Exposure

The whole essence of photography is capturing the light we see before us and fixing it on film or digital sensor.

Our eyes are incredibly good at seeing in near darkness through to bright daylight. Cameras need a bit of help with this. The digital sensor (or film) can only accommodate a narrow band of brightness.

If you let too much light onto the cameras sensor then it will over-expose the image - at best it will be just too bright, at worst it will be nearly all white

If you let in too little light then the image is under-exposed, at best a little murky, at worst completely black.

Interactive Example

Move the slider left and right to see varying levels of exposure. With the slider to the left the image is clearly too dark. Too far to the right and the image is too bright.

But somewhere in the middle are several possibly correct exposures. Only one of these is "technically" correct, but you can choose a level of exposure that has an appealing look.

In this example you would probably want a slightly brighter face than is correct.

Notice how the sky also changes according to the level of exposure.

Dynamic Range

Also the sensor or film can only record a certain range of light. Again our eyes can see detail in deep shadows on a bright sunny day. A camera struggles with this. Too much contrast between the light and dark parts of the scene cannot be recorded.

This is called "Dynamic Range". You may need to decide whether you are going to see details in the shadows, or details in the highlights, but you may not get both.

You control how much light falls onto the digital sensor through the aperture on your lens and the shutter in your camera.

You can also change how sensitive your digital sensor is by adjusting its ISO setting. This electronically adjusts how much light is needed to register an image.

F-Stops

Light is measured by photographers in "f-stops". This scale looks a little odd when you see it since the numbers don't quite make sense. But basically every change upwards in these f-stop numbers means the scene is half as bright.

Each stop increment has half the light gathering area than the previous one.

You don't need to learn the scale; you actually have an infinite variety of stopping points between each of these f-stops anyway. But you will often hear phrases like "it's a stop too bright", "need to stop down a bit", "give it a couple more stops".

"Stopping Down" is the adjustment you make when your image is too bright, you need to stop light getting to the sensor.

"Stopping Up" means your image is too dark; you need to let more light into your sensor.

Exposure Metering Modes

Your camera contains an incredibly accurate and also incredibly clever light meter that does all this measuring of brightness for you.

The computer in your camera will look at the scene and not only measure the total amount of light, but can also determine what TYPE of scene it is, i.e. a landscape, a person etc., and adjust accordingly.

For example, if it recognises a human head against a bright sky it knows you want the face exposed correctly, even if that means the sky is going to be too bright. This computer is nothing like as sophisticated as your eye/brain combination but it is remarkable how often it will get it right first time.

However, if the camera does not match the scene in its "library" of scene types then it will normally just try to make an average of all the light and do its best.

This exposure metering is called TTL (Through The Lens) because it is actually looking at the light being seen by the camera.

Exposure Metering Modes

There are four basic metering modes available to your camera.

Evaluative
Partial
Spot
Centre-Weighted Average

Evaluative Metering

Evaluative metering: analyses the whole scene and selects the "best" exposure based on a "library" of scenes.

Partial Metering

Partial metering: bases the exposure only on the light that falls in roughly a 10% circle in the centre of the picture.

Spot Metering

Spot metering: sets the exposure on the centre spot (about 4%) of the centre of the picture.

Centre-Weighted Metering

Centre-weighted average metering: looks at the light around the whole picture but gives extra emphasis on the light in the centre of the picture.

Usually you can use evaluative metering and your camera will get the right exposure most of the time. However it can be confused by scenes with very dark and very light parts, in which case switching to one of the other modes (usually spot metering) will let you get the correct exposure for your main subject.

How Do I Know What My Camera Meter Reading Is?

On the top of your camera you will have a small LCD screen that will show you key information about your settings, of which aperture and shutter speed are the most important and show largest in the screen.

You will also get the same information shown in your viewfinder, usually at the bottom.

At the left you have the shutter speed and aperture.

On the right you have the ISO and the focus locked indicator.

In the centre is the exposure scale along with an indication of whether the image is considered to be too dark or bright.

The off-centre green arrow indicates the image is over-exposed.

Now I Have An Exposure How Do I Control It?

As we said earlier you can control exposure by adjusting aperture, shutter speed and ISO.

Aperture

Aperture: controls the size of the diaphragm within the lens to make a larger or smaller circle through which the light can pass. f/22 is a very small circle and lets in only a little light. f/2 is a large opening and lets in a lot of light.

Shutter Speed

Shutter speed: determines for how long the sensor is exposed to the light. Normally measured in fractions of a second. 1/10th of a second is considered a "slow" shutter speed, 1/1000th of a second is a "fast" shutter speed.

ISO

ISO: determines how sensitive the sensor will be. ISO 100 is a low ISO and is suitable for daylight pictures. ISO 3200 is high ISO and is suitable for dark locations.

For each of these settings there is a trade-off:

Large aperture (f/2) gives very little depth of field (a narrow range is in focus)
Slow shutter speed (1/10th of a second) does little to freeze motion and camera shake.
Fast ISO (ISO 3200) pushes the electronics so there is more noise (grain) in the picture.

The Exposure Triangle

You can adjust each of these independently, but they also all interact with each other.

To maintain the same level of exposure as you go from aperture f/8 to f/5.6 (getting brighter) you change the shutter speed from 1/100th of a second to 1/200th (getting darker). In this way the two balance out and you get the same exposure.

For example we want to take a picture of runners on a sports track. We want to freeze the action. Set a fast shutter speed (1/500th) and a wide aperture (f/5.6). Our focus will need to be spot on and probably only one runner will be sharp and in focus.

Or, we could get all the runners but with some motion blur, to show a sense of speed. Set a slower shutter speed (1/50th) and a smaller aperture (f/16).

Or, you could keep a fast shutter speed (1/500th) and a small aperture (f/16) and adjust the ISO from 100 to 800 to get greater sensitivity but maybe with some noise and reduced quality.

Each of these examples is about balancing the technical variables to achieve an image based on your artistic choices.

Shooting Modes

Now remember your camera is a clever piece of kit. Rather than having to make all those decisions and calculations for every shot you can let the camera do a lot of the leg work for you.

We talked about camera shooting modes in our general description of the camera. Shooting modes are all about the control of exposure.

Automatic

Automatic: means the camera will make all the decisions for you and come up with an OK, but average choice of exposure settings.

Aperture Priority

Aperture Priority: means you can decide on your aperture setting and the camera will adjust shutter speed to get the correct exposure.

Shutter Priority

Shutter Priority: means you set the shutter speed and the camera adjusts aperture.

Manual

Manual: means you set the aperture and shutter speed. The camera will still give you a meter reading so you can see if your image is too bright or dark.

Notice that the camera will rarely adjust ISO settings. This is because changing the ISO can have a significant impact on the quality of your image. So ISO may adjust slightly, but normally only within the ISO 100 to 400 range, very limited compared to your camera's actual ISO capability.

Special Modes

Portrait, Landscape, Sports modes: all these modes set the camera up so the aperture and shutter speed are more appropriate for the type of shoot. For example sports mode will go for a fast shutter speed to freeze motion, at the cost of wider apertures and reduced depth of field (focus range).

Examples

When I shoot a wedding which is a very fast changing photo shoot that requires constant adjustment, I will be changing modes all the time.

When the bride is walking up the aisle I shoot in shutter priority at 1/125th to freeze the bride and remove any motion blur.

Freeze motion on bride walking in.

Anything might happen on a candid!

When I am shooting candid shots I will often shoot in Automatic mode - the moment I need to capture may be so fleeting I know the camera will calculate an OK exposure for me.

When I am shooting the formal group shots of all the friends and family I know I need maximum depth of field so everybody is in focus. I am in aperture priority with a setting of f/22.

Groups need a wide depth of field for focus from front to back of the group

The rings are sharp, the background has a "pleasing lack of definition"

When I am shooting more artistic and structured moments - a close-up of the bride or of the wedding rings, I will use aperture priority with a wide setting (f/1.2) to get a reduced depth of field and a background that is out of focus.

For the evening reception when things are dark I will turn the ISO right up from my normal ISO 100 to ISO 3200 or even 6400. This lets me get images even in darker rooms or outside at night.

The high ISO has captured the ambient light in the room. A normal flash photo would have looked like a completely dark room.

Understanding the Histogram

Your camera will have an option to view a "Histogram" of a picture. This is essentially a graph of the relative amounts of dark to light in the picture.

You can usually find this option by selecting the "info" button when you have a picture displayed on the rear LCD screen.

Correctly exposed middle tones are displayed in the centre of the graph. Dark tones are on the left. Bright tones are on the right.

You want to see this graph with either even heights across the chart, or with a peak in the centre of the chart.

If the chart is bunched too far to the right then your image will be very bright.

If the chart is bunched too far to the left then your image will be very dark.

Now if you are taking a picture of a bride in a white dress then the histogram will automatically be pushed over to the right because a lot of the image is white. But it should not have a really high peak on the right hand edge - that means it is over exposed.

Similarly if you take a picture of a groom in his dark suit, you would expect an overall shift to the left, but not high peak on the left hand edge.

Light Meters

The old school method of measuring light was to use a light meter. A separate hand-held device for measuring light levels. Since the advent of digital cameras I rarely see a photographer using a meter anymore.

You can so easily see on the back of your camera the various light levels in your picture and how they relate to each other that the need to meter has pretty much gone away.

Bracketing

One last option you may have on your camera is "bracketing". You can use this if you are unsure of exactly the right exposure levels you need. You can get your camera to take 3 or more photos in quick succession each at different exposure settings. So you would get 3 pictures, hopefully one under-exposed, one correctly exposed, one over-exposed. But maybe you really need to see details in the whites, so actually the under-exposed image will show what you need.

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Assignment: Understanding Exposure Controls

Set up your camera so you can take the same shot over and over again.

An indoors still life or a landscape might be good subjects.

Take lots of photos using all the adjustments you can.

Deliberately take pictures which are under- or over-exposed, learn to read how they appear on your rear LCD screen.

Learn how much adjustment is required to go from great, to OK, to poor.

You may want to refer to your camera manual to make sure you understand how to use the various camera controls