What are the components for controlling your exposure in photography? Many who start photography do not understand the relationship between your aperture, shutter speed, and ISO found on a camera. The numbers are confusing with the aperture settings; they are not like the settings on the shutter speed control or even the ISO, which they both seem to follow in a pattern. This Tech Tip article will cover the basic concepts and lay out the foundation for future photography articles. We will explain the relationship for each component and the results of each adjustment.

Lens Aperture

** Let’s first talk about the lens aperture setting, also called the f-stop. The aperture is the physical element that restricts and controls the amount of the light that reaches the digital sensor of the camera. The lens blades make a circular whole called the aperture. The f-stop is based on the diameter of the aperture in relationship to the focal length of the lens; together they control the amount of the light allowed through the lens.

To increase or decrease the amount of light reaching the sensor you must change the overall area of the circular hole (or aperture). The increments between each aperture setting is based on doubling or halving the amount of the light from reaching the sensor.

In the photographic industry this halving and doubling of the amount of light is called “Stops.” Stops can also relate to changes in the shutter speed and ISO sensitivity of the camera. As long as you double or halve the value, then it’s a commonly referred to as a “Stop of Light.”

The f-stop values found on the camera settings can be confusing. If you look at the lens and adjust the f-stop setting: you’ll notice that a large aperture will have a small number corresponding to it, and inversely, a small aperture will have a large number. The math behind this is fairly simple, it is really a ratio of the focal length divided by the diameter of the aperture, which will give you the f-stop value.

Another factor you have to consider is the influence on your “depth of field” by changing the f-stop. “Depth of field,” or DoF, is the amount of in-focus area in a scene and is controlled by the size of your aperture. Larger diameter circular holes, or small f-stop numbers will produce a more shallow depth of field. For more depth of field, you would use a small diameter circular hole and larger F/Stop number. Remember, you will also change the intensity, or amount of light that reaches your camera sensor, so your exposure will vary.

Shutter Speed

The shutter speed of the camera is the easy one. It’s the time, or duration, the light to expose to the sensor. A camera’s shutter speed is typically measured in seconds (or increments thereof), and can also control blur in the image. The faster the shutter speed, the less blur will show in the picture. Slower speed setting gives the shutter longer opening time, exposing the sensor for a more extended period and allowing more to change when the subject moves. Depending on your choice, each shutter speeds can be half as fast, or twice as slow. A “Stop of Light” can be 100 percent change or 50 percent change. If you are shooting a long exposure of 1 second, but change it to ½ second, then that a full stop of light. If you go from 1 second to 2 seconds, that also a full “Stop of Light.”

For most situations, you are shooting faster than a 1 second shutter speed. In full sunlight, you can easily work at 250th of a second. When change your shutter speed to 60th of a second you are slowing down the shutter speed and increasing the duration of the light being exposed to the sensor by two full stops. Remember, it’s either a half or double of the time for each full stop, depending on the direction of the change. Even though the camera will display the shutter speed as whole numbers, we are shooting at fractions of a second. The higher the number, the short the duration of the light to reach the sensor.

Controlling your shutter speed can impact the image's movement or no movement in the picture since we work in fractions of a second and not as whole numbers as the camera states. It's easy to misunderstand that the higher the shutter speed number is faster than a slower speed number. You can apply movement in an image by controlling the shutter speed of the camera. The camera shutter also controls the scene's apparent sharpness. Too slow a shutter speed can lead to camera shake in a low ambient lighting situation, making your image come out soft.

ISO – or Sensor Sensitivity

The third component of exposure control is ISO. It controls the light sensitivity of the camera’s sensor. ISO is a term from the days of using film to make an image. It is the indicator for the speed of the film. But with digital photography, it is the numeric indicator of the sensitivity of the camera sensor. It is a way to darken or lighting an image without changing your aperture or shutter speed.

There is a lot more to ISO than just lighting or darkening an image. Higher ISO settings dictates the sensor does not need to collect as much of the light, compared to a lower ISO setting. It also controls the amount of noise that the camera sensor can produce in the image. Electronic noise is like the appearance of sand when using faster high-speed film. Digital noise is the randomness caused by your camera sensor and internal electronics and appears as imperfections to an image. They usually appear in the shadows and dark tones of an image and are typically caused by increasing the sensitivity of the sensor. Digital noise can also appear when not enough light reaches the camera sensor.

The ISO scale is like the shutter speed scale. Each doubling or halving of the numbers increase or decreases the sensitivity by 100 percent or 50 percent of the light, respectively. 100 ISO is half as sensitive as 200 ISO, but 50 ISO is ¼ as sensitive as 200 ISO. Lowering the ISO number typically means you will have less noise, but you also need light to reach the camera sensor. Higher ISO will increase the sensor‘s sensitivity. Still, it’s a tradeoff with the amount of light the camera needs to gather and the increase in the capture’s.acceptable noise. Each camera has its own ISO sweet spot, where the noise is not apparent, and the tones are smooth and clean.

Reciprocal exposure is the relationship between shutter, aperture, and ISO settings. You can maintain the same exposure but change one or two of the other settings to suit your needs. If you require a certain depth of field and sharp focus, it could result in the image being too dark, but you can increase the shutter speed or the ISO settings to improve the brightness of the image. Sure, the simple way is to change and open up the F/stop, but that will decrease the apparent depth of field and what is in sharp focus. The other options are for you to either increase the ISO setting, or adjust your shutter speed. The concept of “Reciprocal Exposure” today is also called the Exposure Triangle.

The Exposure Triangle is one of the best graphic way to understand the properties of photographic exposure. It shows how the three key factors can affect each other and help determine the proper exposure.