/11
50mm lens set to f/11


When the aperture blades are completely open and out of the way, allowing the maximum amount of light to flow through the lens, the lens is "wide open" or set to its "maximum aperture".  Setting the lens to smaller apertures is called "stopping down". Setting the lens to wider apertures is called "opening up".  If someone says they "opened up one stop", that means they set the aperture (f-stop) one stop wider, doubling the amount of light that flows through the lens.

Here's the aperture series in one stop increments from wider to smaller, going from left to right.

Aperture seriesw in one stop increments.

As you go from left to right, each aperture is one stop smaller than the one before (and lets in half as much light). Going from right to left, the opposite is true.  f/22 is one stop smaller than f/16.  f/11 is two stops smaller than f/5.6. f/4 is two stops wider than f/8, and f/2 is two stops wider than f/4.

Most lenses won't have all of these aperture settings. A typical zoom lens for a film or digital SLR will go from a maximum aperture of f/4 to a minimum of f/22. A "faster" zoom will start at f/2.8.  Some very fast prime (single focal length lenses) will have a maximum aperture of f/1.4 or f/2.  Lenses on point and shoot digital cameras rarely have apertures smaller than f/8.

On most older lenses and a few newer lenses, you set the aperture using an aperture ring on the lens that will look something like this.

Aperture ring on an older lens.
Aperture ring on an older lens.

The aperture ring is at the bottom in this photo of a Canon FD 50mm lens. Apertures range from f/1.4 to f/22 and there is a green letter A for automatic aperture control. This lens also has click stops in between the above indicated apertures so the apertures can be set in 1/2 stop increments. The aperture opposite the orange index line (in this case f/8) indicates the aperture that has been chosen.

Many recent lenses do not have an aperture ring on the lens. The aperture is set electronically by controls on the camera. The camera will have an LCD or other kind of digital display to indicate the aperture in use, as well as other information. The LCD will have information that looks something like this.

LCD display on a digital camera.
LCD screen with the aperture (upper right) set to f/8.

Lenses with a wider maximum aperture than other comparable lenses in the same focal length range are called "fast" lenses, meaning they can let in let in more light than the average lens when they are wide open. Having a fast lens that lets in more light means you have the possibility of faster shutter speeds. If a wildlife photographer is struggling in low light with shutter speeds that are too long to freeze his subject, and says "I need a faster lens", you will know what that means. The same is true of a portrait or wedding photographer working indoors with ambient light.

Most lenses also have aperture settings in between the "full stop" settings in 1/2 or 1/3 stop increments. Some cameras give you a choice of either. Take your camera and go from f/11 to f/16. If you have one aperture in between f/11 and f/16, you have 1/2 stop increments. If you have two aperture settings in between, you have 1/3 stop increments.

It is a good idea to memorize the f-stop series. It will come in handy when you are doing exposure compensation (which is covered in the next article in this series). It will also give you something to do when boring commercials are on TV! 

If it helps to remember the series, the first two numbers in the series are 1 and the square root of 2 (i.e., 1.4). If you keep doubling each of those numbers (with a bit of rounding), you will have every other number in the series:

1 doubled: 1, 2, 4, 8, 16, 32, 64

1.4 doubled (with some rounding): 1.4, 2.8, 5.6, 11, 22, 44

Put the two number series together in numerical order and you will have the full f-stop series.  You will also come up with the tiny apertures that are mostly found only on the lenses that are used by large format photographers: f/32, f/44, and f/64.

If you are wondering where the word "stop" comes from in terms of photography, long ago a photographer would have a set of flat metal plates. Each plate would have a round hole or "stop" drilled in it, and the hole would be a different size for each plate. The photographer would choose the individual "stop" to insert into the camera to control the amount of light flowing through the lens.

The most important thing to remember about f-stops is that they control how much light goes through the lens at any one moment in time. And remember they are fractions, even if they aren't written that way. It is not uncommon to see f4 instead of f/4. Just remember that f/4 (1/4) lets in a lot more light than f/16 (1/16).

Shutter Speeds

Shutter speeds determine how long light flows through the lens to hit the film or digital sensor. Like lens apertures, shutter speeds are measured in stops and fractions of stops.

Shutter speeds are very intuitive and easy to learn since we deal with units of time all of the time. We all know that 8 seconds is twice as long as 4 seconds and 1/4 second is half as long as 1/2 second. With a bit of rounding here and there, shutter speeds in full stops deal with doubling and halving the amount of time, and therefore doubling and halving the amount of light.

Here is an extended list of shutter speeds in full stop increments. (Notice the rounding off of numbers, so the shutter speed before 1/60 is 1/125, not 1/120. The shutter speed after 8 seconds is 15 seconds, not 16 seconds.)

Shutter speeds in full stops incrments.

As you start at the top left and move across each row (and down to the next row), each shutter speed lets in one stop more light than the one before it. Going from right to left (and up to the next row) each shutter speed lets in half as much light.  1/60 second is one stop less light than 1/125 second. 2 minutes is two stops less light than 8 minutes.

The above shutter speed chart indicates an interesting feature of really long exposures . Going from 1 second to 2 seconds is a dramatic change in exposure, doubling the light for a total of one full stop.  Going from 30 seconds to 31 seconds is hardly any change in exposure at all, merely a tiny fraction of a stop. To add a stop to 30 seconds, you need to go to 60 seconds. If you are talking long exposures of several seconds, (or several minutes, a one second change (or one minute change) is not a big deal. If you need a 15 minute exposure to capture some star trails, it is not big deal if you end up using 14 or 16 minutes.

Your camera won't have all of these shutter speeds of course, but the range of shutter speeds continues to grow through the years. From 1/1000 second as the fastest shutter speed to 1 or 2 seconds at the longest used to be typical. A lot of cameras today offer 1/4000 or faster at the top end and 30 seconds on the long end.  Many cameras have a B (for bulb) setting so you can take photos for however long you want to hold down the shutter (or better yet, use a locking cable release). With the bulb setting, you can take photos that are seconds, minutes, or even hours long.  If you haven't taken photos of the night sky (without any moon) using shutter speeds from 4 minutes to 30 minutes long, or longer it's about time you try! If you don't know how, check out the relevant chapters in Digital Photography Exposure for Dummies.

Shutter speed dials omit the fraction when dealing in fractions of a second. 500 means 1/500 second.  60 means 1/60 second. That means things get a little tricky around 1 second. If you see a 2, does it mean 1/2 second or 2 seconds? Cameras have different systems for indicating the difference, ranging from color coding to little marks next to the numbers to indicate full seconds.  Be sure you are aware of how your camera does this. There is a big difference between 1/8 second and 8 seconds.

Here is a typical shutter speed dial for most older model cameras and some more recent retro style cameras.

Shutter spped dial.
Shutter Speed Dial

This shutter speed dial is set to 1/500 second. Shutter speeds longer than a second are in orange. Note the lighting bolt for the flash sync shutter speed (which is 1/90 second for a Canon F-1N), and the B for bulb.

For most recent cameras with electronic controls, you read the shutter speed on the LCD.

LCD screen
LCD screen with the shutter speed (upper left) set to 1/500 second.

On older cameras, shutter speeds were only available in full stop increments. More recent cameras have shutter speeds in 1/2 and/or 1/3 stop increments.

Just like with apertures, it is a good idea to memorize the shutter speeds in full stop  increments. It will come in handy when you want to use exposure compensation.

Reciprocity: Putting Apertures and Shutter Speeds Together

It is the combination of apertures and shutter speeds together that determines how much light reaches your digital sensor or film. A wide aperture and a faster shutter speed can provide the same exact exposure as a smaller aperture and a shorter shutter speed. It is just like filling up a bucket of water with different sized garden hoses. A big garden hose can fill a bucket faster than a tiny garden hose. And a huge hose from a fire truck can fill a bucket with water in almost no time at all.

Using different combinations of apertures and shutter speeds to get the same exposure is called "reciprocity" because there is a reciprocal relationship.

For example, on a cloudy day your camera meter says f/4 at 1/60 second and ISO 100 is a good exposure. Assuming the lighting conditions don't change (and the ISO stays at 100), all of these aperture and shutter speed combinations would provide exactly the same exposure.

f/22   at  1/2 second
f/16   at  1/4
f/11   at  1/8
f/8    at 1/15
f/5.6  at  1/30
f/4    at 1/60
f/2.8  at  1/125
f/2    at 1/250
f/1.4  at  1/500

As the aperture gets one stop wider in each successive row (doubling the light flowing through the lens), the shutter speed gets one stop faster (cutting the time in half), so the combination in each row provides the same amount of light to the sensor or film.

With so many combinations to pick from, all providing the same exposure, which should you choose? That is an artistic decision because the combinations you choose will provide very different artistic "looks".

If you are shooting a landscape, you might choose the f/22 and 1/2 second combination to maximize depth of field (and you would need a tripod, bean bag or other stable camera support).  If you are at a track meet, you would choose the f/1.4 and 1/500 second combination if you wanted to freeze the runners.  On the other hand, if you wanted a blurred runner, you would go with a much slower shutter speed. If you are going to pan with the runner and blur the background, that would require yet a different shutter speed. If you want more information and a lot of illustrations as to how to do this, Digital Photography Exposure for Dummies has several chapters devoted to the artistic side of exposure and the ways you can use different exposure combinations to create unique and creative photos.

Given the above photographic situation, what if you don't have a lens with an aperture any wider than f/4, but you want to use a shutter speed of 1/500 second? that's where ISO comes into play.

ISO Settings

ISO settings, also called ISO "speeds" are simple to remember. Just like apertures and shutter speeds, they are in full stop increments as well as fractions of a stop. Here is the easy to remember ISO speed series in full stop increments.

ISO spped series

Starting at the upper left and moving to the right, each ISO speed is one stop faster than the one before it.  ISO 800 is two stops faster than ISO 200. ISO 1600 is one stop slower than ISO 3200.

Film with ISO speeds at 50 and 25 used to be common and specialty films were available at even lower ISO speeds. Most digital cameras start at ISO 100 or 200 and go to 3200, 6400 or higher. ISO speeds are also available on most digital cameras in 1/2 and/or 1/3 stop increments which are in between the full stop speeds listed above.

On an older cameras, the ISO speed dial looks like this.

ISO (ASA) Speed Dial

"ASA" speeds on older cameras are equivalent to today's ISO speeds. This dial is set to ISO 25.

The numbers on the right side of the dial (1/4, 1/2, 1, 2, 4) are the exposure compensation settings. 1/4 (1/4 the light) equals -2 stops of exposure compensation. 1/2 equals -1 stop. 1 equals no compensation (sometimes indicated by a zero on today's cameras). 2 (2 times the light) equals +1 stop of compensation (the same as on the LCD screen below). 4 equals +2 stops. The marks in between are in 1/3 stop increments. Exposure compensation will be covered in the next article in this series.

On most electronic cameras, the ISO speed is listed somewhere on the LCD screen.

LCD screen.
LCD screen with the ISO (middle right) set to 100.

The simple way to think of ISO speeds is that with each one stop gain, the film or digital camera responds twice as fast to the light. (Technically, that's not true for digital cameras. More about that later.) In practical terms, that means each time you go to an ISO speed one stop faster, you can use a shutter speed one stop faster, or a lens aperture one stop smaller. Reciprocity is at work again!

As an example, if f/4 and 1/60 second gives you the right exposure at ISO 100, if you change the ISO to 200, you can change the shutter speed to 1/125 second. By doubling the ISO (a one stop gain), you can use a shutter speed one stop faster (letting in half the light) and still have the same exposure.

To take the same cloudy day situation that I used at the end of the shutter speed section above, each of these combinations would give you the same exposure, but this time we will change the shutter speed and the ISO, leaving the aperture the same.

f/4  at  1/60    and ISO 100
f/4  at  1/125   and ISO 200
f/4  at  1/250   and ISO 400
f/4  at  1/500   and ISO 800
f/4  at  1/1000  and ISO 1600
f/4  at  1/2000  and ISO 3200

To go back to the question at the end of the shutter speed section, if you are at that track meet on a cloudy day and you don't have a "fast" lens with an f/1.4 maximum aperture, you can't shoot at f/1.4 at 1/500 a second at ISO 100. With your zoom lens with a maximum aperture of f/4, by changing the ISO from 100 to 800 you can shoot at a motion stopping 1/500 second at f/4. Such a deal!

This also applies to apertures. By upping the ISO, you can use smaller apertures. These are all equivalent exposures.

f/4    at  1/60      and ISO 100
f/5.6  at  1/60      and ISO 200
f/8    at 1/60     and ISO 400
f/11   at  1/60      and ISO 800
f/16   at  1/60      and ISO 1600
f/22   at  1/60      and ISO 3200

Using higher ISO speeds sounds like the photographic promised land until you consider there is a penalty involved. If you are using film, you get more grain at higher ISO speeds. If you are using a digital camera, you get more digital noise at higher ISO speeds.

Film photographers have been dealing with grain for years. Higher speed film has to have bigger grain in order to respond more quickly to the light. High speed film really is faster.

In the digital world, turning up the ISO doesn't really make your camera faster (although in terms of photographic results it acts that way). The same amount of photons of light still hit the digital sensor. The sensor doesn't become any faster. In non-technical terms, when you turn up the ISO on your digital camera, the camera amplifies the signal from the sensor, or in other words, it turns up the gain. It is kind of like turning up the volume on an old tube style radio to amplify the signal from a distant radio station. The station gets louder but you pick up a lot of noise or static.  When the camera turns up the gain, you get the right exposure with less light, but you also get a lot of visual noise or static.

High speed film grain looks quaint and artsy. In fact, you can buy software to add the look of film grain to your digital photos.

Unfortunately, high ISO digital noise in a photo looks ugly and blotchy. The good news is that camera manufacturers are all working overtime to find ways to reduce the noise.  ISO 800 digital photos look as good as ISO 400 digital photos from a few years ago. The digital noise situation continues to get better all of the time.

If you shoot digital, the best advice is to use the lowest ISO setting you can get away with and still have the shutter speed and aperture you need for the situation you are shooting in.

One note to film users. Changing the ISO dial on your camera does NOT change the ISO speed of the film in your camera.  If you need a higher ISO, you need to put in a higher ISO speed film.

Putting It All Together

Lens aperture, shutter speed, and ISO (all three together) determine the final exposure of all of your photos. The aperture and shutter speed together determine how much light hits the film or digital sensor.  The ISO of the film in your film camera determines how quickly the film responds to the light. The ISO setting of your digital camera changes how your camera circuits reacts to the light. While not technically correct, using a higher ISO makes your digital camera "act" like it is responding more quickly to the light.

The Water Bucket Theory of Exposure

A simple way to think of all this is to remember my "water bucket theory of exposure". You have a bucket that you want to fill with water. The size of the hose you use is like the aperture of your lens. The time it takes to fill the bucket is the shutter speed.

If you use a really big hose (large aperture) the bucket will fill up more quickly (faster shutter speed). If you use a really small hose (small aperture) it will take a lot longer to fill up the bucket (longer shutter speed). What if it is taking too long to fill the bucket? Switch to a larger hose (larger aperture).

What if you are using the biggest hose you have (largest lens aperture) and it is still taking to long to fill the bucket? Change to a smaller bucket. Switching to smaller bucket is like changing to a higher ISO setting. A small bucket (high ISO) fill up in less time than a big bucket (low  ISO).

Now that you know how this all works, how do you know how much light needs to hit the film or sensor for a given ISO setting? That's where metering comes in, the next article in this series.


Exposure Articles

Why Is Exposure So Important?

Speaking Your Camera's Exposure Language: Aperture, Shutter Speed, and ISO

Using Reflected Light Meters, Part One

Using Reflected Light Meters, Part Two

Simplify Your Life with an Incident Light Meter

Turn Your Camera Meter Into An Incident Light Meter: Using an ExpoDisc

Using a Gray Card, Incident Light Metering on the Cheap

The RAW versus JPEG Exposure Advantage

More Information

The most detailed information about metering, camera meters, and other metering tools is in my book, Digital Photography Exposure for Dummies.

May 20, 2011
Updated May 20, 2013

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