Photography, or “Foto graffi” literally means writing with light in Greek, and exposure is the combination of three factors that determine what the light writes… hence the Exposure Triangle.
Those three elements are: ISO, Aperture and Shutter Speed.
ISO - The ISO rating (which is an international standard) measures the image sensor’s sensitivity to light.
Aperture - The Aperture determines how much light enters the lens and is registered by the image sensor (or film); this is designated by the f-numbers on the lens barrel.
Shutter Speed - The Shutter Speed determines the amount of time the light enters the lens and is registered by the image sensor. Note that the Shutter Speed on your camera is delineated in fraction of a second increment.
The combination of these three elements results in a given subject’s exposure value (EV). What is critical to remember is that any change in any one of these elements will cause a predictable impact on the other and consequently impact the final image (i.e. by changing the Aperture, you change depth of field; by changing ISO rating, you change the amount of light required to obtain an image, and by changing the Shutter Speed, you effect how motion is captured). Such that you will never be able to independently control a given element, because you have to take into account how the other two elements will interact for the final exposure.
Fortunately, the mathematics of photography happen to work in such a way that each element in the Exposure Triangle has a relative “stop of light” value. Such that if you increase the light by one stop by reducing the Shutter Speed, you can regain the original EV by either decreasing the Aperture by the same stop value and/or adjusting the ISO rating accordingly.
Here’s a real world example; I’m at the beach with my family and the sun is going down, and I want to get a shot of my daughters smiling face. I take out my camera, do a quick meter reading with the shutter set 1/60th and get a EV on her face of f/4. I set the aperture to f/4 and take my photo. I look at the image on the display screen, and while I love the way the red and purple light dances on her face, I don’t like the depth of field – I can see too much of the background. I want as shallow a DOF as possible, which means I need to increase my aperture setting. I open the lens wide open to f/1.4. This is a 3-stop difference, which lets in 8x as much light. To compensate, to get back to the same EV that gave me such a pleasing image, I would need to increase the shutter speed by 3-stops – so I crank it up to 1/500th. I quickly take the picture again (that sun is going down)… and viola! I have my photo with the EV that gives me that amazing quality of light AND with the shallow DOF.
Shutter Speed is measured in fractions of a second and it determines how fast the shutter opens and closes, thereby controlling the key element in photography – light, glorious light; specifically the time-frame in which light registers on the image sensor (or on film). The Shutter Speed captures the world in split seconds, but it can also be slowed down to a few seconds (or remain open longer at the photographer’s discretion). This enables all sorts of possibilities in determining what is actually recorded to the image sensor.
Aperture is the opening in the lens that determines the amount of focused light that reaches the image sensor. It’s measured in f-stops. The beauty of the f/stop arithmetic is that regardless of a lens’ focal length, the f/stop measures the same amount of light; such that f/4 on a 50mm lets in the same amount of light as f/4 on a 120mm. The opening’s diameter may differ, but the amount of light is the same because the length of the lens is different.
So what is correct exposure? That’s mainly subjective, but we can agree that it is when the camera effectively reproduces a subject on the image sensor where the most uniform amount of picture information is visible in the highlights, midtones and shadows.
How do you determine the specific exposure you want? All dSLRs have an EV meter in the viewfinder that provides an EV on the subject that you are metering.
An effective way of ensuring a correct exposure is to employ Exposure Bracketing. This is a technique in which you’ll be taking at least 3 exposures – one at the designated exposure value (EV), one 1/3 of an f/stop above, and one at 1/3 of an f/stop below. On some features-laden cameras, you set the ISO, f-stop and shutter to acquire an exposure value (provided by the TTL meter), and press the shutter release. The camera will automatically shoot the upper and lower bracketed exposure. When you review the bracketed exposures, you’ll be able to see subtle, but key differences in the images – most specifically if there is any over- or underexposure. Professionals bracket all the time to make sure they get the best possible negative for later.
What exactly is under- and? It’s when there is excessive loss of image information within the highlights and shadows. There is typically no way of “finding” that lost image information with digital photography in particular (i.e. when the subject emits so much light that the image sensor is overwhelmed, it records that section of the image as zero; and the same thing is true when the subject emits so little light that image sensor believes there is nothing there). No matter how much tweaking you may try with a 3rd party image process, there’s no recorded information to be discovered. Incidentally, this is not always true with film and the photo-chemical process.
So how do you avoid under- or overexposing your pictures, before you master the art and craft of photography? You can use the Automatic Exposure Lock or AE Lock that’s available on most dSLRs. AE-Lock is a feature that, when you have the camera set to one of the automatic modes (i.e. Shutter-priority, Aperture-priority), it enables you to lock the EV and take continuous photos without have to resample the lighting in a given scene.
The wonderful thing about digital photography is that you can continue to experiment at no cost to you as you learn and master the three elements of the Exposure Triangle, going from semi-automatic to full manual.