Every photographer knows that proper exposure is essential for a good photo but far too many picture-takers rely on the Auto or Program modes for exposure control and don't realize the creative possibilities of using manual exposure modes. So what do the auto modes actually adjust to obtain a "proper" exposure and why would anyone want to regress to manual modes?
Exposure is a poorly defined term that describes the effect of light on the media used to capture an image. Today the media is the digital camera sensor. There are three factors that affect the exposure and they are the sensitivity of the sensor, the amount of light coming through the lens, and the duration of time that the lens is open. Each of these elements of the exposure triangle has impact on the image, each has a different effect on the way the final image looks, and each adjustment has pros and cons influencing its use in manual modes of exposure. Let's look at each element and its pros and cons.
In the days of photographic film, the first decision a photographer had to make was what kind of film to load into the camera and, very importantly, how sensitive or "fast" was the film in recording the image. In the olden days, film speed was measured by the ASA of the film. Many remember good old Kodachrome-64 and Kodachrome-25 used for 35mm slides, and Panatomic-X and Tri-X used for black and white photos. Over time, the ASA (American Standards Association) speeds of film were converted to ISO (International Standards Organization) terms with higher numbers meaning greater sensitivity of the film and faster capture speeds. Tri-X film was very fast with an ASA or ISO of 400 (and even higher if special darkroom processing was used.) Fast film (high ISO) could record images in less light or faster shutter speeds needed to capture sports images. The problem was that high ISO meant more graininess in the image and less sharpness for large reproductions or prints. The same ISO is used to describe the sensitivity of a digital sensor, and the same sort of problems occur when a sensor is made faster by increasing the adjustable ISO exposure setting. The major pro for using a higher ISO is that you can use a faster shutter speed to freeze motion or can shoot in darker conditions without supplemental flash. The con of increasing ISO is that when the sensor is made more sensitive, digital "noise" (similar to film graininess) can be introduced into the image. For the clearest image with the least digital noise we want to shoot using the lowest ISO setting on our camera - usually 100 but it may be 50 or 200 depending upon sensor and camera quality.
As you have gathered by now, the second determinant of exposure is shutter speed. Shutter speed tells us how long the shutter is open allowing light to reach the sensor. Shutter speed is generally expressed as fractions of a second but may extend into seconds or even minutes. The shutter speed of old manual film cameras was usually about 1/60 to 1/100 of a second. This speed is often fast enough to capture a still subject image without blur caused by camera shake. But what if the subject is moving as a bird in flight? Usually it takes a much faster shutter speed to freeze the bird's wings - maybe 1/500, 1/1000, or 1/2,500 of a second. The faster shutter speed will freeze the motion but if other factors of the exposure triangle do not change, there will not be time for enough light to hit the sensor to produce a proper exposure. This is the concept of exposure balance. If one factor changes one or both of the other factors must change in the opposite direction.
The third element of the exposure triangle is aperture. The aperture is the size (diameter) of the lens opening that allows light to reach the sensor. Aperture is the most complicated and, therefore, the most useful, of the exposure adjustments. Aperture is measured by the f-stop setting. A larger f-stop number causes a smaller aperture. What? That is right, a larger number causes a smaller aperture and less light reaching the sensor. How can that be? The reason for this has to do with the definition (and calculation) of the f-stop. The f-stop is not a whole number but rather is a fraction relating the smallest diameter of the light pathway in the lens to the focal length of the lens. Starting to sound complicated yet? Well, it's not complicated. Focal length of a lens basically tells you the length from the optical center of the lens to the sensor. A longer focal length produces a larger image on the sensor (more about focal length in the next posting.) A "standard" 50mm lens with an optical diameter of 25mm has a maximum aperture of f/2 (e.g., 50 / 25 = 2.)
The iris of the lens is a cluster of metal leaflets that can open or close to change the diameter of the light pathway and, therefore, the f-stop. If the iris of the 50mm lens described above is closed down to 12.5mm, the resulting f-stop is changed to 50 / 12.5 or f/4. If the iris is reduced further to 6.25mm the f-stop is now f/8. So you can see that by knowing the f-stop is actually a fraction, a greater number will cause a smaller aperture and less light to the sensor. We will discuss the impact of f-stop on the size of lenses in the next posting. For now, recognize that a smaller f-stop means a larger aperture and a bigger lens for any given focal length.
What are the pros and cons of changing aperture? The most important effect of changing the aperture of a given focal length lens at a given distance from the subject is the depth of field (DoF) or the effective range of focus. You will get the greatest depth of field at the smallest aperture (other factors being constant.) So if you are shooting a nice landscape with a perfect foreground object or person close to the camera, you would want a small aperture (larger f-stop) to get the greatest depth of field for the image and keep everything in focus. On the other hand, if you want to isolate a close-up of a flower from its background, you want a large aperture (smaller f-stop) to create a shorter depth of field to softly blur the unwanted background.
So to summarize, if you need to increase exposure in order to capture an image in low light conditions, you can increase the sensitivity of the sensor (ISO), decrease the shutter speed (a longer duration of exposure), or increase the aperture (a smaller f-stop.) It takes experience to decide which adjustment(s) to make to produce the desired effect on the image as well as the exposure. There will be more on all of this in future postings.