Triangulation: The Path To A Proper Exposure

One of the questions that I am often asked is, "Why are my photos so bright" or "Why are they so dark." The answer in almost all cases is that the exposure is not at its optimum. Upon questioning, I sometimes find that the person with the exposure problems may be using one of the camera modes, like Automatic.  This may have an effect on the exposure outcome. In this blog posting, I will attempt to explore exposure. I'll deal with the various camera mode options later.

Exposure is controlled by 3 variables: aperture, shutter speed, and ISO setting. They form what is something called the exposure triangle. All 3 variables control the affect of light on the image sensor. Having the appropriate amount of light registering on the sensor results in a properly exposed image.

These variables are interrelated in that they always work together to achieve the desired exposure. Increase one and another must be decreased; decrease one and another must be increased. Each one controls some part of the process of getting light to the sensor. Each has characteristics that change when they are increased or decreased.


Aperture is a measure of the opening within the lens. This allows light to pass through the lens to the sensor in varying amounts. This opening is actually a ratio of the area of the lens opening compared to the maximum possible opening. The size of the open area is specified by f-stop numbers. It may seem counter intuitive, but the larger the f-stop number, the smaller the open area. So, f16 is much smaller than f8.  Full f-stops allow a doubling or halving of the size of the opening.The range of full f-stops is as follows: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.  In modern cameras, there are fractional openings between the doubling and halving. The range of f-stops will vary with the camera model. Changing an f-stop of 4 to an f-stop of 5.6 will decrease the opening by half or 50%. Moving the f-stop from 4 to 2.8 will increase the opening by 2 times doubling the size of the opening. If the other legs of the triangle remain constant, changing the aperture will increase or decrease the total exposure. There is a trade off. Changing the aperture will affect the depth of field in a given image. An f-stop of 2.8 will have a much smaller depth of field when compared to an f-stop of 16. So, there can be a depth of field (area with acceptable focus) consideration when altering an aperture opening.


Shutter speed is the amount of time the light passing through the lens is allowed to strike the sensor. The numbers describing shutter speed may be easier to understand as the are expressed as seconds or fractions of a second. Increasing the amount of time the shutter is open, will increase exposure. Decreasing the amount of time the shutter is open, will decrease the exposure. Doubling or halving the shutter speed has the same effect as increasing or decreasing the aperture by full stops. The trade off with shutter adjustments is motion blur. If freezing motion is the objective, then a faster shutter speed is desirable, for example, freezing the action of kids playing sports. If a blur is desired, then slowing the speed will yield the desired result. An example would be capturing the motion of water in a waterfall. For the shots where a blur is desired a tripod might be required. The motion of camera shake will occur at longer shutter speeds. Camera shake is a result of the movement inherent in hand holding the camera. 


The third side of the exposure triangle is ISO.  This denotes is how sensitive the image sensor is to the light passing through the lens and past the shutter. As sensitivity increases, less shutter speed or aperture opening is needed for a proper exposure. ISO settings are represented by numbers like 100, 200, 400, 800, 1600. These numbers are a representation of doubling or halving the sensitivity of the image sensor in the camera. Moving the setting from ISO 200 to ISO 400 makes the sensor twice as sensitive. Moving the setting from ISO 200 to ISO 100 makes the sensor half as sensitive. The effect of increasing ISO is more noise in the image file. Noise manifests itself like grain in film captured photos. Low ISO means less noise.


If any one of the exposure triangle's settings is fixed, adjustments to the other 2 can result in a proper exposure, but one must be aware of the trade offs.  


If you have your camera set to Auto, you will not have access to the legs of the exposure triangle. The camera makes all of the decisions. Sometimes it's okay, and sometimes it's not. More about the camera modes in the next blog post.