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Exposure and The Exposure Triangle - In Plain English
In the previous lesson we took an overview of the key features of digital cameras. Now it's time to start getting practical and learn how to create great pictures by controlling the points on the exposure triangle. First we'll clear up some facts about how exposure is measured, then we'll de-mystify the manual exposure controls on your camera. It's time to leave the automatic mode behind and start discovering what your camera can really do...
Before we look at how to control exposure, let's straighten out a few facts about what exposure actually is. In brief, exposure is the amount of light that is gathered by the camera to form an image. When too much light is gathered a picture would be described as 'overexposed'. Conversely, when too little light is gathered it woild be called 'underexposed'.
The basic process of photography is about exposing the image sensor (or film) to just the right amount of light to create a picture of the correct brightness. Hence why photos are often called 'exposures'. The level of brightness is measured in "Exposure Value" (EV), which works on a scale of "stops".
So when you increase the EV by 1 stop you are making the photo brighter. Each increase of 1 stop in EV doubles the brightness of an image, whilst a decrease of 1 stop halves the brightness. So if you hear a photographer saying they're going to "stop up" for their next shot, it means they're going to create a brighter picture, because their last shot looked too dark.
EV can be adjusted by any of the 3 points on the exposure triangle, which we'll discover in just a second. Getting the 'correct' exposure for a picture often requires a bit of trial and error. Our eyes are very clever at adjusting for a wide range of brightness in the world around us. Cameras, on the other hand, can register a much smaller range, so we need to carefully choose the settings to get the best exposure possible.
Introducing the exposure triangle
The exposure triangle is made up of 3 settings: aperture, shutter speed and ISO. Each of these settings determines how much light enters the camera, in other words the EV of a photo. So creating an accurate exposure is all about choosing the right balance of all 3 points on the exposure triangle. Changing 1 alters the overall exposure, meaning 1 of the other 2 would have to be changed as well to maintain a constant exposure.
Aperture, shutter speed and ISO don't just determine how much light enters the camera; they also effect the visual characteristics of pictures. So normally we begin by choosing the settings for 1 point, to produce the visual style we want, and then simply adjust the other 2 to balance out the exposure. Confused? Don't be, it will seem very simple soon! Here's a brief summary of each of the 3 points:
Aperture: The aperture is a hole in the lens that determines how much light enters the camera during an exposure. Its size is adjustable and measured in "f stops" or "f numbers". The smaller the f number the larger the hole. The aperture doubles/halves in size with each step up/down on the scale of f numbers. Aperture also determines "depth of field", which is the area either side of the focal point that is acceptably sharp. The smaller the aperture (higher the f number) the larger the depth of field and vice versa.
Shutter speed: Shutter speed is the length of time for which the image sensor is exposed to light entering via the aperture. The speed of the camera's shutter is measured in seconds, or more often fractions of seconds (e.g. 1/200th). Shutter speed also determines the appearance of motion. A fast shutter speed freezes action, whilst a slow shutter speed is able to recored the path of an object's movement, which is rendered as blur.
ISO: ISO is the sensitivity of the image sensor to light. The sensor's ability to convert light into an electronic image is "amplified" with higher settings. ISO is measured on a simple scale of numbers, usually beginning at 100 and extending to tens of thousands. So with a higher ISO an image is formed more quickly, regardless of the amount light entering the camera. The higher the ISO speed, the greater the amount of noise (a grainy texture) that appears in the image.
The water in a bucket analogy
Here's a handy analogy that will help the principles of the exposure triangle stick in your mind. I'm pretty sure it's been used by photography teachers for about as long as cameras have existed! (This only works as an explanation of how aperture, shutter speed and ISO influence exposure, not their accompanying side-effects. But it's still useful)
Image by peasap
So, let's imagine we have a garden hose and and old, slightly knackered and leaky bucket. We want to the fill the bucket up to a specific level. In this analogy, the width of the hose is like the aperture, the length of time it is turned on is like the shutter speed, the water is like the light, the bucket is like the image sensor and the desired water level is the correct EV.
So the wider the hose the more water will be entering the bucket, and the longer it is left on the more water enters too. If we try filling up the bucket the first time and the water doesn't reach the desired level (the picture is too dark), we could either keep the hose on for longer or increase its width (decrease shutter speed or increase aperture). Alternatively, we could plug some of those leaks (like increasing the ISO) and the bucket would fill up quicker even without increasing the amount of water entering.
Using the controls on the exposure triangle
Let's take a look at a quick example situation to see how the points of the exposure triangle relate to each other. There is normally 1 key thing that you need in a photo, like a narrow depth of field or fast shutter speed (achieved with 1 point on the exposure triangle) and the other settings can work around this to balance the exposure.
So, let's say we want to take landscape photo with as much of the scene in sharp focus as possible (a large depth of field). For this we are going to need a really narrow aperture, like f13.
But as we change the aperture to f13, we are both increasing the depth of field and reducing the amount of light that can enter the camera. So, to compensate, we'll have to either slow down the shutter speed or increase the ISO. Which would be the best option...?
Well, ideally we will be able to decrease the shutter speed without capturing any unwanted blur or camera shake. Using a tripod is a good way to ensure this. But if that's not possible then we'll have to increase the ISO, which will give us a bright, sharp picture, albeit with some visible noise.
So there is a constant trade off between aperture, shutter speed and ISO. There are lots of different ways of balancing the 3 elements to create a picture of the same brightness, but with differing visual characteristics.
We briefly looked at exposure modes in the previous lesson, and we'll explore them more fully later. For now, it's just worth knowing that you don't have to choose between total control of exposure (manual mode) and none at all (program mode). There are also 2 very useful semi-automatic modes: aperture priority and shutter priority.
Many photographers set their camera to aperture priority as a default mode, because aperture is the thing they want to control the most. Depth of field is often the most important charatceristic of pictures, so there's no need to waste time setting shutter speed too if you don't have to.
Having said that, in some situations it's the shutter speed that is paramount, like when freezing action at a sports game. At those times shutter priority is a really useful mode to be able to use. So, try to find the best balance of creative control and convenience when dealing with the points on the exposure triangle.
That brings to an end lesson 3 of this Digital Photography Basics series. The exposure triangle is a really simple and effective system once you get the hang of it. Just by controlling aperture, shutter speed and ISO you can create well exposed pictures all of the time, as well as fine tune depth of field and the appearance of motion. In the following three articles we will look in more depth at each point on the triangle and how they can each be used to improve your photography.