Aperture, Shutter speed, and ISO
The mechanics of photography involve getting light through a lens and onto a medium that will convert it to colors. Before the digital era, the light was directed to a film that reacted differently to different colors and strengths of light. In digital photography, the film is replaced by a sensor that saves the color and intensity information as a computer file.
The information that follows will skip past the (very, very important) contribution of the lenses and focus on the interaction between the aperture (a part of the lens assembly), shutter speed (part of the camera) and ISO settings (also part of the camera or inherent to the film for non-digital applications).
Light is PhotonsIn the discussions that follow, light is going to be discussed in terms of its particle nature. Though light (and other electromagnetic energies) behaves both as a wave and as a stream of particles, for the sake of understanding how cameras work, it is sufficient to just consider light as a stream of particles, not unlike a stream of droplets that travel from the object through the lens to the sensor (film).
Each photon carries (for sake of understanding how cameras work) color information and brightness information. These photons strike the sensor (film), and the color and brightness is captured.
While this is an oversimplification of the physics, it is sufficient for what follows.
ISO SettingISO settings are quite simply how sensitive the sensor or film is to incoming light. The more sensitive the setting, the less light brightness that is needed to record the information.
The higher the ISO setting, the more sensitive the sensor (film). An ISO setting of 400 will respond to light half as strong as an ISO setting of 200.
ISO is inherent to film, but is adjustable on digital cameras. When you buy film, you do so with an understanding that the ISO will influence the range of possible uses you will have for that roll. Likewise, when the ISO is set on a digital camera, the possible combinations that other settings can have are influenced.
It might seem sensible to just set the ISO to most sensitive and go with that. There are numerous reasons that this is not a good practice.
Foremost is the fact that in both film and digital, higher sensitivity comes with a loss in quality. In film, this sometimes results in larger "grain," For digital photography, higher ISO settings can lead to loss in color integrity and result in color "noise" within an image.
The better practice is to use the LOWEST ISO setting that will let you take the pictures you need to take.
This will make more sense after the next two factors are explored.
Shutter SpeedShutter Speed is exactly what it sounds like. Cameras operate to let light reach the sensor or film by quickly opening a shutter, then closing it again. Shutter Speed is how quickly that happens. (On phone cameras, the "shutter" is an electronic timer that turns on and off the sensor.)
Shutter speeds range from some number of seconds to some tiny fraction of a second. Bigger numbers represent slower speeds.
For instance, a shutter speed of 1/60 sec. is slower than one of 1/2000 sec. A speed of 2 sec is slower than 1/60 sec.
The information display on some cameras might leave off the numerator of the fraction, however. Some will simply show 60, 200, 1600, or 2000 instead of the full fractions 1/60, 1/200, 1/1600 or 1/2000.
The shorter the amount of time the shutter is open, the less light that is allowed into the camera and onto the sensor (film). As long as the shutter is open, light enters, so if an object is moving or if the camera is shaking, slower shutter speeds result in blurred images (which is different from out of focus images).
To "freeze" motion, it is necessary to use a fast shutter speed. Sports images rarely are clear below 1/500 sec shutter speed, and settings like 1/2000 are common. However, it is sometimes desirable to intentionally include blur, and this is accomplished with slow shutter speeds.
All considered, (when in the 1/x form) the smaller the number for shutter speed, the less light that passes through the lens. A shutter speed of 1/2000 lets in less light than a speed of 1/200.
ApertureOne last setting related to the amount of light entering the camera is aperture. The actual aperture mechanism is part of the "lens." While it is accurate to say that a lens is the glass through which the light passes, it is common to call the whole device that includes the glass lenses as well as the focusing mechanism and aperture assembly the lens. In this use of "lens," the aperture is the part that determines how much light will pass through.
Aperture settings are called f-stops, and (as a working definition) represent the denominator of a fraction of the light allowed through. It actually has to do with ratios of the lens opening and other technical measures.
It is conceptually okay to think of it the fraction of the light entering the opening of the lens that makes it out the back and into the camera. A setting of 4 means 1/4 of the light goes through. A setting of 1.4 means 1/1.4 of the light goes through.
It is accurate to say that a setting of 4 lets in twice as much light as a setting of 8 (1/4 vs 1/8).
The effect of aperture is not limited to the amount of light that passes, but it also has an effect on how much of the image is in focus (depth of field). This topic will be covered separately, but here is it worth saying that depth of field goes up as the f setting goes up. So, an f-stop setting of 22 has more depth of field than one of 2.8.
With regard to the amount of light, however, the smaller the f-stop setting, the more light that passes through the lens.
All TogetherThus, the three settings work in combination to determine how an image will appear. Changing ISO "on the fly" was impossible back in the days of film, since the sensitivity was a characteristic of the film and, once loaded, was established for the duration of the role. As such, even in the digital world, changing ISO "on the fly" is not typical in the use of DSLRs. However, phone cameras lacking an aperture have only shutter speed and ISO to manipulate, and automatically change them to match the available light.
Where all three can be set, it is normal to set the ISO, then adjust the aperture and shutter speed as conditions warrant.
Thus, a little forethought goes a long way. If you are shooting sports, you want fast shutters speeds (less light) and deep depth of field (large aperture = less light), so you want to set ISO as high as you can, remembering that, at some point, quality will begin to degrade. Once set, you can adjust your shutter speed to something suitable for sports (like 1/1000 or 1/2000 or 1/500) and your camera can then pick the aperture that matches.
On the other hand, if you are shooting architecture or still lifes, motion is not a consideration, but depth of field and quality of the image are. Thus, you would pick a high-quality ISO, put your camera on a tripod (so it won't shake and blur), then set the aperture to the highest setting and use whatever shutter speed is needed for a good exposure.