We all know that photography is about capturing light, right? Well, capturing reflected light, to be more precise. Capturing just the right amount of light is what you or your camera has to do to get the right "exposure". In the case of a film camera it is a matter of how much light the film is exposed to and the same holds true in digital photography, except that it's the sensor that is being exposed to light, not film.
There are three aspects of your camera that control the exposure process: your sensor (or film) sensitivity, the aperture setting and the shutter speed.
The sensitivity of the film or sensor to light (also called film speed) is rated in a number of different ways but the most common is the ISO rating. When I was learning photography we used ASA. In both cases, the higher the number, the more sensitive the film or sensor is to light. On the surface, one would think “great, get the highest sensitivity possible”. But, we all know how things work, “there is no free lunch”. It turns out that the higher the film speed, the grainier (film) or noisier (the digital equivalent of grain) the image. Although this can be a desirable effect in some cases, it generally results in a loss of detail.
The second problem with film speed is a little more insidious: it changes the range of settings needed for the other exposure components, shutter speed and aperture. If your camera has a top shutter speed of 1/2000 of a second and the smallest aperture is f/22 then a more sensitive film speed, in bright sunlight, might require 1/2500 of a second at f/32 resulting in an over exposure. If you have ND (neutral density) filters available, you can work around it – but that’s another subject. More importantly, what if you want that very slow shutter speed to have a smooth water effect on a waterfall or the wide aperture to make the background out of focus? You no longer have those options if your film speed is too high.
Conversely, a film speed that is too low will force you to use a wide aperture setting and risk losing focus in the foreground/background of an image or it may force you to use a slower shutter speed and risk a blurred image from camera or subject motion.
In the digital world, your camera can usually change the film speed for you, if needed, based upon your aperture and shutter speed selections. You simply select AUTO or (A) as the film speed. But, back in the day (I love that expression), setting film speed was usually a manual affair. In fact, some cameras even provided film tag holders to keep the box label on the camera to remind you of what film and film speed you had loaded in the camera. As film cameras became more advanced some could even read the code on the film canister that was loaded and automatically set the film speed for you.
There was a simple rule that we used to “guestimate” exposure: the inverse of the film speed should be used as the shutter speed with an aperture of f/16 in bright daylight conditions. So, if you were using ASA 100 then, on a sunny day, your aperture would be f/16 and the shutter speed of, approximately, 1/100th of a second. This was pretty handy since, knowing that shutter speed and aperture are inversely proportional to each other I could get the same exposure by setting 1/200th of a second at f/11.
This probably all seems a little useless at this point – after all, who really cares as long as my camera takes good pictures? If that's what you are thinking at this moment, then you need to ask yourself one question: “Am I the photographer or is it just the camera?”. If you are the photographer then you need know how to be in charge – even if you let the camera figure it out most of the time.
So, now that we know a little bit about film speed let’s talk about shutter speed. The shutter speed represents the amount of time that the film or sensor is exposed to light. Shutter speeds usually come in standard increments as well: 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2500, 4000 but these will vary from manufacturer to manufacturer. Of course, each increment represents the inverse in terms of time (e.g. 125 is 1/125th of a second). Again, in digital cameras, things are a little different because you can usually select from more increments.
On the other end of the scale you have shutter speeds that are measured in full seconds. Usually from 1 to 30 seconds; these will display with a quote mark next to them so that you are aware that 4" is really 4 seconds and not 1/4 of a second.
Lastly, there is the “B” setting. The “B” stands for “bulb” and harkens back to the days of rubber compression bulbs where you would open the shutter by compressing the air in the bulb and close the shutter by releasing the bulb. And that is, effectively, how it still works. Pressing the shutter release will open the shutter until you release the shutter button, at which time the shutter will close. This can be used for many interesting effects which are, unfortunately, beyond the scope of this article.
So why is shutter speed so important? Because it controls whether or not an action will appear frozen or blurred in the image. Depending on what effect you want, you may choose to have a slower or faster shutter speed. If you are shooting a sports event, you probably want to freeze the action and capture a sharp image of that moment. On the other hand, you may want to have a slower shutter speed to emphasize that objects and people are in motion.
The last component to exposure is aperture, sometimes referred to as an “f/stop”. The aperture refers to the size of the lens opening with respect to the focal length of the lens. To illustrate, think of a paper towel tube. Hold it up to your eye and you can see the opening at the other end. Even though the opening is about an inch and a half in diameter, it looks small because it is about a foot away from your eye. Now, cut the tube in half, and hold it up again. The opening looks much bigger even though it is still the same one and a half inches. There is more light but nothing has changed except for the length of the tube.
The size of the opening represents the maximum aperture so, if we want less light, we need a way to make the opening smaller, in effect, reducing the aperture. This is done with an “iris”, a circular ring with multiple leaves designed to open and close together forming a series of smaller openings and allowing less light to reach the film or sensor.
Depending on the lens, aperture settings generally range from 1.2 to 32 with larger numbers representing the smaller opening. This is because, once again, we are dealing with a ratio of the diameter of the lens opening to the focal length of the. In the case of the 1.2 aperture, the size of the opening is roughly 80% of the focal length of the lens. For example, a 50mm lens at f/1.2 would have a 40mm maximum opening. Clearly, as the focal length increases, to allow the same amount of illumination we need a larger opening and thus, a larger piece of glass on the end of the lens. This can become prohibitively heavy and very expensive which is why you see smaller maximum apertures on lenses with longer focal lengths.
The math can be affected by some of the optics used but, in general terms, the size of the aperture represents the same amount of light from lens to lens. That is a 200mm lens at f/4 lets in the same amount of light as a 50mm lens at f/4 even though the physical diameters of the lens openings will be different.
Like shutter speed, aperture is also important for a very specific reason: it drastically affects what is known as “depth of field”. This is the range of distance from your camera to infinity that will appear in focus when the image is captured. It is very effective for giving depth and perspective to an image by forcing the background (or foreground) to be out of focus – the viewer’s eye will naturally go to the sharp detail of the in focus part of the image.
In an effort to simplify this measurement of light, the EV (Exposure Value) was born. An EV unit of 1 represents a full stop so, in effect, changing from f/16 to f/11 will change by an EV of +1 because we have increased the amount of light by 1 full stop. Similarly, changing the shutter speed from 125 to 250 will have an EV impact of -1 because we have decreased the amount of exposure time by one full stop.
Why did I put you through all of this? Because your camera, ultimately, is just a tool. While it's true that photography is part luck, it is also true that "knowledge is power" and, with the right knowledge, you can make your own "luck".
If you are taking "snapshots" then you can probably get by with setting everything on automatic (sometimes called "green box" mode) and letting your camera do the work. On the other hand, if the artist in you is struggling to be set free or you just find yourself in a "difficult" lighting situation, then knowing what your camera is trying to do and how will let you switch over to "Manual" mode so that you can coax it in the right direction and get the effect you really want by making light work for you.