Technical (beginner to intermediate) - Understanding EXPOSURE
With the advancement of technology, we now find a lot of photographers relying on a camera's ability to choose the correct settings for the circumstances. Although this works pretty well for much of the time, it is still important to be familiar with how the three main exposure controls work, and how they relate to each other.
"Exposure" simply refers to the amount of light captured, whether by an electronic sensor, film, or (for any Victorians out there) a light-sensitive coating on a glass plate. Actually, I remember using glass plates to photograph the night sky during my astronomy course at university, so perhaps I should gloss over that Victorian comment!
Three camera settings control the exposure of the photos we take: aperture, shutter speed, and ISO. These three terms should at least be familiar to anyone using a camera, even if they are not fully understood by all. As with most things in photography, it helps to break everything down to smaller, simpler steps and understand each of these before putting it all back together.
Aperture - cameras lenses can control how much light reaches the recording medium (film or light-sensitive sensor) by changing the size of the hole through which light enters the camera. We use aperture to control the depth of field.
Shutter speed - the camera can alter the length of time the shutter is open: the longer it is open, the more light gets through. We use shutter speed to control the amount of blur in moving objects.
ISO - this is the least obvious setting, and one that some may not even be aware of. In digital cameras, it refers to the sensitivity of the light-sensitive sensor, which can be altered electronically. It equates to the film speed in older, film-based cameras. Its use is a little less obvious.
Each of these three settings has a different effect on the resulting image, and it is up to the photographer to decide which of these should take priority. Shutter speed will affect the amount of blur a moving object has, so can be used to freeze action (fast shutter speed, typically 1/1000th sec or faster) or to give the impression of flow or movement (slow shutter speed, typically 1/20th sec or slower). Aperture controls the range of distance over which our photograph is acceptably sharp. ISO is not really used creatively in the way that shutter speed and aperture are. Rather, it gives us some latitude in what settings we can use, depending on how much light is available. It is important to note, however, that a low ISO (less sensitive, requiring more light) gives better results than a high ISO (more sensitive, not requiring as much light). Using a high ISO creates more "digital noise" or, with film, larger grains, so in either case, some detail is lost. With film, that has been used creatively but digital noise is generally unwanted. The "film grain" effect, if desired, can be created in the photo manipulation stage by digital photographers.
As any one of these settings changes, we need at least one of the others to be altered to retain the same exposure, which is where the phrase "exposure triangle" comes from. Our cameras are designed so that the increments in each have the same effect on exposure.
We refer to "stops" in photography. A full stop (or 1 stop) is an adjustment to the camera settings which doubles (or halves) the amount of light landing on the light-sensitive material (the sensor, in digital cameras). So, we might talk about going up one stop (doubling the amount of light) or going down one stop (halving the amount of light).
A lower exposure can be achieved by a faster shutter speed, or a smaller aperture, or a lower ISO setting. Each of these reduces the amount of light captured. Similarly, a higher exposure can be achieved by a slower shutter speed, a larger aperture, or a higher ISO setting. Each of these increases the amount of light captured. The increments our cameras allow for adjusting each of these are designed to be the same, and may be measured in stops or fractions of a stop, normally a half, a third, or a quarter stop. One increment in the appropriate direction for shutter speed, aperture, or ISO will have the same effect on the exposure.
One more slight complication I should mention here is that most cameras allow us to set an "exposure compensation". Often this will be used in unusual lighting conditions such as a lot of light reflecting off a snowy scene.
With all these available settings, it is very tempting to let the camera make the decisions for us. A lot of the time cameras will do that pretty well (assuming you understand the modes they offer you). However, I think that the attempt to "automate" these settings has done two things. First, it has taken away the understanding of what is really going on, which leaves us vulnerable when the camera doesn't manage to make a good decision. Second, the "automated" settings have themselves become more complex, with all kinds of "scene modes" or other terms entering the fray. With a proper understanding of exposure, and how the three basic elements that control it relate to each other, it may actually be quicker and easier to set up a shot than it is to find an appropriate "automatic" setting.
Cameras, especially DSLRs, usually have a couple of "semi-automatic" settings that most proficient photographers use a lot of the time. You can set "aperture priority" which means you choose the aperture and the camera selects the shutter speed to give the correct exposure. Alternatively, you can select "shutter priority" where you choose the shutter speed and the camera selects the aperture. In neither case have I mentioned ISO. Usually this will be set by you, and altered only by you, but some cameras can alter the ISO setting automatically. To discuss all the permutations would just be confusing and pointless so I strongly suggest you consult you camera's instruction book, or look for advice about your own camera on-line if it can change the ISO for you.
There is one more setting commonly used by proficient photographers, and that's the manual mode, where the photographer chooses every setting. This is gives ultimate control of creative effects - if you know what you're doing!