the Next Step: Exposure

Exposure

Mastering photography is like playing a musical instrument.  Anyone can make noise with an instrument, but to really make music, it is important to read music, learn proper technique, and practice.  Learning how to control exposure is a similar discipline.  Many people don’t think it is fun, but if you ever want to get beyond ‘click and hope for the best’  it is absolutely essential.

What is exposure?

The term ‘exposure’ refers to the amount of light to which the film, or the sensor, is exposed.  The most common photographic errors come down to allowing in too much light (over-exposure) or too little light (under exposure).

The human eye can simultaneously process a sunlit lawn and a log within the deep shade of a tree.  Cameras are becoming better at duplicating this, but the sensor is still nowhere near the capabilities of the human eye.

To achieve proper exposure, photographers consider the “Exposure Triangle”.  This triangle consists of Shutter Speed, Aperture & ISO.  They work in tandem, and to master them is half the battle of becoming a good photographer. 

Let’s look at the first element: shutter speed.

Think of Shutter Speed as a door.  The longer the door stays open, the more light it lets in. 

Shutter Speed is the amount of time that the shutter is open.  A camera’s shutter normally remains closed.  When the button is pressed the camera’s shutter opens for a given period of time.  Shutter speed is measured in seconds, or in most cases fractions of seconds (1/X). The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30). 

Photography is a compromise.  We balance time (shutter speed) versus quantity of light (aperture).

Therefore, along with the aperture setting (I will discuss later), the shutter is controlling the light reaching the sensor or film.  The human eye “sees” at 1/60 of a second.  ‘Camera shake’ (blurred photos) happens at speeds of slower than 1/60^th of a second.  If you’re using a slow shutter speed (anything slower than 1/60) you will need to either use a tripod or some type of image stabilization.

Shutter speeds will double with each setting.  A shutter speed of 1/250 second will allow twice the amount of light on the sensor as 1/500 second.  Doubling is handy to keep in mind as aperture settings also double the amount of light that is let in.  As a result increasing shutter speed by one stop and decreasing aperture by one stop should give you similar exposure levels.

The faster the shutter speed (those indicated on the left of the exhibit: 1/4000, 1/2000, etc.) will freeze motion.  The slower the shutter speed (indicated on the right of the exhibit: 1 second, 2 seconds, 4,8,15 seconds) will typically blur or show motion.  To freeze movement in an image you’ll want to choose a faster shutter speed.  To let the movement blur you will want to choose a slower shutter speed.  Some cameras also give you the option to shoot in ‘B’ (or ‘Bulb’) mode.  Bulb mode lets you keep the shutter open for as long as you hold the shutter button down.

When considering what shutter speed to use in an image you should always ask yourself whether anything in your scene is moving and how you’d like to capture that movement.  If there is movement in your scene you have the choice of either freezing the movement or letting the moving object intentionally blur.

Motion is not always bad. For example: When you’re taking a photo of a waterfall and want to show how fast the water is flowing, or when you’re taking a photo of a racing car and want to give it a feeling of speed, or when you’re taking a photo of a star scape and want to show how the stars move over a longer period of time you would set a long shutter speed. 

The image of the waterfall on the left was taken with a shutter speed of 1/15, relatively slow.  The second waterfall image on the right was taken with a shutter speed of 1/80.  The first image shows motion, very similar to what was seen while photographing the waterfall.

However in all of these cases when the shutter speed is longer than 1/60 second, you need to use a tripod or you’ll run the risk of ruining the shots by adding camera movement.  A fast shutter speed will use a larger aperture (small F-stop number) to avoid an under-exposed image. A slow shutter speed requires a small aperture (large F-stop number) to avoid over-exposure.

Shutter Priority:  You set the shutter, and the camera will attempt to deliver a good exposure.  You use Shutter Priority mode when you’re shooting subjects in motion.  Use a fast shutter speed to freeze motion.  Use a slow shutter speed to render an object in motion with an artistic blur.  The shutter speed needed to freeze action depends on how fast the subject is traveling, how far you are from the subject and what direction the subject is moving.

B Mode:  Set your camera to B (Bulb) mode when you want to capture pictures of night scenes, fireworks, and any other scene that requires a lot of light to properly exposure the image.  When you shoot in this mode, the shutter stays open as long as you have the shutter button pressed. This is called a time exposure.  When you shoot a time exposure, place the camera on a tripod to ensure a blur-free shot.  You’ll also need a cable release or a remote trigger, which lets you open the shutter without pressing the shutter button.