The three common variables that contribute to exposure value (EV; how bright or dark your picture turns out) and their interactions with each other can be a little confusing to novice photographers. There are lots of mnemonic devices and analogies out there, but the cutest one I’ve heard is the monkey analogy, so I’ll go ahead and repeat that to you right now.
So your digital camera’s sensor, or your film camera’s film, is made up of thousands of little tiny monkeys sitting at little tiny desks. They each have a pen and a bunch of paper, and their job is to write down exactly what they see. As soon as the camera’s shutter opens, they write like mad and then when the film is developed, we see what they wrote. Remember, these are simply trained monkeys, so if you tell them to write forever, they’ll just keep writing forever and you’ll end up with a completely white picture.
So now that the stage is set, onto the variables. First up, shutter speed, since that’s the easiest to conceptualize. This is quite simply the amount of time that the shutter is open and the monkeys can see the scene they’re describing. If you keep the shutter open for a full second, the monkeys have plenty of time to write down what they see, and if it’s just 1/1000th of a second, each monkey will only be able to write down a limited amount of information.
Next, this thing called aperture. That’s how wide open the hole is that the monkeys see out of. The wider the hole, the more monkeys can see out, so the more information they will write down in a given amount of time. The confusing thing about aperture is that it’s measured in “f stops”, which is the reciprocal of the fraction of the lens that is open. So f/16 is a really small aperture, and f/1.8 is really big. Technically, f/1 is as wide open as it is scientifically possible to get (1/1), but there are some lenses that go even wider than that and are insanely expensive. [And not really worth it for the benefits they bring, IMHO.]
Finally, the hardest concept of all, ISO (which is numerically equal to ASA, and also some film manufacturers use DIN, which is a straight calculation from the ISO/ASA). This refers to the “sensitivity” of the film or the digital sensor, so therefore in our analogy it translates to how fast the monkeys can write. At a really low ISO, like 50 or 100, the monkeys are writing pretty slow, a few words a minute or something. At really high ISOs (3200, 6400, and some digital cameras go up to 12800 and even higher) the monkeys are writing at a blazing fast speed. BUT … just like in real life, the faster you write, the more mistakes you tend to make. And the same is true with our poor little monkeys. At ISO100, they’re pretty much describing the scene EXACTLY the way it is. At 6400 and beyond, one monkey here fucked up, a monkey over there fucked up, and you end up with splotches of the “wrong” color. That’s what we call “noise” or “grain” (well, it’s not grain, but when the color is wrong you really notice that it’s a grain).
So now that you know the concepts, how are they related? Obviously, for a given EV, if your shutter speed is really short, you’ll want a wide aperture to make up for that, OR a high ISO. Same way, if your aperture is really small, you’ll have to up the shutter speed or ISO to compensate. If you’re stuck shooting a really low ISO film, expect to use wide apertures and longish shutter speeds.
But how do you know exactly how much to compensate? Here’s the cool thing, each of the three variables is measured in “stops”, and it’s as easy as “one stop less of this, one stop more of that”. The stops themselves don’t always make logical sense, especially aperture, which is given in square root of twos. ISO and shutter speed are (approximate) multiples of two.
ISO: 100, 200, 400, 800, 1600, 3200, 6400, 12800…
shutter speed: 1sec, 1/2″, 1/4″, 1/8″, 1/15″, 1/30″, 1/60″, 1/125″, 1/500″…
aperture: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22…
So if your camera meters f/16 for a 1/4″ shot at ISO100, and you want to increase the shutter speed to 1/30″ (because you’re like me and can’t hold the camera steady that long), then that’s 3 stops faster, so either make the aperture 3 stops faster (f/5.6) or increase the ISO 3 stops (800). Or a combination of the two.
Ok, if you’ve seriously gotten this far, then I might as well tell you the tradeoffs to changing each of the values. A slow shutter speed obviously means that you’re more prone to get motion blur from moving subjects, and at the higher extremes, blurriness just from not holding the camera still (I’m very guilty of this). On the other hand, in order to freeze a runner or a dancer in his or her tracks, you’ll want to use a very fast shutter speed. ISO we’ve already gone over, the higher the ISO the more grainy and error-prone the photo. Aperture is the neatest variable in this sense, because changing it changes your depth of field (DoF). If the aperture is wide open (smaller f-number), then the amount of subject that’s in focus is a very small slice, and everything closer than that or farther away is out of focus. This gives a very pleasing blurriness that focuses (no pun intended) attention on the subject. [The background blurriness, btw, is referred to as “bokeh”. You get lots of photographer points if you use this word in general conversation.] On the other hand, a smaller aperture (larger f-number) makes a lot more of the scene in focus, so if you don’t have time to focus perfectly, it’s a lot more forgiving.
I think I’ve reached about the end of this discussion. But remember to feed and take care of the monkeys on your film/sensor, and they’ll write for you for a long time!