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Home    Lesson Plans    #1: Understanding Exposure

What is a Light Meter?

For your digital SLR camera to generate the correct combination of aperture, shutter speed and ISO to produce a properly exposed photograph, it needs to know how bright the scene is before it can calculate an accurate combination of aperture, shutter speed and ISO.

Your DSLR camera measures the light through the lens. It collects light that passed through the camera's lens and measures its intensity. The processo f measuring the light intensity is known as Metering. Modern DSLR cameras uses 3 types of metering: Center weighted, Partial metering and Spot Metering. At this point, I will not explain the differences between the 3 types because it is not important in this lesson. Metering will be explained in detail in future lessons. For now, just know that your camera uses a metering system that accurately (most of the time) produces the correct combination of aperture, shutter speed and ISO.

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Tying Everything Together

As we have learned in the previous articles, we know that aperture, shutter speed and ISO setting work together to control the final exposure of a photograph. We also briefly discussed your camera's light meter which can accurately measure the amount of light in your scene and produce a combination of aperture and shutter speed setting based on a given ISO setting.

However, the aperture, shutter speed and ISO combination provided by your camera is not the only correct combination. There are actually many different combinations which can produce the same exposure because you can always change the value of 1 element and it will always have a corresponding value of another element that will produce the same exposure. However, because each of the elements (aperture, shutter speed and ISO) has their own unique effect on the photograph, the combination you choose is normally based on the creative effect you wish to convey on your photo.

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The Sunny 16 Rule

The Sunny 16 Rule is a general rule of thumb which states that when using f/16 on a sunny day, you can use a shutter speed that is roughly equivalent to the ISO setting to get proper exposure of your scene. So, if we use ISO 100, we can set the aperture to f/16 and set shutter speed to 1/125 sec which is roughly equivalent to ISO. Given this as your baseline combination, all the other combinations below will produce the same exposure.

f/5.6, 1/1000 sec, ISO 100
f/8, 1/500 sec, ISO 100
f/11, 1/250 sec, ISO 100
f/16, 1/125 sec, ISO 100 - baseline
f/22, 1/60 sec, ISO 100

Notice how these combination remind you of this general rule of thumb?

  1. Double your shutter speed for each full-stop decrease in f-stop OR halve your f/stop for each full-stop increase in shutter speed
  2. Halve your shutter speed for each full-stop increase in f-stop OR double your f/stop for each full-stop decrease in shutter speed

Below are sample photos using the different combinations of aperture, shutter speed and ISO above.

Exposure 1
F/2.8, 1/25 sec, ISO 1600
Exposure 2
F/4, 1/13 sec, ISO 1600
Exposure 3
F/5.6, 1/6 sec, ISO 160
Exposure 4
F/8, 1/3 sec, ISO 1600
Exposure 53
F/11, 1.6 sec, ISO 1600

FIGURE 1: Exposure Combinations

Notice how all 5 photos above have the same exposure but different depth of field? This is because of the different apertures used in each image. Remember that Depth of Field is a unique effect of Aperture. Also, notice how it shows the relationship between aperture and shutter speed (see general rule of thumb above). Each time the aperture is halved, the shutter speed is doubled.

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Camera Setting: Manual Mode

Manual Mode is a setting on your camera that allows you to control both the aperture and shutter speed value. This gives you full control over the exposure of your photograph. Sometimes when lighting is a little tricky and your camera's light meter is unable to produce the exposure that you want so you would want the ability to override the exposure setting values. You can also override the aperture and shutter speed values suggested by your camera's light meter for your own creative needs. There are times when you want to underexpose parts of your scene for creative purposes. One example is when taking silhoutte shots shown below.

FIGURE 2: Sample Silhoutte shot

Camera Setting: Aperture Priority Mode

Manual Mode Dial

The Manual Mode is usually represented by a M on your camera dial. See illustration on your right. Consult your camera's product manual if you are unable to find the setting for Manual Mode. When set to Manual Mode, your camera's viewfinder will show you an Exposure display which tells you if your aperture, shutter speed and ISO combination will generate an optimal exposure (good exposure), an underexposure, or overexposure. The images below illustrates this.

Nikon Viewfinder - Manual Mode
Figure 3: Nikon Viewfinder
Canon Viewfinder - Manual Mode
Figure 4: Canon Viewfinder

Different camera manufacturers have different viewfinder but it should be similar to the Nikon viewfinder shown in Figure 3. The image on Figure 4 is a sample viewfinder from a Canon camera.

  • It's time to tinker with your camera! This time, take a few minutes to figure out how to set your DSLR camera to Manual Mode.
    Consult your manual if you do not know how.