Understanding Exposure

Photography is all about capturing light, and if you don’t understand how this happens inside your camera, you will have a difficult time grasping many of the lessons we cover in this book. Depending on the shooting mode you are working in, the camera can and will make many of the important decisions. But even if you are comfortable with this, it is important to know what choices it is making and why.

So what is exposure? By definition it is the process whereby the light reflecting off a subject reflects through an opening in the camera lens for a defined period of time onto the camera sensor. Within this process there are three core elements that determine the exposure:

• ISO: Determines how sensitive the camera sensor is to light. ISO stands for International Organization for Standardization, but the acronym is used to describe the sensitivity of the camera sensor to light. The higher the sensitivity, the less light is required to properly expose an image. In the film photography days, each roll of film carried an ISO rating, forcing you into that ISO strength for the duration of the roll. Now you can change ISO on the fly.

• Aperture: Also known as f-stop, it determines how much light passes through the lens at once.

• Shutter Speed: Controls the length of time that light is allowed to pass through the lens and hit the sensor.

It is through these three variables that exposure is determined for a photograph. The catch is that each variable is interconnected with the others, so it can become a balancing act in order to find the exposure that gives you the best results. So how does it work? By choosing an ISO, you are setting the level of sensitivity for the camera sensor. In order to have a balanced amount of light in your image—so that it is not too bright and not too dark—the lens needs to adjust the aperture (the size of the hole that allows light to enter the camera via the lens) while balancing the amount of time (shutter speed) that the light is allowed to hit the sensor. If you allow too much light in, you will lose details in the image and it will be overexposed. Allow too little light in, and the image will be underexposed and you will again lose details.

ISO numbers on the 6D start at 100 and end at 25600, covering a wide range of light sensitivity. It is important to note that every time you move up in ISO with the default base ISO increments, you are doubling the sensitivity. So 200 is twice as sensitive as 100, 400 is twice as sensitive as 200, and so on. In your custom settings, you have the ability to set the ISO to move in 1/2- or 1/3-stop increments, giving you finer control. You also have a wide variety of shutter speeds at your disposal—from as long as 30 seconds down to 1/4000 of a second. Note that when you see the shutter speed on your camera, it is referring to a fraction (1/x). So when you see 320, that actually means 1/320 of a second. By the time you have slowed down your shutter speed to get closer to an actual second, you will notice “ appear. For example, 0”5 means half a second and 1” means 1 second. The aperture settings vary slightly depending on the lens you are using. Each lens has different minimum and maximum apertures. The standard apertures that you should have access to are f/4, f/5.6, f/8, f/11, f/16, and f/22. If you have a fast lens, you might have the ability to shoot at f/1.2, f/1.4, f/1.8, and f/2.8.

Exposure is about finding the right balance of light via these three core elements: ISO, shutter speed, and aperture. Change one of these variables, and you will need to adjust one or both of the others in order to balance the light hitting the sensor. This is known as a reciprocal change. For example, if you increase the amount of time the shutter is open (shutter speed), to balance everything out you will need to either decrease how sensitive to light the sensor is (lower the ISO) or decrease the aperture opening.