Join Ben Long for an in-depth discussion in this video Understanding how your camera controls exposure, part of Introduction to Photography.
- If I hold my hand up in front of my face, in front of that light fixture that's up there, I shield myself from some of that light. Now from where you are, you can see that I'm casting a shadow onto my face with my hand. But that's not how I think of it. I simply think of it as, blocking light that was striking my eyes. If I open and close my fingers or move my hand forward or back or in and out, I can block more or less light, in other words, I can control the exposure of my eyes to that light source.
My eye has a built-in mechanism for doing this called the pupil or iris. You've all probably seen somebody elses' pupils open and close or watch your own in a mirror. As the pupil closes, less light can enter the eye and strike the light-sensitive cells on the back of the eyeball. In this way, as light levels in the room brighten, my eyes can lower their exposure to that light. Conversely, as light levels dim, my pupils will get much larger to let in more light. I can also put on sunglasses to lower light levels.
There are many different mechanisms available to me for controlling my eyes' exposure to light. In the last video, we talked about controlling your camera's exposure to light to render images brighter or darker. As there are many ways to control the light that strikes your eyes, your camera has three different mechanisms. First, it has a shutter. This is simply a door that sits in front of the sensor. It can open and close and depending on how long it stays open, the image sensor in the camera will be exposed to more or less light.
This is known as shutter speed. We might refer to an image and say that image was shot with a shutter speed of 1/60 of a second, meaning that the shutter was only open for a 60th of a second when we took the picture. Shutter speeds can be very fast. This camera can shoot with shutter speeds up to 1/8000 of a second or they can be very slow, astrophotographs are sometimes shot with shutter speeds that measure in minutes or even hours. Shutter speeds pretty intuitive. The longer that little shutter is open, the more light that will hit the camera's image sensor and as more light strikes the sensor, the image will get brighter.
Your camera also has an iris that functions like the iris in your eyes. It's a series of interlocking blades that create a circular opening that can be made bigger or smaller. As the opening gets larger, more light passes through and your image gets brighter. We refer to this as the aperture or iris and its size is measured in f-stops. Understanding the relationship between the f-stop scale and the size of the aperture can be a little confusing so we're not going to go into that in this course.
The third parameter is called ISO and it's simply a measurement of the sensitivity of your camera's image sensor. We can adjust that sensitivity to make the sensor more or less responsive to light. Anytime you take a picture, the light will be focused through your camera's lens where it will be passed through the aperture which will be open to whatever size you have specified to allow more or less light. Then the shutter will open and close at the speed that its currently set for letting that light pass through where it will strike the image sensor.
How much of that light will get recorded is dependent on the sensitivity of the sensor. These three parameters are things you'll think about anytime you take a picture. Each one has several different effects on your final image and the three parameters together have a powerful intertwined relationship. A deep understanding of exposure requires a fair amount of study and we have an entire course set aside for it. Fortunately, your camera's auto-mode can take care of a lot of the exposure decisions for you. This is great for the beginning photographer because it means you can go ahead and start taking good pictures.
You can begin practicing composition and shooting technique and you can slowly begin to chip away at an understanding of exposure. Those are the things we're going to be doing in the rest of this course.
Then it's time to take to the field and examine the rest of the factors that influence the quality of your photographs, including light metering, focus, composition, and flash. Ben also introduces techniques for shooting portraits and shows what you can do with an image editor in post. Last but not least, he'll provide a roadmap for learning more with the lynda.com extensive library of photography training. The path to becoming a better photographer begins with the first step. Start here!
- Exploring cameras and lenses
- Understanding media
- Controlling exposure
- Composing with autofocus
- Shooting portraits
- Understanding form and geometry
- Exporting and editing digital images