Exposure

Paoay Church, Ilocos Norte, Philippines

In order to capture an image in a digital camera, first and foremost, we need light to reach the image sensor.  Image sensor was discussed here.  The amount of light needed ideally depends on the intention of the photographer.  If the photographer wants a bright photo, then the camera must allow more light to reach the image sensor. Conversely, less light is needed to produce dark photo.

What if the camera is set in AUTO shooting mode (discussed here), who decides how much light goes through the sensor?.....  You are absolutely right!  It is the camera "who" will determine, based on its own logic, the amount of light that should reach the image sensor.  The end result may not be always acceptable to the photographer, but that is how AUTO mode works, the camera is in full control.

You might be wondering at this point why this blog post is entitled "Exposure".  It is because in photography, the amount of light that reaches the image sensor during the process of taking the picture is called "exposure".  An exposure may be too much, too little or just right.  Whereas from the human point of view, establishing a correct exposure is totally subjective, while a digital camera itself has its own definitive standards of exposure.

It is not very practical for photographers to measure the actual exposure needed to properly capture an image, so we leave that complex calculations to your digital camera.  However, with few adjustment in your camera's settings, you can easily control the exposure.

When I wrote about the ISO Sensitivity Value in my previous blog, I mentioned three elements that determines the brightness or darkness of your photo.   The brightness or darkness of a photo depends on the exposure.  Accordingly, exposure is affected by these three camera elements:

1.  Shutter Speed
2.  Aperture
3.  ISO Sensitivity

Understanding how these three elements work is fundamental in digital photography.


Shutter Speed 

The classic definition of "shutter speed" is the length of time the camera's shutter is open, allowing light to reach the sensor.  In dSLRs (as well as in film cameras), "shutter" is a mechanical device, usually placed between the lens and the image sensor, that opens and closes to control the light that passes through.  Most point and shoot cameras, however, use electronic shutter that turns on (open) and off (close) the light reading capability of the image sensor.

The shutter sits between the lens and the image sensor

A sample mechanical shutter (source: fareastgizmos.com)

Shutter is closed/off when the camera is not in use and opens up when the shutter release button is pressed.  The shutter remains open/on for the duration of time dictated by the shutter speed.  Shutter speeds in camera are expressed in "seconds" or in fractions of a second, e.g. 1/4000s, 1/1000s, 1/125s, 1/60s, 1/30s, 0.5", 0.8", 1", 1.6", 10", 30".

Let us say  your camera's shutter speed is set at five seconds (5").  When you press the shutter release button, the shutter will open up while the camera collects the light that passes through the lens and reaching the  sensor.   The shutter will then close after five seconds and the camera will start processing the captured image. If you know how fast light travels, you can imagine how much amount of light was collected by your camera in five seconds.  Yes, you may say it is too much.  But remember, the amount of light needed depends on the photographer's intention and/or the lighting condition.  If you need less light, then shutter speed should be faster.   In a well lighted situation, shutter speed of 1/125 seconds (one over one hundred twenty fifth of a second or 1/125s) is generally enough to get a "proper exposure".

From the above example, you can already conclude that slower shutter speed allows more amount of light to reach the sensor and therefore the brighter the photo becomes.  Conversely, faster shutter speed will allow lesser amount of light to reach the sensor, resulting to a darker photo.

In AUTO shooting mode, the camera sets the appropriate shutter speed which it thinks will capture a "proper exposure".  You cannot control the shutter speed setting of your camera in AUTO shooting mode.  What you can only do is hope that your camera's preference is the same as yours.

If you do wish to control your shutter speed, set your camera's shooting mode to one of the creative or semi-automatic modes (P, S or Tv, TAv) or in Manual mode (M).  An introduction to shooting modes was discussed here.  Read your camera's user manual on how to adjust the shutter speed.  These modes, however, may not be available in some point and shoot cameras.

Apart from controlling the exposure, shutter speed can also be used to produce creative photos, which I will discuss in my succeeding blog post.


Aperture

Aperture is the hole or opening in the lens that allows light to travel through before reaching the image sensor. While the shutter and image sensor are placed within the body of the camera, the aperture is located within the lens.  This understanding is particularly important if your camera is a dSLR.

It is the "size" of the aperture which determines the amount of light that can reach the sensor.   The larger or wider the aperture, the more amount of light can travel through the sensor, thus resulting to a brighter photo.  Conversely, a smaller or narrower aperture will allow less amount of light to pass through, producing a darker photo.

The aperture size is expressed in "f-number" or "f-stop" (e.g. f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22).  Get ready for this.  The higher the f-number, the smaller the aperture.  Conversely, the lower the f-number, the larger the aperture.

Diagram of decreasing aperture sizes (increasing f-numbers)

An aperture of  f/2 is larger than f/5.6, therefore more amount of light will pass through in f/2 than in f/5.6.  Aperture f/16 is smaller than f/8, therefore lesser amount of light will pass through in f/16 than in f/8.   Very simple logic but enough to confuse beginners.

Like shutter speed, you cannot control the aperture size in AUTO shooting mode.  Aperture size adjustment can only be possible in creative or semi-automatic modes (P, A or Av, TAv) or in Manual mode (M).  An introduction to shooting modes was discussed here.  Read your camera's user manual on how to adjust the aperture.  These modes, however, may not be available in some point and shoot cameras.

Apart from controlling the exposure, different apertures can also be used to produce creative photos, which I will discuss in my succeeding blog post.

Obtaining a "proper exposure" is a setting combination of shutter speed and aperture.  Although ISO sensitivity is another factor that affects exposure, it is advisable to use this as a last recourse.  Always try setting the ISO to the minimum level in order to prevent digital noise to appear in your photo.  More on digital noise here.

In the second part of "Choosing the Appropriate Shooting Mode" article, I will provide tips on how to obtain a "proper exposure".

Keep on shooting!