Photography basics - Exposure
In this lesson, I'll talk about getting a "balanced" exposure. Not underexposed or overexposed. We will start with talking about some specific functions of our cameras, shutter speed, aperture, and ISO. These three functions directly related to getting "proper" exposures. They also have specifics that allow us to be more creative, such as freezing or blurring motion, allowing for more elements to appear to be in focus or to isolate a subject by blurring out backgrounds.
Shutter speed is just what it says, how long the shutter is open. What is the shutter? Think of the shutter as a curtain that opens to expose the film or digital sensor to light allowing use to record our scene.
For most photography, we are dealing in fractions of seconds. The range of shutter speeds varies by camera make and model. On my old Canon 35mm film camera, the range was from 1/1000th of a second to 2 seconds and then a setting called "B" for bulb (we will talk more about this setting at a different time). On my current Nikon, a D750, the range is from 1/4000th of a second to 30 seconds and then bulb (again we will discuss the bulb setting at a different time).
As you can see by the chart above shutter speed can stop or blur motion. This also pertains to camera movement. Ever try holding your camera steady at 1/2 second? You probably got a blurry photograph, didn't you? The faster something moves the faster our shutter needs to be to freeze the motion. We can also slow the shutter speed to create a sense of movement and speed.
There is also a rule of thumb pertaining to the slowest shutter speed that we can use the camera handheld, that is without the support of something like a tripod. This rule of thumb goes like this; use the inverse to the focal length of the lens. For example, if I am using a 50mm lens, I typically need a shutter speed of, at least, 1/50th of a second. If the focal length of your lens is 200mm you need a focal length of, at least, 1/200th of a second. If you look at the shutter speeds above you will see there is no 1/50th or 1/200th. This is because the examples above are using whole shutter speeds, commonly referred to as stops.
Below is a list of shutter speed typically found on our cameras.
Aperture often referred to as an f-stop, is essentially the internal opening in the lens to allow light to exposure the film or sensor. For simplicity sake, we will just use somewhat of an analogy with aperture. Think of the aperture (f-stop) as a fraction. The reason we will use this analogy is because the smaller the aperture number the bigger the opening. For example, f/4 is a bigger opening than f/11, because 1/4th is bigger than 1/11th.
As the aperture is opened up, moving to a smaller number, the less of a scene or subject is in focus, often referred to as the depth of field. Opening up the aperture (moving to a smaller number) also allows more light to enter the lens and ultimately the camera body. Stopping our lens down, moving to a larger number, closes the size of the opening, allowing less light into the lens and gives us a greater depth of field (more of a scene or subject in focus).
In another lesson, we will discuss aperture in more detail.
ISO - "Film Speed"
ISO is essentially how sensitive our sensor or film is to light. The higher the number the more sensitive to light.
One of the first topics I teach my students is exposure. Getting proper exposures are the foundation of learning photography. We need to learn to get the best exposure we can before we think about editing programs.
There are three factors to exposure. Shutter speed, aperture, and ISO (film speed). This is often referred to as the exposure triangle. Today's cameras have a built-in meter to help us achieve a proper exposure. If we put our camera in auto mode, the program built-in will read the light coming into the camera through the lens using a light meter. The camera will adjust shutter speed, aperture, and ISO to achieve an exposure that is "zeroed". In other words, the meter is in the middle.
While many people don't mind letting the camera decide it doesn't always give us the results we want. By adjusting our cameras ourselves we gain a greater level of creative control. We may want to freeze or blur the action. We may want more control over how much of the scene is in sharp focus, depth of field.
If we don't understand exposure and how shutter speed, aperture, and ISO work together, often called the exposure triangle, we will often get underexposed or overexposed images.
First, and introduction on metering. Cameras have had built-in light meters for several years. Back when I started in the early 1980's Light meters in the camera were similar to this one.
This is as seen through the viewfinder.
Today, light meter in our digital cameras look more like this:
As seen through the viewfinder.
Both of these meters measure reflected light i.e. the amount of light being reflected off a subject back to our camera's meter. NOTE: For the purpose of this lesson we will discuss assume the use of the camera's built-in meter and measuring reflected light.
Above is a more detailed view of the light meter built into our cameras. As you can see, there is a neutral point (zero) and a + and -. As you move towards the minus the image will become underexposed. As you move towards the plus image will be overexposed.
As I mentioned earlier, three settings control exposure. Those three settings are the shutter speed, aperture, and ISO (film speed or in our case now the relative sensitivity of the camera's sensor).
If a balanced exposure (meter is centered not indicating over or under exposure) is achieved by setting the camera to 1/250th of a second shutter speed at an aperture setting of f/8 with an ISO, film speed, of 200 it would commonly be written as 1/250th f/8 ISO 200. The same balanced exposure could be achieved with the following settings.
Example 1. 1/500th f/4 ISO 200
Example 2. 1/500th f/8 ISO 400
Example 3. 1/125th f/11 ISO 200
In Example 1 when we adjust the shutter speed to 1/500th of a second, we made the aperture larger (smaller number) which allowed more light to strike the film (sensor) with the same light sensitivity.
Example 2 we increased the shutter speed and left our aperture set to the original f/8 but we made our film (sensor more sensitive to light.
Example 3 we slowed the shutter speed to allow for a longer exposure of the film (sensor to light) but we decreased the size (larger number) of the aperture with the film/sensor at the original sensitivity to light.
This all may sound confusing but it isn’t as confusing as it sounds. Think of the light being water. The diameter of the hose is the aperture, the time to fill the bucket is shutter speed and the pressure of the water is ISO. We have a bucket we want to be full of water (a full bucket is a balanced exposure).
If I use a small diameter hose with normal pressure water it will take more time to fill the bucket (example 3). If I increase the diameter of the hose with the same normal pressure water it will take less time (example 1). If I decrease the diameter of the hose (f/11) it will take more time to fill the bucket (example 2).
In future lessons, I will dive deeper into exposure and discuss more advanced topics.
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