How exactly does the camera get the exposure perfect every time (in optimal conditions, mind you), and what are those numbers that are constantly changing on my display before I press the shutter button to snap the photo? If you’re like most people out there, you had the exact same questions the first time you tried using a camera more advanced than a smartphone. Even compact cameras have come a long way and have become very capable little machines in recent years and many offer similar setting and options to professional level cameras! So lets look at what is really going on inside your camera to get you to that perfect exposure.
There are three main functions that really determine the outcome of a photograph. They are, in no particular order, the aperture, shutter speed, and ISO. Many people refer to these as the exposure triangle. In essence, all three of these settings must be cooperating with each other to achieve a well rounded exposure. If just one of these settings is out of line with the others, the entire exposure can be compromised. When one setting is changed, another needs to change the same amount in the opposite direction to compensate. I’ll quickly break down what each of the three settings do for a better understanding of how exactly they work together, along with an example for each. I’ll go into more detail on each setting at a later date, but for now here is the cliff notes version.
Aperture – This refers to the iris within the camera lens that determines how much light is able to reach the sensor. You will also hear aperture referred to as “F-stop”. The camera will display your aperture setting in an X.X format within your viewfinder or LCD (1.4, 2.8, 4.0, etc…). The smaller the number, the larger the opening. The larger the opening the more light that enters the camera body. f/1.4 is a very large opening, letting in a lot of light, where f/22 is a tiny opening that will allow very little light to reach the sensor. The aperture is also what control the depth-of-field (amount of the photo in focus) of an image.
Shutter Speed – This refers to the amount of time that the sensor is gathering light. The longer the camera is able to gather light, the brighter the image becomes. Shutter speed can vary greatly, from thousandths of a second, to several seconds, minutes, or longer. A “fast” shutter speed is one in which the shutter opens and closes very quickly (gathering light for a short amount of time), whereas a “slow” shutter speed means that it stays open for longer (gathering light for a longer amount of time). A fast shutter speed will freeze things in motion and a slow shutter speed will allow moving objects to become blurry.
ISO – This refers to the sensitivity of the image sensor, and how easily it will gather and record light. ISO can range from a setting of 50 (low ISO) to well over 100,000 (high ISO) in some modern professional cameras. As the ISO number increases, so does the sensor’s sensitivity. With increased sensitivity comes more digital noise (similar to grain on film) that degrades the overall quality of the image. It is always best to shoot with the lowest ISO that will allow you to get a proper exposure.
Each of these setting are measured in something called stops. Most cameras by default measure stops in 1/3 stop increments. For example, an adjustment from f/1.4 to f/2.0 is considered one full stop (this goes back to film days where there were no 1/3 stop increments). So on an old camera lens, if you were on f/1.4, the next option was f/2.0, then the next would be f/2.8, and so on. However, with the advent of digital, we are able to break each full stop into smaller 1/3 stops in between for more precision. So now, from f/1.4 you have f/1.6, f/1.8, then the full stop of f/2.0. When talking about exposure, many people refer to stops as “clicks”. This comes from the clicks that you get from most cameras when changing settings with a dial on the camera. So it would take “3 clicks” for every full stop (if you’re at f/1.4 in our previous example, you would have to move the dial three times, or clicks, to reach f/2.0). Shutter Speed and ISO are measured the exact same way, and also for the purposes of the example below I will refer to each by the amount of clicks we are moving.
For the following example we will assume you are shooting in full manual mode.
Example: For a perfect exposure, you determine that the aperture should be f/4.0, the shutter should be 1/125 and the ISO at 800. Then, you decide that you want more of the photo out of focus which requires a larger aperture (smaller f number). You move the aperture three clicks to the left, landing on f/2.8. If you changed nothing else and took the picture you would be allowing one extra stop of light to enter the camera, since f/2.8 has a larger opening than f/4.0. This would result in your image coming out overexposed. So, to fix that we must change one of the other settings to compensate for the additional light coming into the body. You could either move your shutter speed three clicks to the right landing on 1/250 (which keeps the shutter open for a shorter amount of time than at 1/125, therefore less light gets to the sensor), or reduce the sensitivity of the sensor by lowering the ISO three clicks to ISO 400 (which allows the sensor to become less sensitive to the light that enters). You could also do a combination of moving the shutter and ISO both, but we won’t get into that here.
As you can see, any change in the exposure triangle requires an equal change in the opposite direction (more light gathering, or less light gathering depending on which setting you need to change) to compensate for the change in light that hits the sensor to achieve a technically correct exposure. Play with these setting individually in Manual mode to better understand how each one impacts the image.
There are several books that I have personally read that go into much greater detail. My personal favorites being Understanding Exposure by Bryan Peterson, and Exploring the Light by Rick Sammon. I will link them below. Check them out and get a much deeper look into what makes a perfect exposure!