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The correct Exposure - a tutorial - Part 1: Shutter, Aperture and ISO
Determining the "correct" exposure for an image can be a challenging task. We could choose to simply let our camera decide for us, and use the fully automatic settings that are quite sophisticated and often do a reasonably good job. However, this also means to give away part of the creative process to your camera. I am writing this tutorial with the beginning photographer in mind. The principles outlined in this article should become a second nature to you. I encourage you to practice and to ask questions. In many ways, this is like riding a bike. Once you learned how, you can focus your attention on your surroundings.
I am not advocating using the manual settings of your camera at all times. Instead, I am a big fan of the creative zone settings such as aperture priority Mode, Shutter Priority Mode and Program mode. I use these automatic modes 95% of the time and I rarely resort to Manual Mode.
Aperture settings determine the Depth of Field (how much of your scene is in focus). Landscape photographers often strive for a very large Depth of Field while Portraits often require a shallow Depth of Field. Being in control of the aperture, the photographer can direct attention and guide the viewer through the image.
Shutter speed controls how motion of the subject or your camera will look like in your pictures. A slow shutter will blur fast moving subjects and a fast shutter will freeze them.
The three exposure controls: Shutter Speed, Aperture, ISO
If all this sounds confusing and if you feel overwhelmed, you are in good company. There are too many degrees of freedom and too many choices to make, but you can become a master through practice.
Introducing ISO sensitivity
The ISO sensitivity (ISO rating, ISO speed) characterizes the sensor or film sensitivity to light. Formerly called ASA rating, digital cameras continue to use the same system introduced a long time ago. It is common to use the term "stop" in photography. One stop in terms of ISO refers to a doubling of our sensitivity. ISO 200 is twice as sensitive as ISO 100 and is therefore "one stop higher." In film photography, we would have to make a decision on the type of film needed before going out on a shoot. One of the great advantages of digital photography is the capability to change the ISO settings for each frame.
Use higher ISO settings with caution. High ISO film is wonderful for low light photography, but they have much more grain then low ISO films. A high ISO setting on your digital camera will also produce more noise in your image.
Modern SLR cameras have very low noise and I would not hesitate to use even higher ISO settings to "get the shot." Digital SLR cameras have less noise than point and shoot cameras at the same ISO setting, since their larger sensor means they are more light sensitive (a large window lets in more light than a small one).
The shutter is covering the film or sensor and only when the shutter-release is pressed, do we open the shutter for a pre-determined time to let light pass through our lens to the sensor. We also use "stops" to measure shutter speeds. Each stop doubles the time the shutter remains open and thus doubles the light sensitivity of our camera (e.g. 1/160s is twice as long as 1/320s). For handheld shooting, we require at least 1/(focal length) or shorter to avoid camera shake (if no Image Stabilization is used). For a Rebel XSi with 50mm lens, this means 1/80s or longer (the rebel has a focal length multiplier of 1.6, so 50x1.6=80).
The aperture is the size of your lens opening. It controls how much light we let pass to our sensor. Aperture also determines Depth of Field and is measured in stops (f-stops for aperture). Each stop doubles the amount of light we let pass (f/4 is two stops larger then f/8).
Using Shutter Speed, Aperture and ISO controls for Exposure
Now that we understand ISO, Shutter Speed, and Aperture, we need to learn how to combine them to create an exposure. This is the most important thing to understand in photography and the reason I spent so much time creating the diagram.
The combination of Shutter Speed, Aperture, and ISO as well as the focal length and focus point fully control how an image looks like.
Any given brightness level, requires a certain sensitivity to capture it. In my diagram, I encoded the brightness levels of a scene with colors. A dark scene requires a sensitive camera system (slow shutter speed, wide-open aperture, and/or high ISO). A small f-stop number (e.g. f/2) corresponds to a wide-open aperture that is very bright. Lenses that open up to f/2 or wider are often quite expensive.
The message to take away from this diagram is the possibility to trade off. For any given brightness, an increase in shutter speed (e.g. to freeze motion), requires a wider open aperture (less depth of field) or an increase in ISO speed if the aperture cannot be increased anymore.
Trading off shutter speed for aperture is one of the basic rules. A small aperture (large Depth of Field) often requires a tripod, as the shutter speed decreases. Landscape photographers often work with tripods even during the day for exactly this reason. Increasing ISO is not an option, due to the added noise.
On the other hand, a tripod is not always an option. I did many of my indoor shots of old missions with a higher ISO setting and a short focal length. The short focal length helps me to capture everything in the scene, but it also lets me shoot at slower speeds handheld without camera shake becoming a problem
A very bright scene lets us shoot at small apertures (large f-stop) and still shoot handheld at fast shutter speeds. The diagram only shows a small portion of the shutter speed scale that ranges from 1/8000s to bulb (hours if necessary) for most SLR cameras.
Polarizing filters reduce the brightness of up to two stops (depending on the rotation of the filter). Sometimes this is beneficial (blurring motion) but often it is an unwanted side effect to remember.
The "Sunny 16 Rule"
The light meter in your camera may fail or you might be in a hurry. Whatever the reason, sometimes it is good to know that at f/16 (an aperture with lots of depth) your exposure on a sunny day should be 1/(ISO Rating) to keep the highlights from clipping. If you have your camera set to ISO 100, this means your exposure should be 1/125 (the closest value to 100). One stop reduction in light due to a faster shutter (to 1/250) requires opening the aperture by one more stop (going to f/11 from f/16).
In my next tutorial, I will show you how to play Shutter, Aperture and ISO against each other to control Depth of Field, Motion Blur and your inner artist.