Exposure is one of the most basic elements of photography, and most important. In its most basic sense, exposure is how light or dark everything appears in the image. This process begins when light from a source reflects off the objects in your scene. That light enters your camera through the lens and is absorbed by the sensor. Your camera’s sensor is made up of millions of photosites (or pixels) that absorb light. The more light that is absorbed, the lighter the image is going to be. Inversely, if you restrict the light that is let into the camera, the image will be darker.
Although cameras have come a long way since their inception 75 years ago, the way exposure works is still exactly the same. Exposure isn’t just one simple adjustment. There are three separate parts of every camera that work together to control the overall exposure. They are aperture, shutter speed, and ISO. Learning how to adjust these manually will not only help you to understand how your camera works, but you’ll learn how to take creative control over your photography. By learning how to set the aperture, ISO, and shutter speed, you can make creative decisions like how to show depth-of field and how to control motion in an image.
Inside all lenses are a set of rounded blades in a circle called aperture blades. These blades form an octagonal shape opening inside the lens that can open and close to control the amount of light entering the camera.
If we were to take a picture and the image was too dark we could simply open up the aperture wider to let more light in. If we closed the aperture all the way, very little light would reach the sensor and the image would be very dark. All lenses have a maximum aperture at which the blades are all the way open, letting in the maximum amount of light (a.k.a. shooting wide open).
Aperture is measured by a scale of f-stops. A small f-stop number such as f/1.4 means that the aperture blades are wide open; and when the f-stop number is large like f/16 or f/22 this indicates that the aperture blades are closed down very small letting only tiny amounts of light in. Every full stop up or down the scale lets in exactly double or half the amount of light as the previous stop.
In addition to controlling how much light enters through the lens, the aperture also controls the depth-of-field in your images. Depth-of-field is a term that refers to how much of the scene is in focus. The wider that you open the aperture (smaller f-stop number), the more shallow your depth-of-field. The more you close the aperture down (higher f-stop number), the wider your depth-of-field. The wider your depth-of-field is the more space in the image can be in focus at the same time.
In many portrait images you see the subject of the photo in sharp focus while the background is a blur of out-of-focus shapes and colors. When the background blurs like this it is often referred to as bokeh. Images like this are taken using a lens with a wide open aperture. The wider that you open the aperture, and shallower your depth-of-field, the more out of focus the areas behind and in front of the subject will appear.
The shutter is inside the camera body. After the light passes through the lens and the aperture it hits the shutter. The shutter is like a curtain that opens letting the light passing through the aperture hit the sensor. The slower the shutter speed (the longer the shutter is open), the more light hits the sensor, and the lighter the image. A faster shutter speed reduces the amount of light that hits the sensor resulting in a darker image.
Shutter speeds are expressed as fractions of a second. Shutter speeds that are one second or longer are simply expressed as 1”, 2”, 10”, etc.. Most cameras have shutter speeds that are as slow as 30”. To get a longer exposure than 30 seconds you will most likely have to use a cable release. Shutter speeds less than a second that are expressed as fractions such as 1/8, 1/32, 1/125 are usually displayed on the camera as simply 8, 32, 125, leaving off the top half of the fraction to save room.
In addition to simply helping control light and overall exposure, the shutter speed is what controls how we see motion in still images. Faster shutter speeds help to stop or freeze motion. For example taking a photograph of a bird in flight would require a shutter speed around 1/4000 of a second to freeze the action of the wings flapping rapidly. A slower shutter speed would result in motion blur and you would only see the wings as a blur of color with no detail. There are also lots of reasons that one might want to use a slow shutter speed to blur motion. A common example is moving water. Waves on a lake or water moving down a stream can be blurred to have a beautiful glass-like appearance.
The motion within the frame is not the only motion that we have to worry about. The small motions made by holding the camera can create blur and softness in your photograph, especially when using slower shutter speeds or photographing in low-light situations. While using a lens of normal focal length most people can hand-hold a camera as slow as 1/125 down to 1/40 without introducing any motion. With practice and proper holding techniques that number can improve. Lenses with vibration control can also help to improve that number even further.
The focal length of the lens also plays a role in how fast of shutter speed is required. The longer focal length the lens is (higher mm), the higher shutter speed you will need if you intend to hand-hold the camera. A general rule for this is to divide one by the focal length of the lens. If you are hand-holding a 300mm lens then you would want to use a 1/300 shutter speed (the closest equivalent is probably 1/320). Telephoto lenses are where vibration control features make the most impact. Canon calls their vibration feature Image Stabilization (IS) and Nikon uses Vibration Reduction (VR). These features vary but usually allow you to hand-hold your camera at a shutter speed between 2 and 4 stops slower than you would otherwise be able. This can make a huge difference in what you are able to shoot, especially in lower light situations. If, however, you are absolutely unable to hand-hold and shoot at the shutter speed required then you will need to use a tripod or some sort of support to stabilize the camera.
ISO, in very basic terms, determines how sensitive your camera is to light. The sensor inside your camera is what gathers light and translates it into an image. The sensor is made up of millions of photosites (or pixels). The higher the ISO number the more sensitive those pixels are to light and the more they absorb. A low ISO number like ISO 100 means that the sensor is much less sensitive to light. The higher that the ISO number goes the more light sensitive the sensor becomes.
In the past, with film cameras, film came in different ISO values. Once you loaded a roll of film into the camera you were stuck with that same film until you finished the entire roll. Today with digital cameras, ISO is controlled within the camera digitally, allowing you to instantly change the sensitivity from one image to the next. This is especially helpful in places where the ambient light changes frequently.
Every camera has a base ISO (the lowest number, typically ISO 100) at which the camera produces the best image quality possible. With a higher ISO it becomes possible to take pictures in low-light situations without having to use a flash. The downside to raising the ISO is that at the quality of the image degrades slowly the higher you set the ISO. This happens because the camera is working to magnify the signal coming from the sensor. The result is called digital noise. How much noise depends on a lot of factors including the size of your camera’s sensor, the number of pixels on your sensor, and how far apart those pixels are. Cameras that have larger sensors tend to do better in low-light situations. This is one of the primary reasons that most pro photographers upgrade to a full-frame camera at some point.
The part that you should take away from all this is that it is always best to set the ISO at the lowest possible setting the situation will allow. Typically you will want to set the aperture and shutter speed first, then set the ISO as low as possible for the exposure that you want to achieve. This way the digital noise is eliminated as much as possible and you have the highest quality image possible.
How does one setting effect the others?
Aperture, ISO, and shutter speed are all set according to a scale of f-stops. When you increase or decrease one, the total exposure increases/decreases by one stop. If one of them is increased by one stop, one of the other two has to be decreased by one stop to keep the exposure the same.
If we wanted to add depth-of field by opening up the aperture by two stops, there are several options. To balance the exposure back to zero, we need to decrease overall exposure by two stops. We can do this by raising the shutter speed two stops, lowering the ISO by two stops, or by adjusting each of them by one stop. The specifics of the subject being photographed will determine which option is the best. If there is movement, or very little light, the shutter sped may not be able to be reduced without adding blur to the image.
Putting it all together
The first question that I had when I was learning this was: What exactly does it mean to have a good or perfect exposure? When taking a photograph, you will obviously want to have the exposure fall within certain limits so that you retain details in the important areas of the photograph. It’s also important that the image is of a similar representation to what you see with your eyes. I don’t, however, believe that exposure is about right or wrong. What one person sees as a good exposure may not look the same to someone else. There is nothing wrong with purposely underexposing an image to create more feeling and intense vibrant color. The same is possible on the other end. Slightly overexposing an image to produce high-key colors and even purposely leaving some areas blown out and void of detail is absolutely okay.
This post is much too short to cover everything, but hopefully you have a basic understanding of how exposure works in your camera. Being able to manually set exposure is one of many valuable tool that can really improve the quality of your work. If you still aren’t totally comfortable with using the full manual exposure mode, using the aperture and shutter priority modes are a great first step. Take your time and picture the scene in your head, imagine what you want the final image to look like, and decide on the settings required to make it happen.
Please feel free to download, save, or share the cheat sheet below. Enjoy!