Exposure Guide
This and the related pages are an "exposure guide" to help you understand the basics of exposure. Exposure is a key concept in photography and to take good pictures you need to understand and manipulate it.
The articles below discuss various aspects of exposure - they are the complete exposure guide, and the rest of this page provides an overview for general understanding.
Read this page first then dive into the articles!
Shutter Speed
Shutter speed refers to the length of time the shutter remains open. The longer it remains open, the more light reaches the sensor and the exposure increases.
Aperture Size
The aperture is a part of the camera which varies in size to physically control the amount of light reaching the lens. It works very much like the pupils in your eyes.
Film Speed (ISO)
In film photography, the film speed refered to how much each film was affected when exposed to light. In digital cameras, the same effect is achieved by using electronic amplification (gain) of the signal sent by the sensor. It is a purely electronic effect.
What is Exposure?
Exposure in photography refers to the amount of light that reaches and affects the sensor (or film). If you don't let enough in, the photo will be too dark. If you let too much in, it will be too bright. These are known as "under-exposed" and "over-exposed" respectively.
Exposure is correct when there are no "blown out" regions. A sensor has a range in which it is able to store information. If you expose it to too much light, the the pixels just record the area as being pure white, meaning that all the pixels in a blown out region will be completely white (or completely black). This is undesirable, as large areas of a single colour contain no information about the image.
Consider you take a photo of the sky, including the sun. The sun itself will just appear as a circle of white pixels (as well as producing a variety of other undesirable effects). Or, consider you take a photo at night with no light source. You'll just get a photo of black.
Preserving Detail
Photography is about recording the information which is provided by light. When we take a photograph, we want to capture as much information, or detail of the scene, as possible. This means that in areas of shadow and light, we can still see what's there.
To illustrate this, let me take you from this exposure guide to Venice.
Here I've taken a photo from a trip to Venice and used software to adjust the exposure (this is done using the RAW file format, a great advantage of digital photography). The top photo is under-exposed, the centre is correctly exposed and the bottom photo is over-exposed.
Note how the detail vanishes from the shadow in the under-exposed photo and from the light regions in the over-exposed photo. The centre one however contains all the detail from all regions.
So how do we achieve this?
We want to get as much right as possible about a photo at the time - in camera, without having to rely on software which may not do as good a job.
Controlling Exposure
Exposure is made up of three parts: shutter speed, aperture size and film speed (commonly known as "ISO"). These three elements can be independently changed to affect the resulting exposure.
Please note that "exposure" is not something that can itself be adjusted in a camera, but is changed when one of these three elements are changed, with varying effects.
Below is an overview of each component, but for a full description please click on each link.
Measuring Exposure
Exposure is measured in "stops". When you look at the screen of your camera (or through the viewfinder) you see a horizontal bar with notches. As you adjust the exposure, an indicator moves left or right. The centre of this bar is "0" and then each large notch either side is ±1.
For example, in my Venice montage, the top image is under-exposed by -2 stops and the bottom one is over exposed by +2 stops. The centre one is actually at 0 (which means I got it right at the time.) Very often you'll find that you'll have to change the exposure in software to "get it right".
Automatic Exposure
Digital cameras are very clever and have full automatic modes which can do all this for you. To be honest, if you're using the full auto mode on a dslr all the time, you probably should have bought a compact digital camera. The whole point is to take control!
That said, if you feel you're not confident with adjusting the exposure, then do use this function. If you want the camera to adjust the aperture size while you adjust the shutter speed, then use "Shutter Priority" mode on your camera, written usualy as "Tv" or just "S".
If you want to adjust aperture and have the camera adjust shutter speed to keep exposure correct, use "Aperture Priority" mode, written usually as "Av" or just "A".
I hope this mini exposure guide and the related articles help you understand more about exposure.