To make a properly exposed image, the photographer (you!) have to control how much light falls on the light sensitive surface (sensor or film). The longer this light falls on the sensor, the more light there is.
So, let’s use a handy metaphor: The garden hose. Turn it on high, and you have a wide aperture — lots of water. Close it down a good deal and you have a narrow one — just a trickle. Continuing the metaphor, if you water for 1 minute with the hose wide open, your plant might get enough water. But with the hose closed most of the way down, it could take 10 minutes. So you see that to get the “right” amount of water, you can either water really hard for a short while or drip the water in for a longer time.
Bringing this back to cameras and lenses, you can use a small aperture (larger numbers like f/16) for a longer time (say, 1/30 sec.) or a large aperture (say, f/4) for a shorter duration (say 1/500 sec.). And these would deliver the same amount of light to the sensor.
ISO
We’ll get around to why you might care about all this in a minute. First, what does ISO have to do with it all? The ISO is sort of like how thirsty the plant is. So, an ISO of 100 would be a pretty thirsty plant and ISO 800 would be, shall we say, drought-resistant. The point is, the larger the ISO, the less light needs to fall on the sensor to make a properly exposed image.
But wait, you say, my meter figures all this junk out for me. True enough, grasshopper, but do you really understand the decisions your meter is making and control its recommendations? Let’s tie the whole thing together.
Big ISO numbers work well for lower light situations or when you need to stop motion. However, sensors operating at higher ISOs tend to produce what’s called digital noise. The lower the ISO you can use, the higher your image quality will be. Your camera manufacturer will not engineer you out of this responsibility.
Shutter Speeds
Fast shutter speeds (higher denominators like 1/1000 second) will stop motion, but they allow less light in than longer shutter speeds. Longer shutter speeds, especially on a tripod, will allow you to achieve artistic effects like motion blurs.
Aperture
Aperture is the real kicker. The wider the aperture, the “shallower” the depth of field. What, you ask, is depth of field? It’s the part of the image that is tack-sharp in focus. The smaller the aperture, the more of your image will be in focus. Sometimes you need tack sharpness from edge to edge, so you will tend toward f/16 or f/22. Other times, you are deliberately selecting a particular image component to be sharp and stand out from the more diffuse background. For these images, use a wider aperture like f/5.6. And, of course, as you vary your aperture, you will have to vary your shutter speed in the inverse to keep that magical “correct” exposure. Depth of field varies from lens to lens, and by the distance between the camera and the subject. Learn how you lenses behave.
The images below are of the same set of batteries in the same light, shot at (from left to right) f/4, f/8, and f/22, respectively. Focus was on the second battery, and you can see that at f/4, the second battery is pretty much all that is in focus, as the aperture gets smaller, the surrounding batteries come more into focus.
So… how to apply this to the modern DSLR? Use either the shutter priority mode (often denoted as TV) to pin your shutter speed at a given point. I know I can hand-hold a moderate lens like 80mm or shorter at a 60th of a second, so if I’m working fast, I’ll put the camera on TV at 1/60 second and just watch to make sure that the aperture doesn’t get so wide that my subject will be out of focus (too shallow depth of field). However, if I’m shooting a landscape on a tripod, I might set the camera on AV at f/22 and let the camera’s meter figure out all the rest of the good stuff. By the way, I use ISO 100 unless I simply cannot get sufficient depth of field without increasing.
