Guide to Exposure

Exposure; it’s what photography is all about. But getting it right can be difficult and requires a reasonable level of understanding.

Well, I’m here to help! I’ve been taking pictures for years and have developed a solid understanding of exposure as I have been exposing myself since I was a boy.

To start with you may find the concept of exposure a difficult one to grasp. Most of you will currently rely on your camera to expose for you, particularly if you have other, more experienced exposers watching; nobody wants to expose themselves incorrectly in front of others.

But by following this guide you should begin to get a good grasp of the nature of exposure and before long you will no longer be relying on your cameras to do the exposing for you.

Read, absorb, and let your confidence grow. Cast off your shackles of uncertainty and expose yourself, no matter who’s watching!

Ben.

What is exposure?

Ok, so jokes aside, exactly what do we mean when we talk about exposure?

Well, all cameras essentially record an image in the same way. Light passes in through the lens and hits a device used to record these light rays. This can be a frame of light sensitive film or, far more commonly these days, a digital sensor.

Put simply, the exposure is the amount of light that must be received by the film or sensor to record the image correctly. Too little light and the recorded image is too dark. Too much light and the image becomes “burnt out”.

Under Exposed

Too dark (under-exposed)

Correct Exposure

Correct Exposure

Over Exposed

Too bright (over-exposed)

How do I control exposure?

There are three different factors that control the exposure: Aperture, Shutter, and I.S.O.

The first two control the amount of light that is allowed to fall on to the sensor. The Aperture controls the volume of light and the Shutter controls the amount of time that the light is able to fall on to the sensor for.

Picture it this way: imagine the sensor is a bucket and the light is water falling into that bucket from a pipe. The Aperture is the size of the pipe and the Shutter is the tap that turns the flow on and off. The amount of water required to fill the bucket is the Exposure.

Aperture-Shutter speed tap ananaly

 

With a bigger pipe more water can pass through, so the tap has to be open for less time for the bucket to fill up with water than it would if the pipe was smaller.

In exactly the same way, a larger Aperture means more light is let through so the Shutter can be open for less time, or to put it into more commonly used language, the Shutter Speed can be faster.

Aperture-Shutter speed tap ananaly

small aperture + short exposure = under exposed

If the taps are shut off when the larger pipe has filled the bucket, the lower flow of water through the smaller pipe means the bucket is under-filled.

A Shutter Speed that lets enough light through a larger Aperture for a correct exposure will not give the same results with a smaller Aperture. Not enough light will be let through and the image will be under-exposed.

Aperture-Shutter speed tap ananaly

large aperture + large exposure = over exposed

Small Aperture/Slow Shutter Large Aperture/Slow Shutter If the taps are left on long enough for the bucket to fill from the smaller pipe, the higher amount of water flowing from the larger pipe will cause the bucket to overflow.

If a Shutter Speed provides the correct exposure with a smaller Aperture, the Same Shutter Speed with a larger Aperture will allow too much light on to the sensor and the image will be over-exposed.

The I.S.O. determines the sensitivity of the film/sensor to light. The more sensitive the film/sensor, the less light is required to expose an image. Unlike film, on a digital sensor this sensitivity can be altered with each shot.

Picture our bucket, pipe and tap again. Imagine that I.S.O. refers to stones in the bucket; the higher the I.S.O., the more stones you put in the bucket.

ISO-buckets and stones

      Under-filled                            Correct                               Over-filled                        Over-filled

With a few stones added in to the bucket, less water is required to fill it up, so the tap does not need to be left open for so long, even with a small pipe. Put too many stones in or increase the size of the pipe and the bucket may still overflow.

By increasing the I.S.O. on a camera it is possible to use faster Shutter Speeds with a smaller Aperture. If the I.S.O. is too high or the Aperture is also increased, the image may be over-exposed. There are many methods of getting the correct exposure by balancing the Aperture, Shutter and I.S.O. settings. Below are just a few examples.

more ISO-buckets and stones

 

Enough of the buckets! So what is the Aperture?

The Aperture works like the iris of your eye (the coloured bit around the pupil). Under bright light conditions, the iris contracts, making the pupil smaller, so less light can pass through your eye. Under dim light conditions, the iris expands, increasing the size of the pupil and letting more light pass through. When it is dark, your iris fully expands, making the pupil as large as possible to absorb the maximum amount of light.

Bright Light

 

Normal Light

 

Low Light

 

The Aperture can also be expanded and contracted to let more or less light in as required. Aperture sizes are given in “f/stops”. The higher the f/stop, the more the Aperture contracts and the less light is allowed to pass through the lens. The lower the f/stop number, the wider the Aperture is and the more light is admitted.

f2.8

f/2.8

f4

f/4

f5.6

f/5.6

f8

f/8

f11

f/11

f16

f/16

f22

f/22

The Aperture itself is usually housed within the lens of the camera.

Depending on the Aperture setting, more or less light from a scene passes through the back of the lens and on to the sensor (or film) in the camera where the image is recorded.

As already mentioned, Aperture settings are quoted in f/stops. Now, although these f/stops look like random numbers with little or no meaning, there is a specific sequence. It goes something like this:

f1

f/1

f1.4

f/1.4

f2

f/2

f2.8

f/2.8

f4

f/4

f5.6

f/5.6

f8

f/8

f11

f/11

f16

f/16

f22

f/22

f32

f/32

f45

f/45

These are all the WHOLE f/stops from f/1 up to f/45. The most commonly found f/stops are from f/2.8 up to f/22, and the exact range differs from lens to lens and camera to camera. You will also come across f/stops other than those above. Some cameras and lenses utilise halves or thirds of a stop, so you may find f/stops such as f/1.8, f/4.9 or f/7.1. Each whole f/stop change halves or doubles the amount of light being let through. For example, changing from f/2.8 to f/4 will half the light. Changing from f/8 to f/5.6 will double the amount of light.

aperture light diagram

 

Hope this has all gone in, because you’ll be needing it later on…

So, if lower f/stops let in more light, why don’t we just use wide apertures all the time? Well, in addition to controlling the amount of light that passes through the lens, the aperture also controls the “depth of field”.

Depth of field can be quite a tricky concept to grasp, but it basically refers to how much of a scene is in focus from front to back. Have a look at this delightful little vista:

mountain scene

The pink flower is the closest object and appears in the very foreground of the image. The snow-capped mountain is the furthest away and in the middle distance there’s a tree. Between the tree and the mountain are some sheep and between the flower and the tree are some ducks. Beautiful isn’t it?

Now, let’s suppose you wanted to take a picture of the tree. Maybe it’s a very beautiful tree; maybe it’s a very rare tree; maybe you just like trees. You focus on the tree and take your photo. The aperture you are using will determine how much of the rest of the scene is in focus. Now, the lower the f/stop, the wider the aperture. The wider the aperture, the smaller the depth of field. The smaller the depth of field, the less that will be in focus in-front or behind the tree. The higher the f/stop, the narrower the aperture. The narrower the aperture, the greater the depth of field. The greater the depth of field, the more that will be in focus in-front or behind the tree.

mountain scene with aperture settings

 

If you took the picture with a wide aperture of f/2.8, the resulting image would have the tree in focus, but very little else.

With an aperture of f/8, the tree is still in focus, but the sheep slightly further away than the tree and the ducks which are slightly closer are also in focus.

At f/22 everything from the flower in the foreground to the mountain in the background is sharp, despite the tree still being the actual point of focus.

f16

f/16

f8

f/8

f2.8

f/2.8

These pictures show how this works in practice. The first image was taken with an aperture of f16. Although not entirely in focus, the background is clearly visible and distracting. The second image is improved due to an aperture of f/8, but the backdrop is still too noticeable. The third image was shot at f/2.8. This has blurred the background to the extent that it no longer interferes with the subject of the photograph, the blossom flower.

 

Seal
Flower

The shallow depth of field given by wide apertures is usually preferred for portrait and some wildlife photography. Using a low f/stop means that the foreground and background are blurred and do not distract from the main subject. The photograph of the seal on the left and the flower on the right both demonstrate this technique.

Water
Venice

Using a high f/stop brings more of the image into focus and is therefore often used by landscape and architectural photographers who require everything from the closest objects to the most distant part of the scene to be in focus. The image on the left shows both the foreground rocks and background trees in focus. The image on the right has all the columns of the building sharp.

Ok, I get Aperture. What about Shutter Speed?

Shutter Speed is probably an easier concept to understand, but you’re still going to need to concentrate.

We know that a photograph is created by light falling on to a digital sensor or film. Well, this film or sensor is hidden in the camera behind a shutter. When you take a photograph, this shutter opens to allow light from the lens to fall onto the sensor, much like a blind being opened to allow light in to a room.

Shutter Closed

With the shutter closed, the light from the lens can not reach the sensor.

Shutter Open

When the shutter button is pressed, the shutter opens and the sensor is exposed.

The Shutter Speed refers to the amount of time the shutter is open. The longer the shutter is open, the more light reaches the sensor.

Store Queue example

Let’s put this in to a real life type example; it’s the morning of the start of a well known high-street store’s summer sale. People have been queuing up since midnight to make sure they’re the first through the door to grab the best bargains. As soon as that door is opened, the budget conscious fashion connoisseurs will pour into the store in an excited shopper frenzy.

In an attempt to minimize the amount of hair pulling, eye scratching and handbag slapping as customers fight for that perfect outfit, security have decided that they will only open the doors for short bursts at a time.

Store Queue example - two by two

When the doors are opened, people start to enter the store. If the door is closed again almost immediately, only a couple of people have passed through.

Store Queue example - two by two

If the doors are left open a little longer before they are closed, more people have time to enter the shop.

Store Queue example - two by two

If the doors are left open twice as long as before, then twice as many people are able to enter the store and commence the Battle of the Boob-Tube.

Light passing through a shutter and onto a sensor works in much the same way. In the above examples, the amount of time that the door is open for would be the Shutter Speed. The slower the Shutter Speed, the longer the door is open and more light will be absorbed by the sensor.

Shutter speeds are primarily measured in seconds. Listed below is a typical range.

Shutter Speeds

These times are how long the shutter will stay open for when the shutter release button is pushed. In other words, it is how long the sensor will be exposed to light for.

In addition to these, some cameras also have a Bulb and/or timer setting. These are usually shown as a “B” or “T” setting on the camera. With the Bulb setting, the shutter stays open for as long as you hold the shutter release button. , With the Timer setting, the shutter release button is pushed once to open the shutter and again to close to it. These are useful for long exposures which require more time than the longest setting on the camera (usually 30 seconds).

As with the Aperture values, the difference between each of these shutter speeds is either double or half the amount of light. Changing from 1/500 second to 1/250 second will double the amount of light permitted to reach the sensor, whereas shutter speeds other than those above, such as 1/80, 1/200 or 1/3000, where camera use halves and thirds of a stop to obtain the most accurate exposure.

In addition to limiting the amount of light that reaches the sensor, the shutter speed will also have an effect on how the final image appears. As well as being an important part of the exposure calculations, varying the shutter speed can have other effects.

Hand Print Exposure

 

The image above shows 3 hand prints.

When the first print was made, the hand was not in contact with the paper long enough and the print is unclear. If the shutter is open for too shot a time, the sensor will not receive enough light and the image will be under exposed. The second hand is correctly printed. The hand was placed on the paper for just long enough for all the detail to be printed. If the exposure is correct and the shutter open for just the right amount of time, the resulting image will have full detail. In the final version, the hand has been in contact with the paper for too long. All the detail has been over printed and lost. If the shutter speed is too slow and the shutter open too long, the image will be over exposed and detail will be lost. If either the photographer or the subject of the photograph is moving, then using the correct shutter speed becomes even more important. Let’s look at the hand-print example again.

Hand Print Blur

 

If the owner of the hand is moving and the paper is also moving, then the print needs to be made quickly to avoid smudging. The second and third prints above show the result if the hand is left in contact with the paper for longer. The print becomes increasingly smudged and detail is lost.

If the shutter speed is too long, then the image recorded on the sensor will also be blurred if either the photographer or the subject is moving. This is usually not desirable, but on occasion may be the required effect.

1/40 second

1/40 second

1/200 second

1/200 second

1/500 second

1/500 second

These images of a water feature show the effect of changing shutter speeds. The first image above used a relatively slow shutter speed and the moving water has been blurred. The second picture used a faster shutter speed and although there is more detail, the water is still quite blurred. In the third picture on the right, the movement of the water has been almost frozen by using a faster shutter speed still. It is possible to see some of the individual water droplets as they fall.

shutter speed example 1

 

shutter Speed example 2

 

Shutter Speed example 3

 

These 3 images show how different shutter speeds can be deliberately used to get different image effects. The first image shows a slow shutter speed being used on a stationary camera. The static buildings are clear, but the milling crowds are slightly blurred to show motion. The second picture also uses a slow-ish shutter speed, but this time the camera is also being moved to track the subject. This results in the static background of the slide being very blurred, but the girls on the slide are much clearer. This effect has been used to try and give a feeling of speed and is also often used in motor-sports photography. The third image uses a high shutter speed to freeze the motion of the subject and the splash of the water.

How do I use Aperture and Shutter Speeds together?

If you’ve chosen a particular Aperture you want to use, then you’ll have to adjust the Shutter Speed to get the correct exposure. Likewise, if you want to use a specific Shutter Speed, you’ll need to work out the correct Aperture to allow you to do so and get the correct exposure.

Let’s look again at a range of Aperture and Shutter Speed values.

Shutter Speed and aperture values

 

We know that slowing the shutter speed or opening the f/stop increases the exposure, and that increasing the shutter speed or closing the f/stop reduces exposure. It therefore makes sense that if you want to increase the shutter speed you need to reduce your f/stop and vice-versa.

Example: Reducing the depth of field

Shutter Speed and aperture values

f/8 @ 1/30 second

The photograph on the left was taken at 1/30 second at f/8. The picture is sharp from front to back. If you wanted to reduce the depth of field so that only the badge was sharp you would need to use a wider aperture and reduce the f/stop.

Shutter Speed and aperture values

f/2.8 @ 1/30 second

However, simply reducing the f/stop to f/2.8 results in the picture on the right. Because the shutter speed has not been changed and was left at 1/30 second the image has been over exposed.

Shutter Speed and aperture values

f/2.5 @ 1/250 second

By using a faster shutter speed as well as increasing the aperture, over exposure is prevented. The larger aperture has started to blur the foreground and background, but none of the detail has been burnt out.

Example: Increasing the shutter speed

Shutter Speed and aperture values

f/8 @ 1/30 second

Let’s start with the same image again, but this time you want to use a faster shutter speed because you are having trouble holding the camera still and want to make sure that the image isn’t blurred.

Shutter Speed and aperture values

f/8 @ 1/250 second

Changing the shutter speed to 1/250 second but leaving the aperture at f/8 results in the image being under exposed and appearing to dark.

Shutter Speed and aperture values

f/2.5 @ 1/250 second

To correctly expose the image, the aperture must be expanded to counteract the reduction in light caused by using a faster shutter speed.

The table below shows how the image becomes darker or lighter as the aperture and/or shutter speed values are amended. The frames in the green boxes are those which have been correctly exposed. As you can see, in order to maintain the correct exposure, as the shutter speed becomes faster, a wider aperture must be used, and as the shutter speed becomes slower it is necessary to use a smaller aperture. Likewise, as the aperture increases in size, so the speed of the shutter must be increased to prevent over-exposure, and if the aperture is reduced, a slower shutter speed is required to keep the exposure correct.

As mentioned in previous sections, the difference between whole aperture f/stops is double or half the exposure, (e.g. f/4 lets in half the amount of light as f/2.8, and f/5.6 lets in twice as much light as f/8). We also know that increasing or decreasing whole shutter speed values has the same effect (e.g. 1/60 second lets in half as much light as 1/30 second, and 1/125 second lets in twice as much light as 1/250 second). As such, to maintain correct exposure, any change in aperture value requires an equal but opposite change in shutter speed, and vice versa. Each whole step of value change is called a “stop”.

f-stop shutter speed chart

 

Looking again at the table above we can see that an aperture of f/5.6 with a shutter speed of 1/60 second gives the correct exposure. If the f/stop is reduced by 1 stop to f/4.0, the shutter speed must be increased by 1 stop to 1/125 second. If the shutter speed is decreased by 2 stops from 1/60second to 1/15 second, the f/stop must be increased by 2 stops from f/5.6 to f/11.

Obtaining the correct exposure is all about balancing the shutter speed and aperture values to keep the light levels correct whilst achieving the required image effect.

Hang on, what about I.S.O.?

I.S.O. stands for International Standards Organisation, which means pretty much nothing to you or me. What is important though, is that I.S.O. is a measurement of how sensitive a film or sensor is to light.

In the olden days before the advent of digital sensors, photographers used a now rare and undervalued substance called “film”. Different films required different amounts of light to expose an image. These characteristics were dependent on the films sensitivity to light, so the I.S.O. labelling system meant it was easy to know what sort of film you were getting. Low I.S.O numbers such as 50, 64 or 100 required more exposure to record the scene, whereas higher I.S.O. films such as 800 or 1600 required much less light.

However, there was a trade off in terms of quality. The higher I.S.O. films could record images with less light so allowed for the use of fast shutter speeds, even under relatively dim conditions. As such they were ideal for reportage photography, but they could not match the lower I.S.O films for detail, meaning that these low I.S.O. films were preferred for landscape or portrait photographers. High I.S.O. films had a pronounced grain and higher contrast which meant that subtle detail in shadows and highlights could be distorted or lost altogether.

These pictures show how grain increases and detail decreases as the I.S.O. gets higher and the film becomes more sensitive to light.

ISO 100

ISO 100

ISO 100

ISO 200

ISO 100

ISO 400

ISO 100

ISO 800

ISO 100

ISO 1600

ISO 100

ISO 3200

ISO 100-3200 comparison

ISO 100 / 3200 comparison

Now, in the modern era with most cameras in use being digital, the need to select a film on the basis of its I.S.O. is almost irrelevant. However, I.S.O. is still very much relevant.

Digital cameras enable you to alter the sensitivity of the sensor shot by shot if required, and the I.S.O. numbering system is still used. Although sensors don’t have grain as such, image quality is still affected by changes in sensor sensitivity. When a high I.S.O. is used, digital images can suffer from something called “noise”. This is similar to film grain, but is usually coloured and appears in the darker areas of an image.

ISO 100-3200 comparison

ISO 100

ISO 100-3200 comparison

ISO 200

ISO 100-3200 comparison

ISO 400

ISO 100-3200 comparison

ISO 800

ISO 100-3200 comparison

ISO 1600

ISO 100-3200 comparison

ISO 3200

Although not necessarily immediately obvious, the noise in the above images becomes evident when the darker areas are enlarged.

ISO 100-3200 comparison

Once blown up it’s easy to see the colour distortion in the darker areas of the image. This is noise. A typical I.S.O. range on a modern digital camera is from 200 to 1600, with some models extending that range to 100 to 3200 I.S.O. However, it is also possible to find I.S.O. as low as 50 and as high as 6400 on some of the newer, high performance cameras.

As you might expect, an I.S.O. of 50 is half as sensitive as an I.S.O. of 100 and therefore requires twice as much exposure. An I.S.O. of 400 is twice as sensitive as an I.S.O. of 200 and requires half as much exposure. It is therefore possible to use changes in I.S.O. alongside changes in aperture and shutter speed to obtain the correct exposure. With film you were limited to whatever I.S.O. the film was until the film had been finished, but with digital cameras these changes can be made on a shot by shot basis.

ISO 100-400 comparison

1/125 second, f/5.6, I.S.O. 100

ISO 100-400 comparison

1/250 second, f/5.6, I.S.O. 200

ISO 100-400 comparison

1/250 second, f/8, I.S.O. 400

So how do I do all this on a camera?

Cameras come with a variety of exposure modes. Some offer only automated modes, whereby the camera determines the best exposure method, whereas others allow some or total manual control. Listed below are some of the more common modes, what they do and how they work.

Standard Exposure Modes:

Auto

Automatic mode. The camera will automatically choose the best combination of aperture, shutter Speed and I.S.O. depending on subject and lighting conditions.

Program

Program mode. The camera will automatically choose the best combination of Aperture and Shutter Speed, but it is possible to adjust these and the I.S.O. manually.

Aperture Priority

Aperture priority. The user selects the aperture and the camera works out which shutter speed is required. I.S.O. can be manually altered.

Shutter Priority

Shutter priority. The user selects the shutter speed and the camera works out which aperture is required. I.S.O. can be manually altered.

Manual

Manual. The user selects the aperture, shutter speed and I.S.O. required.

Many cameras also offer a variety of automatic modes based on the subject matter. These are known as Scene Modes and they vary from camera to camera. When the camera is put into one of these scene modes the camera automatically selects the optimum aperture, shutter speed and I.S.O. combination for the type of picture selected. Below is a selection of some available modes:

Portrait  Portrait
Night Portrait  Night Portrait
Landscape  Landscape
Sport  Sport
Indoor / Party  Indoor / Party
Beach / Snow  Beach / Snow
Sunset  Sunset
Museum  Museum
Night Landscape  Night-time
Firework Display  Fireworks
Backlit  Backlit
Underwater  Underwater

But how do I know what the correct exposure is?

With older, manual cameras it was necessary to have a separate external light meter to make these measurements, with the exposure details then being entered into the camera. Modern equivalents of such meters are still used for studio and flash photography. Even when exposure meters first appeared on cameras they were of the needle and dial variety and simply gave a value which could then be used to manually control the aperture and shutter speed of the camera.

Backlit

Older exposure meter

Backlit

Modern exposure meter

Backlit

Early viewfinder exposure meter

These days nearly all cameras will have some kind of exposure meter built in. These exposure meters measure the light falling on to the sensor and allow either the user or the camera to work out the best exposure. In automatic modes the camera uses this information to calculate the correct exposure, but in manual modes the user need to see this metering information to determine what exposure to use. The reading is displayed in the viewfinder, on an LCS panel on top of the camera or on the main screen on the back, and may look a little like this:

Digital Viewfinder Exposure meter

As the exposure is altered by changing the shutter speed or aperture, a needle or illuminated bar moves up and down the scale to show whether the current settings will under or over expose the image.

This scale is usually displayed even when the camera is in control of exposure, but when the user is manually setting the exposure they must rely on the cameras metering to obtain the correct settings.

Metering? How does that work?

Metering is basically how the camera works out the exposure. It measures the light levels entering the camera and works out how much exposure is required to make an image.

The problem with this is that different parts of a scene may be brighter than others. The camera doesn’t know this and just measures the total amount of light. As such some parts may be under exposed and others may be over exposed. However, if the user tells the camera which parts of the scene are most important the camera can make sure that these areas are exposed correctly.

Multi metering example

The image on the right was taken using Average or Multi Metering. The camera has measured the total amount of light falling on to the sensor and has worked out an average exposure. The bright sky and dark foliage have confused the meter and as a result the main subject, the rather attractive flame haired weeble, is under exposed.

Centre-Weighted example

With the metering mode changed to Centre-Weighted, the camera again measures the total amount of light from the scene, but biases the exposure based on the light levels in the centre of the frame. This has improved the exposure of the weeble. Although still under-exposed, it is not as dark as the previous image and the overall effect is far more satisfactory.

Spot metering example

This image was taken using Spot Metering. The camera bases the exposure on the meter reading from a very small area in the centre of the frame and ignores the rest of the image. The weeble is now correctly exposed, but the rest of the scene is far too bright as a result. The sky is completely burnt out and the foliage is bleached.

Average Metering

Average Metering

Centre-Weighted Metering

Centre-Weighted Metering

Spot Metering

Spot Metering

Even with the camera in full control of the exposure, using the various metering modes available means that the user can still take some control of the image by determining which parts of a scene the camera should measure.

Some modern cameras have even more sophisticated metering systems which can work out the types of scene being photographed simply from the light falling on to the sensor, while others will also meter colour temperature or take distance to subject in to account when metering. However, the three modes mentioned above are the most commonly used and can be found on all types of cameras from compact pint and shoot up to pro-spec SLRs.

Thanks for that. I’m ready to expose myself.

Well done you. Despite the reams of information presented here, the basics are really quite straightforward.

And remember, should you have any more queries you can usually find someone to ask in our call centre, someone who’s more familiar with exposing themselves.