Photography - Exposure - Getting it right

By 'exposure' we mean the amount of light that falls onto the film, or CCD if you are using a digital camera. In modern cameras the exposure is usually set to automatic by default and, most of the time, it can be left there and will produce beautiful pictures. There are times though, when the lighting conditions are difficult or we want to produce a particular effect and it would be nice to understand what is going on 'under the hood'.

The problem with all types of film and recording media is that they cannot record the entire range of contrast (black to white) that the eye can see. Especially when you take into account that the eye is constantly adjusting to cope with high contrast. On a sunny day if you look into the shadows of a scene then into the bright areas, the iris in your eye will quickly adjust so you can see detail in both.

Faced with the task of recording as much information as possible, the camera will try to average out all the light levels and expose the film accordingly. As burnt out highlights are normally considered uglier than black shadows, the camera, left to it's own devices will normally err on the dark side. Which is no good if you are shooting someone's face against a bright sky. It's the person's face you want to see, and you don't really care if the sky is white.

A tricky lighting situation requiring delicate control to ensure the right amount of detail in the highlights and the shadows.

Auto Exposure

The camera manufacturers have come up with all sorts of ingenious metering systems to try to help, there are now multi mode metering systems, which give you a choice of 'centre weighting', 'spot metering' or 'multi spot metering' on many of the better cameras, but none can guarantee to give you what you want every time.

The temptation to think that your camera 'knows what it's doing' is great, even for more experienced photographers, which is my main complaint against automatic cameras. It's easy to get lulled into a false sense of security, switch your brain to more interesting things and end up with a pile of rubbish.

Click here for an example of a tricky lighting situation

Tip - using auto exposure to your advantage.

If you have a modern camera, the chances are that the default metering system is 'centre weighted average', which means that, although it takes an average reading of the whole scene, it takes more notice of what is in the middle of the frame. Which is good news for us. The other good news is that it takes this reading at the time when you take 'first pressure' on the button to take your picture. When you push it halfway down and it beeps at you, not only is the focus now set (on an auto focus camera) but the exposure reading is taken and the aperture and shutter speed are set. So, if your main point of interest is not in the centre of the frame, it's a good idea to put it there temporarily while you focus and take your light reading, then move the camera whilst still holding the button halfway down and compose the picture the way you want it to be. A common use for this technique is when you are taking a close up shot of two people and there is space between their heads, if you're not careful the camera will focus on the wall or trees behind them. If the background is very dark or very light this can alter the exposure significantly and result in faces that are too dark or too light.

Skin tones are what most meters are set up to consider an 'average tone', they are also usually the part of the picture that we most want to get right. If I am photographing a group of people in difficult circumstances, like bright sunlight for instance, I will often move close in to the group and take a light reading from someone's face or, if we are all standing in the same type of light, I will take a reading from the back of my hand. This is no good, of course, if the subject is in bright sunlight and I am in the shade. Brown parcel paper is also an extraordinarily accurate surface to take a light reading from.

Manual Exposure

Now it's time to turn that dial away from 'programme' mode and have a look at the dreaded 'manual' mode. There are also a bewildering array of other choices such as 'aperture priority', 'shutter priority', 'exposure compensation' etc., but once you understand the basics you will be able to select the most suitable mode.

The amount of light falling on the film is governed by three things.

• The amount of light reflected from the scene which, if you are outdoors, you can do very little about.
• The 'shutter speed' which is the amount of time the shutter is open, measured in fractions of a second.
• The 'aperture setting' which is the size of the hole through which the light enters. If you look at the lens of your camera you will see a diaphragm in the middle of the glass which the camera adjusts according to the light. This does exactly the same job as the iris in your eye. Aperture settings are measured in 'f stops'. For an explanation of 'f stops' click here.

The shutter speed and aperture settings have other quite separate effects on the photograph which we will discuss in another article, but for the purposes of exposure, making the picture darker or lighter, they are interchangeable. Make the hole twice as big and open the shutter for half the time and you will expose the film the same amount.

Why use manual exposure?

The advantage of manual exposure is that the settings do not keep changing as your scene changes. Let's suppose that you are taking close up photos of cars passing by. Some of the cars will be black or dark colours and some will be light colours or white. If you are filling the frame with almost nothing but car, the meter will be trying to render each car as mid grey. Although it will probably not succeed, what you will notice is that the background is a different shade in each photo.

I often have to take portraits of people, some are wearing very dark clothes and some are wearing white. If I am not careful with my light readings the skin tones will be affected by the clothes.

Although it is by no means always necessary to use manual exposure, an understanding of how it all works will save a lot of disappointment.

For a more in depth look at exposure try this book.