Exposure refers to the camera reproducing the image we see with our eyes with the correct levels of brightness to make a good photograph. There are three camera setting that work in combination to produce a properly exposed image – not too light, nor too dark. These components are:
- ISO (the camera’s sensor sensitivity)
- Aperture (the lens opening that light passes through)
- Shutter Speed (The amount of time the camera allows light to make contact with the sensor)
*** CLICK ON THE LINKS ABOVE FOR DETAILS ON EACH COMPONENT ***
Briefly, ISO is measured with an arbitrary value typically ranging from 100 to 1600 (though newer cameras extend up to 6400, and beyond). The scale doubles as it moves up from 100 (100, 200, 400, etc.) and each adjustment represents a doubling of camera image sensor sensitivity to light. Moving down the scale represents cutting the sensitivity in half.
The aperture number is represented as a ratio, such as 1:5.6. Sometimes its is expressed as f/5.6, and in many digital cameras it is simply listed as 5.6. An aperture f-stop (or ratio) is the focal length of the lens (e.g. 50mm) divided by the diameter of the lens opening (e.g. 9mm). Dividing 50 by 9 gives a ratio result of roughly 5.6. As with ISO, there are set values (stops) the either double, or cut in half the amount of light striking the sensor. In the case of Aperture, moving from a small number to a larger number is a decrease by half, whereas moving from a large f-number to a smaller f-number is a doubling of light. This is reverse of ISO.
Shutter Speeds are typically expressed in fractions of a second (although with a tripod some photography situations call for full seconds of exposure). A common shutter speed for general use is 1/125th of a second. As shutter speeds get faster – doubling the speed to 1/250th of a second for example – the amount of light hitting the sensor is cut by half. That is, the camera reduces by half the amount of time light is allowed to make contact with the sensor. Selecting a shutter speed twice as long (e.g. 1/60th second) therefore doubles the amount of time light contacts the sensor.
If you think of shutter speed only in terms of the denominator (the bottom number – such as 125 in the above example) note that for both Shutter Speed, and Aperture, moving from a smaller number to a larger decreases the amount of light by half, and moving from a larger number to a smaller one doubles the amount of light,
STOPS
With all these settings (including ISO) each movement – up or down the scale – is referred to as a STOP. The term STOP is shortened from f-stop, which refers to focal-stop; which is the ratio of the focal length of the lens divided by the lens opening. While this may be confusing at first, remember that each STOP – on any of the three scales – represents either a doubling of light striking the image sensor, or cutting the amount of light in half.
As we photographers move from letting the camera have control over exposure toward us taking control of exposure, manipulating these elements can often times remove the frustration we sometimes experience when photos are too light, too dark, or too blurry, or when the camera won’t take the image for some reason.
Understanding how each or the three settings works is extremely helpful to knowing what setting to change and when in order to take the best possible photos.
This video does an exceptional job of explaining how the different elements work, and work together to make a perfect exposure.
[youtube https://www.youtube.com/watch?v=6-NhJua5NFA&w=560&h=315]