Photography Basics: Controlling Exposure
We have covered this topic several times and it still is one of the most common emails we get so we are reposting this older article to help new people be able to find it easier.
What is exposure?
Exposure is, simply put, the amount of light that enters the camera and hits the sensor. The more light that hits the sensor the brighter the scene will be. If too much light hits the sensor, parts of the image will be “blown out”, this is considered to be over-exposed. A scene that is too dark and the details in the shadows are lost is under-exposed.
How to control exposure
There are three camera settings that control the exposure, these are:
- ISO speed
- Controls the sensitivity of the sensor to light
- The aperture controls the size of the diaphragm over the shutter, the smaller the aperture number the more open the diaphragm is, allowing more light to enter.
- Shutter speed
- The slower the shutter, the more light will enter the camera
We will look at these settings in more detail to see exactly how the play together later. First, you need to start thinking of light in terms of “stops”. A stop is not an exact amount of light, it is a relative amount of light, each stop of light doubles the amount of light. For example, if we have a single lightbulb as our base light source, adding a second lightbulb (doubling the amount of light) increases the light by one stop. If want to increase the light by another stop we have to again double the amount of light to four lightbulbs. Understanding this is going to be the key to understanding how the different camera settings affect our exposure.
As already mentioned, the ISO speed will control the sensitivity of the sensor. The lower the number, the less sensitive. For low light you will need a higher ISO number to make the sensor more sensitive. The downside is that the higher ISO the more digital noise (grainy look) will show up which will degrade the image quality.
Each time you double the ISO speed you increase the exposure by one stop, so ISO 400 will give you one stop of exposure more than ISO 200, and ISO 800 will give you one more stop over ISO 400 etc. Some cameras have some in-between ISO settings to provide a finer level of control.
You currently see high ISO performance as being a major selling point for new DSLR cameras as this allows them to shoot in lower light conditions with less noise.
Pro: Higher ISO allows shooting in lower light conditions
Con: Higher ISO settings result in more digital noise
Aperture is a little confusing because the larger the number the smaller the size. The largest aperture lenses are f/1.0 although most of the good prime lenses start at f/1.4 – f/1.6. Zoom lenses typically are not available that can do more than f/2.8.
So besides letting in more or less light, what does the aperture actually do? The more light that comes in at a particular shutter speed, the shallower the depth of field. If you want more depth of field you use a smaller (higher number) aperture setting.
Pro: Larger apertures allow more light in and thus allow shooting in lower light conditions
Con: Larger apertures reduce the depth of field which may not be desirable
The faster the shutter, the less light enters the camera, the slower the shutter the more light will enter the camera. If you are shooting in low light conditions, you may need a slower shutter speed, however, too slow of a shutter and you can introduce blur from the subject moving, accidental camera movement, or both.
Sometimes a little blur can help portray movement and action, a sharp bike rider with a blurred background will make the rider look like he/she is moving fast where a tack sharp image shows no motion at all and would make the same image look boring. While blurring on other images can ruin the image.
Pro: The right shutter speed can really make an image more dramatic. Fast shutter speeds can freeze action, slow shutter speeds can add a dramatic motion blur.
Con: Fast shutter speeds require lots of light, slow shutter speeds are harder to control
By understanding the three variables that control exposure along with the pros and cons of each one, you will be able to better control the different options that are available to you.