The Fundamentals of Exposure

Exposure Settings

There are three settings we can adjust to alter the exposure:

• Aperture (Av, Aperture Value, or F-stop), an example is F2.8
  The size of the lens opening, controls how much light passes through the lens
• Shutter speed (Tv or Time Value), an example would be 1/8 or one eights of a second 
  The amount of time the sensor is exposed to light
• ISO (Sensitivity), an example would be ISO 100
  The sensor gain amplification

If you are not familiar with these, have no worries!  We've devoted an entire page to each.

Any combination of these three settings represents an Exposure Value (EV). One EV is commonly referred to as a "stop".  We often use this term to refer to the relative value such as when we're off the correct exposure setting: "You're off by half a stop".

We need to set aperture, shutter speed and ISO so that the image gets correctly exposed. In other words, the Exposure Value (EV) represented by these three settings combined must match the light value (LV) of the scene. This is where the light meter in our camera comes to the rescue. Most modern DSLRs will in manual exposure mode display a scale and a bar on the top LCD, in the viewfinder and/or on the rear LCD monitor and all we have to do is to dial in a combination of aperture, shutter speed and ISO that will reduce the light meter indicator to just one tick mark in the middle of the scale. We do not need to know what the EV actually is.

Here are examples of what the light meter scale may look like when the exposure mode is set to M and a correct exposure is dialed in (Pentax K-S2 to the left, Pentax K-5 IIs to the right):

Light meter scale on rear LCD	Light meter scale on top LCD

We do not need to know what the exposure value is. We just center the tick mark and we then know that the exposure value (whatever it is) matches the light value (brightness) of the scene.

The three exposure settings (aperture, shutter speed, and ISO) are also shown on the display:

Shutter speed: 1/8 of a second Aperture F4.5 ISO 200	Shutter speed: 1/45 of a second Aperture F2.8 ISO 160

Next we show the light meter read-out in a number of cases from 3 stops underexposed to half a stop overexposed. Note how the brightness of the rear monitor on our Pentax K-S2 in Live View reflects the exposure (older models do not necessarily do this). Note also that the scale is callibrated in "stops" (or EVs) away from the correct exposure with the center tick representing "+/- 0" or "spot on":

3 stops underexposure	One stop underexposure
Correct exposure	Half a stop overexposed

Now a couple of examples of the light meter read-out on the Pentax K-3 top panel:

Two stops underexposed	Correct exposure

Several combinations of aperture, shutter speed and ISO will center the light meter indicator and result in the one and the same exposure value. As an example let's imagine a scene with a brightness of LV = 14 (like the one shown on the previous page). We had set the aperture to F22, the shutter speed to 1/30s and the ISO to 100 to obtain a correct exposure (the EV of this combination is 14). There are several ways of achieving EV 14, some of these are:

For the low light example on the previous page with a light value of 9 we could use any one of the below settings (and many more of course):

If you have the camera set to one of the fully automatic modes the camera will pick a combination of shutter speed, aperture, and ISO that will give an exposure value matching the brightness of the scene. But this tutorial is about you taking control and having you set a combination to your liking so that the resulting image gets the look you want. Read on to see what we mean by this, but first we have some homework for you.