Photography Explained: Exposure
It's probably the most commonly used term in photography jargon, but what exactly is meant by exposure?
Simply put, it describes how bright your picture is although to be more accurate, it's a measure of how much light is recorded by your camera's sensor which in turn determines how bright the photo will be. If it comes out darker than you wanted, it's underexposed; if it's too bright, it's overexposed.
It's worth emphasising that there isn't necessarily such a thing as correct exposure other than what you intend it to be, but to demonstrate let's use a common scenario; taking a photo of a famous building on a bright, sunny day. You probably want the picture to be more or less as bright as you perceive the scene, so it's likely you'll know straight away whether or not the shot is exposed how you want it.
In this situation your camera's auto setting will probably do a very good job. In auto mode the camera will assess the amount of light available and select what it thinks are the optimal settings to achieve the best exposure. But if that doesn't quite give the result you wanted for whatever reason, you'll need to know how to override the camera and control the exposure yourself. It's time to leave auto mode.
The Exposure Triangle
Three settings on the camera have an effect on exposure: aperture size, shutter speed and ISO.
The aperture is the hole in your lens which lets in the light. As it gets bigger, more light is allowed to hit the sensor and the picture is brighter. It's measured in units called f numbers (or f stops) and an increase in the f number actually corresponds to a smaller aperture. So an aperture setting of f5.6 is bigger and gives more exposure than f16.
The sensor is covered by what are known as curtains which move out of the way (or open) when you press the button to take a shot. The shutter speed is the length of time they stay open. It's measured in seconds or fractions of a second and a faster setting allows less time for the light to hit the sensor so the resulting picture is darker.
Here's something just to make a mental note of for now. Adjustments to the aperture size and shutter speed will affect more than just exposure. They have other functions, mainly in relation to focus. Right now we're only concerned with how they influence exposure but we'll be learning about their other effects in the Focus Tutorial which is handily coming up next.
The third part of the exposure triangle is ISO (don't worry about what it stands for) which determines the sensor's sensitivity to light. As you increase this value, the sensor becomes more sensitive so the image is brighter. However, unlike opening up the aperture or slowing down the shutter speed to increase exposure, hiking up the ISO comes with a cost to image quality.
'Noise' as a photography term is the amount of 'graininess' evident in an image. Sometimes you might want a bit of noise but in most cases you probably want it kept to a minimum and that means using the lowest ISO setting you can get away with. (The Bonus Section at the end of this tutorial explains a bit more about noise and how to decide which ISO level to use.)
Metering
I'm going to introduce just one more new term for this tutorial; metering. This is the name given to the process by which the camera measures the amount of available light. You might have come across the phrase 'meter the scene' which basically means point the camera at what you want to photograph and see what light intensity it indicates.
This information is displayed on the LCD screen (or through the viewfinder) using the exposure meter which looks something like this.
It's usually a good starting point to have the exposure meter reading '0' for the first shot which you can then check to see if the camera is making good decisions for your scene. If you're in auto mode, the settings will automatically adjust to achieve that but there are 3 other modes which give you partial or complete control over exposure: Aperture Priority, Shutter Speed Priority and Manual. To summarise:
- Auto: The camera decides which aperture, shutter speed and ISO combination is best.
- Aperture Priority: You set the aperture size you want and the camera will decide the rest.
- Shutter Speed Priority: You choose the shutter speed and the camera decides the rest.
- Manual: The camera takes a break. You decide everything.
The exposure meter will only display when you're in manual mode (it's not necessary in the other modes because the camera always makes sure the final reading is 0) and as you change the aperture, shutter speed or ISO, you'll see the indicators move along the meter bar to reflect how your adjustments are influencing the exposure.
Remember that you, as a human being, don't always have to agree with what the camera's machine intelligence thinks is the correct exposure. For example, your subject might be dominated by shadows which the camera will interpret as being too dark so it will set itself to brighten up the image and you'll lose the contrast you wanted.
Conversely, if you're taking a picture of the snowman you've just proudly built, all that white snow will make the camera think the image is going to turn out too bright so it will compensate by making its '0' exposure setting darker than you might expect. The resulting snowman will look more grey than white so you will need to increase the exposure yourself.
So, you've taken a picture and you're unhappy with the exposure. You now know the three possible ways it can be corrected. But...which one should you choose, or indeed, what combination of the three should you change? This choice depends largely on what you're shooting and what, if any, creative effects you're trying to achieve. Which means the next question you need to ask yourself is 'What is the main focal point of my picture?'
The following tutorial explains how focus can be set and controlled.
Bonus Section: Controlling Noise
Most modern cameras do a very good job of keeping noise levels down as ISO settings are increased but you'll certainly notice the effect in a picture taken at the highest ISO, especially if you zoom in. It's a good idea to know, for your particular camera, what ISO setting begins to introduce a level of noise you're no longer happy to accept.
To measure this find a dark, featureless surface. Make sure the focus is good. Auto-focus won't work unless you include a coin or a drawing pin in the corner to give it something to hook on to. Go into Manual mode and choose the highest aperture and the lowest ISO setting. Set the shutter speed to give a reading of 0 on the exposure meter and take a shot. Now increase the ISO by one setting and adjust the shutter speed until the exposure meter reads 0 again, keeping aperture size the same for the next shot. Repeat this for all ISO settings on your camera.
Load the images onto your computer and zoom in 100% to the same section of each one. As you tab through them you'll notice the noise levels becoming more prominent. You can then make a call on which image and its corresponding ISO is the last one before the quality becomes unacceptable to you.
Personally I don't worry about ISO up to 400. After that I start to consider if there might be a better way of increasing exposure but it depends on what kind of picture I'm taking and how it'll ultimately be used. If you want to make a high quality print for your mum's birthday present you won't want any noise at all if possible but if you only intend to share it on facebook, a medium quality image might be acceptable.
Sometimes though, there's just no way to avoid using a high ISO. For example, if you're trying to capture a singer moving around on stage in a dark venue, you'll most likely have to use the highest ISO setting your camera has and accept there will be some degree of noise. You might then have the option of using an image editing package to reduce the noise once the images are on your computer.
