What Are The Basics Of Exposure?



The Exposure Triangle:

Three main elements that need to be considered when playing around with exposure by calling them ‘the exposure triangle’.

Each of the three aspects of the triangle relates to light and how it enters and interacts with the camera.

The three elements are:

  1. ISO – the measure of a digital camera sensor’s sensitivity to light
  2. Aperture – the size of the opening in the lens when a picture is taken
  3. Shutter Speed – the amount of time that the shutter is open

Check This Image

Camera Exposure:

A photograph’s exposure determines how light or dark an image will appear when it’s been captured by your camera. Believe it or not, this is determined by just three camera settings: aperture, ISO and shutter speed (the “exposure triangle”). Mastering their use is an essential part of developing an intuition for photography.

Understanding Exposure:

Achieving the correct exposure is a lot like collecting rain in a bucket. While the rate of rainfall is uncontrollable, three factors remain under your control: the bucket’s width, the duration you leave it in the rain, and the quantity of rain you want to collect. You just need to ensure you don’t collect too little (“underexposed”), but that you also don’t collect too much (“overexposed”). The key is that there are many different combinations of width, time and quantity that will achieve this. For example, for the same quantity of water, you can get away with less time in the rain if you pick a bucket that’s really wide. Alternatively, for the same duration left in the rain, a really narrow bucket can be used as long as you plan on getting by with less water.

In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the width, time and quantity discussed above. Furthermore, just as the rate of rainfall was beyond your control above, so too is natural light for a photographer.

Exposure Triangle: Aperture, ISO & Shutter Speed:

Each setting controls exposure differently:

Aperture: controls the area over which light can enter your camera

Shutter speed: controls the duration of the exposure

ISO speed: controls the sensitivity of your camera’s sensor to a given amount of light

One can, therefore, use many combinations of the above three settings to achieve the same exposure. The key, however, is knowing which trade-offs to make, since each setting also influences other image properties. For example, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.

The next few sections will describe how each setting is specified, what it looks like, and how a given camera exposure mode affects their combination.

Shutter Speed:

A camera’s shutter determines when the camera sensor will be open or closed to incoming light from the camera lens. The shutter speed specifically refers to how long this light is permitted to enter the camera. “Shutter speed” and “exposure time” refer to the same concept, where a faster shutter speed means a shorter exposure time.

By the Numbers. Shutter speed’s influence on exposure is perhaps the simplest of the three camera settings: it correlates exactly 1:1 with the amount of light entering the camera. For example, when the exposure time doubles the amount of light entering the camera doubles. It’s also the setting that has the widest range of possibilities:

Typical Examples

Speciality night and low-light photos on a tripod To add a silky look to flowing water

Landscape photos on a tripod for enhanced depth of field

To add motion blur to the background of a moving subject

Carefully taken hand-held photos with stabilization

Typical hand-held photos without substantial zoom

To freeze everyday sports/action subject movement

Hand-held photos with substantial zoom (telephoto lens)

To freeze extremely fast, up-close subject motion

Shutter Speed

1 – 30+ seconds 2 – 1/2 second 1/2 to 1/30 second 1/50 – 1/100 second 1/250 – 1/500 second 1/1000 – 1/4000 second

How it Appears. Shutter speed is a powerful tool for freezing or exaggerating the appearance of motion:

With waterfalls and other creative shots, motion blur is sometimes desirable, but for most other shots this is avoided. Therefore all one usually cares about with shutter speed is whether it results in a sharp photo — either by freezing movement or because the shot can be taken hand-held without camera shake.

How do you know which shutter speed will provide a sharp hand-held shot? With digital cameras, the best way to find out is to just experiment and look at the results on your camera’s rear LCD screen (at full zoom). If a properly focused photo comes out blurred, then you’ll usually need to either increase the shutter speed, keep your hands steadier or use a camera tripod.

For more on this topic, see the tutorial on Using Camera Shutter Speed Creatively.

APERTURE SETTING:

A camera’s aperture setting controls the area over which light can pass through your camera lens. It is specified in terms of an f-stop value, which can at times be counterintuitive because the area of the opening increases as the f-stop decreases. In photographer slang, when someone says they are “stopping down” or “opening up” their lens, they are referring to increasing and decreasing the f-stop value, respectively.

By the Numbers. Every time the f-stop value halves, the light-collecting area quadruples. There’s a formula for this, but most photographers just memorize the f-stop numbers that correspond to each doubling/halving of light:

Relative Light Example Shutter Speed

1X16 seconds

2X8 seconds

4X4 seconds

8X2 seconds

16X1 second

32X1/2 second

64X1/4 second

128X1/8 second

256X1/15 second

Aperture Setting

f/22 f/16 f/11 f/8.0 f/5.6 f/4.0 f/2.8 f/2.0 f/1.4

The above aperture and shutter speed combinations all result in the same exposure.

Note: Shutter speed values are not always possible in increments of exactly double or half another shutter speed, but they’re always close enough that the difference is negligible. The above f-stop numbers are all standard options in any camera, although most also allow finer adjustments of 1/2 or 1/3 stops, such as f/3.2 and f/6.3. The range of values may also vary from camera to camera (or lens to lens). For example, a compact camera might have an available range of f/2.8 to f/8.0, whereas a digital SLR camera might have a range of f/1.4 to f/32 with a portrait lens. A narrow aperture range usually isn’t a big problem, but a greater range does provide for more creative flexibility.

Technical Note: With many lenses, their light-gathering ability is also affected by their transmission efficiency, although this is almost always much less of a factor than aperture. It’s also beyond the photographer’s control. Differences in transmision efficiency are typically more pronounced with extreme zoom ranges. For example, Canon’s 24-105 mm f/4L IS lens gathers perhaps ~10-40% less light at f/4 than Canon’s similar 24-70 mm f/2.8L lens at f/4 (depending on the focal length). How it Appears. A camera’s aperture setting is what determines a photo’s depth of field (the range of distance over which objects appear in sharp focus). Lower f-stop values correlate with a shallower depth of field:

Wide Aperture f/2.0 – low f-stop number shallow depth of field

Narrow Aperture f/16 – high f-stop number large depth of field

ISO SPEED

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture and shutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds dramatically increase image noise. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren’t otherwise obtainable.

Common ISO speeds include 100, 200, 400 and 800, although many cameras also permit lower or higher values. With compact cameras, an ISO speed in the range of 50-200 generally produces acceptably low image noise, whereas, with digital SLR cameras, a range of 50-800 (or higher) is often acceptable.

Camera Exposure Modes:

Most digital cameras have one of the following standardized exposure modes:

Auto (green rectangle), Program (P), Aperture Priority (Av), Shutter Priority (Tv), Manual (M) and Bulb (B) mode. Av, Tv, and M are often called “creative modes” or “auto exposure (AE) modes.”

Each of these modes influences how aperture, ISO and shutter speed are chosen for a given exposure. Some modes attempt to pick all three values for you, whereas others let you specify one setting and the camera picks the other two (if possible). The following table describes how each mode pertains to exposure:

How It Works: Camera automatically selects all exposure settings. The camera automatically selects aperture & shutter speed; you can choose a corresponding ISO speed & exposure compensation. With some cameras, P can also act as a hybrid of the Av & Tv modes.

You specify the aperture & ISO; the camera’s metering determines the corresponding shutter speed.

You specify the shutter speed & ISO; the camera’s metering determines the corresponding aperture.

You specify the aperture, ISO and shutter speed — regardless of whether these values lead to the correct exposure.

Useful for exposures longer than 30 seconds. You specify the aperture and ISO; the shutter speed is determined by a remote release switch, or by the duration until you press the shutter button a second time.

Exposure Mode Auto () Program (P) Aperture Priority (Av or A) Shutter Priority (Tv or S) Manual (M) Bulb (B)

In addition, the camera may also have several pre-set modes; the most common include landscape, portrait, sports and night mode. The symbols used for each mode vary slightly from camera to camera, but will likely appear similar to those below:

How It Works: Camera tries to pick the lowest f-stop value possible for a given exposure. This ensures the shallowest possible depth of field. The camera tries to pick a high f-stop to ensure a large depth of field. Compact cameras also often set their focus distance to distant objects or infinity.

The camera tries to achieve as fast a shutter speed as possible for a given exposure — ideally 1/250 seconds or faster. In addition to using a low f-stop, the fast shutter speed is usually achieved by increasing the ISO speed more than would otherwise be acceptable in portrait mode.

The camera permits shutter speeds which are longer than ordinarily allowed for hand-held shots, and increases the ISO speed to near its maximum available value. However, for some cameras, this setting means that a flash is used for the foreground, and long shutter speed and high ISO are used to expose the background. Check your camera’s instruction manual for any unique characteristics.

Exposure Mode Portrait Landscape Sports/Action Night/Low-light

However, keep in mind that most of the above settings rely on the camera’s metering system in order to know what’s a proper exposure. For tricky subject matter, metering can often be fooled, so it’s a good idea to also be aware of when it might go awry, and what you can do to compensate for such exposure errors (see section on exposure compensation within the camera metering tutorial).

Finally, some of the above modes may also control camera settings which are unrelated to exposure, although this varies from camera to camera. Such additional settings might include the autofocus points, metering mode and autofocus modes, amongst others.

ISO Setting In Digital Photography.

What is ISO?

In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.

In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.

Higher ISO settings are generally used in darker situations to get faster shutter speeds. For example, an indoor sports event when you want to freeze the action in lower light. However the higher the ISO you choose the noisier shots you will get. I’ll illustrate this below with two enlargements of shots that I just took – the one on the left is taken at 100 ISO and the one of the right at 3200 ISO (click to enlarge to see the full effect).

100 ISO is generally accepted as ‘normal’ and will give you lovely crisp shots (little noise/grain).

Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also.

When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well-exposed shot. For example – if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.

Questions to Ask When Choosing ISO

When choosing the ISO setting I generally ask myself the following four questions:

  1. Light – Is the subject well lit?
  2. Grain – Do I want a grainy shot or one without noise?
  3. Tripod – Am I using a tripod?
  4. Moving Subject – Is my subject moving or stationary?

If there is plenty of light, I want little grain, I’m using a tripod and my subject is stationary I will generally use a pretty low ISO rating.

If it’s dark, I purposely want grain, I don’t have a tripod and/or my subject is moving I might consider increasing the ISO as it will enable me to shoot with faster shutter speed and still expose the shot well.

Of course, the trade-off of this increase in ISO will be noisier shots.

Situations, where you might need to push ISO to higher settings, include:

  • Indoor Sports Events – where your subject is moving fast yet you may have limited light available.
  • Concerts – also low in light and often ‘no-flash’ zones
  • Art Galleries – many galleries have rules against using a flash and of course being indoors are not well lit.
  • Birthday Parties – blowing out the candles in a dark room can give you a nice moody shot which would be ruined by a bright flash. Increasing the ISO can help capture the scene.

ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. Experiment with different settings and how they impact your images today – particularly learn more about Aperture and Shutter Speed which with ISO are a part of the Exposure Triangle.

Shutter Speed In Digital Photography:

Previously I’ve introduced the concept of the Exposure Triangle as a way of thinking about getting out of Auto Mode and exploring the idea of manually adjusting the exposure of your shots.

The three main areas that you can adjust are ISO, Aperture and Shutter speed. I’ve previously looked at making adjustments to ISO and now want to turn our attention to shutter speed.

What is Shutter Speed?

As I’ve written elsewhere, defined most basically – shutter speed is ‘the amount of time that the shutter is open’.

In film photography it was the length of time that the film was exposed to the scene you’re photographing and similarly in digital photography shutter speed is the length of time that your image sensor ‘sees’ the scene you’re attempting to capture.

Let me attempt to break down the topic of “Shutter Speed” into some bite-sized pieces that should help digital camera owners trying to get their head around shutter speed:

  • Shutter speed is measured in seconds – or in most cases fractions of seconds. The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30).
  • In most cases, you’ll probably be using shutter speeds of 1/60th of a second or faster. This is because anything slower than this is very difficult to use without getting camera shake. Camera shake is when your camera is moving while the shutter is open and results in a blur in your photos.
  • If you’re using a slow shutter speed (anything slower than 1/60) you will need to either use a tripod or some type of image stabilization (more and more cameras are coming with this built-in).
  • Shutter speeds available to you on your camera will usually double (approximately) with each setting. As a result, you’ll usually have the options for the following shutter speeds – 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8 etc. This ‘doubling’ is handy to keep in mind as aperture settings also double the amount of light that is let in – as a result increasing shutter speed by one stop and decreasing aperture by one stop should give you similar exposure levels (but we’ll talk more about this in a future post).
  • Some cameras also give you the option for very slow shutter speeds that are not fractions of seconds but are measured in seconds (for example 1 second, 10 seconds, 30 seconds etc). These are used in very low light situations when you’re going after special effects and/or when you’re trying to capture a lot of movement in a shot. Some cameras also give you the option to shoot in ‘B’ (or ‘Bulb’) mode. Bulb mode lets you keep the shutter open for as long as you hold it down.
  • When considering what shutter speed to use in an image you should always ask yourself whether anything in your scene is moving and how you’d like to capture that movement. If there is movement in your scene you have the choice of either freezing the movement (so it looks still) or letting the moving object intentionally blur (giving it a sense of movement).
  • To freeze movement in an image (like in the surfing shot above) you’ll want to choose a faster shutter speed and to let the movement blur you’ll want to choose a slower shutter speed. The actual speeds you should choose will vary depending upon the speed of the subject in your shot and how much you want it to be blurred.
  • Motion is not always bad. I spoke to one digital camera owner last week who told me that he always used fast shutter speeds and couldn’t understand why anyone would want motion in their images. There are times when motion is good. For example when you’re taking a photo of a waterfall and want to show how fast the water is flowing, or when you’re taking a shot of a racing car and want to give it a feeling of speed, or when you’re taking a shot of a starscape and want to show how the stars move over a longer period of time. In all of these instances choosing a longer shutter speed will be the way to go. However, in all of these cases, you need to use a tripod or you’ll run the risk of ruining the shots by adding camera movement (a different type of blur than motion blur).
  • Focal Length and Shutter Speed – another thing to consider when choosing shutter speed is the focal length of the lens you’re using. Longer focal lengths will accentuate the amount of camera shake you have and so you’ll need to choose a faster shutter speed (unless you have image stabilization in your lens or camera). The ‘rule’ of thumb to use with focal length in non image stabilized situations) is to choose a shutter speed with a denominator that is larger than the focal length of the lens. For example, if you have a lens that is 50mm 1/60th is probably ok but if you have a 200mm lens you’ll probably want to shoot at around 1/250.

Shutter Speed – Bringing it Together

Remember that thinking about Shutter Speed in isolation from the other two elements of the Exposure Triangle (aperture and ISO) is not really a good idea. As you change shutter speed you’ll need to change one or both of the other elements to compensate for it.

For example, if you speed up your shutter speed one stop (for example from 1/125th to 1/250th) you’re effectively letting half as much light into your camera. To compensate for this you’ll probably need to increase your aperture one stop (for example from f16 to f11). The other alternative would be to choose a faster ISO rating (you might want to move from ISO 100 to ISO 400 for example).

Aperture In Digital Photography:

What is Aperture?

Put most simply – Aperture is ‘the size of the opening in the lens when a picture is taken.’

When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in – the smaller the hole the less light.

Aperture is measured in ‘f-stops’. You’ll often see them referred to here at Digital Photography School as f/number – for example f/2.8, f/4, f/5.6,f/8,f/22 etc. Moving from one f-stop to the next doubles or halves the size of the amount of opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also – this means if you increase one and decrease the other you let the same amount of light in – very handy to keep in mind).

One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. So f/2.8 is in fact a much larger aperture than f/22. It seems the wrong way around when you first hear it but you’ll get the hang of it.

Depth of Field and Aperture

There are a number of results of changing the aperture of your shots that you’ll want to keep in mind as you consider your setting but the most noticeable one will be the depth of field that your shot will have.

Depth of Field (DOF) is that amount of your shot that will be in focus. Large depth of field means that most of your image will be in focus whether it’s close to your camera or far away (like the picture to the left where both the foreground and background are largely in focus – taken with an aperture of f/22).

Small (or shallow) depth of field means that only part of the image will be in focus and the rest will be fuzzy (like in the flower at the top of this post (click to enlarge). You’ll see in it that the tip of the yellow stems are in focus but even though they are only 1cm or so behind them that the petals are out of focus. This is a very shallow depth of field and was taken with an aperture of f/4.5).

Aperture has a big impact upon depth of field. Large aperture (remember it’s a smaller number) will decrease depth of field while small aperture (larger numbers) will give you larger depth of field.

It can be a little confusing at first but the way I remember it is that small numbers mean small DOF and large numbers mean large DOF.

Let me illustrate this with two pictures I took earlier this week in my garden of two flowers.

The first picture below (click them to enlarge) on the left was taken with an aperture of f/22 and the second one was taken with an aperture of f/2.8. The difference is quite obvious. The f/22 picture has both the flower and the bud in focus and you’re able to make out the shape of the fence and leaves in the background.

The f/2.8 shot (2nd one) has the left flower in focus (or parts of it) but the depth of field is very shallow and the background is thrown out of focus and the bud to the right of the flower is also less in focus due to it being slightly further away from the camera when the shot was taken.

The best way to get your head around aperture is to get your camera out and do some experimenting. Go outside and find a spot where you’ve got items close to you as well as far away and take a series of shots with different aperture settings from the smallest setting to the largest. You’ll quickly see the impact that it can have and the usefulness of being able to control aperture.

Some styles of photography require large depths of field (and small Apertures)


For example in most landscape photography you’ll see small aperture settings (large numbers) selected by photographers. This ensures that from the foreground to the horizon is relatively in focus.

On the other hand in portrait photography it can be very handy to have your subject perfectly in focus but to have a nice blurry background in order to ensure that your subject is the main focal point and that other elements in the shot are not distracting. In this case you’d choose a large aperture (small number) to ensure a shallow depth of field.

Macro photographers tend to be big users of large apertures to ensure that the element of their subject that they are focusing in on totally captures the attention of the viewer of their images while the rest of the image is completely thrown out of focus.

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