A correct exposure in photography terms is a simple combination of the three important factors: aperture, shutter speed, and ISO. Since the early days of photography, these three factors have always been at the heart of every exposure, whether that exposure was correct or not. The same stands today, nothing really changed - even in the digital world. The three elements are referred to as the exposure or simply the photographic triangle.
Before reading on, I have always liked the following analogy. Well imagine this; you are standing behind the window curtains in your sitting room looking at the street. The opening in your curtains is your Aperture and the speed of which you open and close the curtain is the Shutter Speed.
Whether you turn on the lens itself (older versions), push a button, turn a wheel, or rotate a ring on the lens, you'll see a series of numbers coming up in the viewfinder or on the LCD screen. The numbers you see may be 4, 5.6, 8, 11, 16, and maybe 22. Each one of those numbers correspond to a specific opening in your lens, and these openings are called f-stops. So in the photography world the 4 is called f/4, and the 5.6 is f/5.6, and so on.
The primary function of these openings in the lens is to control the amount of light coming in and hitting the sensor of your camera (film negative in legacy terms).
The smaller the f-stop number the larger the lens opening; the larger the f-stop number, the smaller the lens opening.
For the ones who are technical minded out there, the f-stop is a fraction that represent the diameter of the aperture. The f stands for the "focal length" of the lens, the slash ( / ) means divided by, and the number represents the stop in use. For example, if you were shooting with a 50 mm lens set at an aperture of f/1.4, the diameter of the actual lens opening would be 35.7 (diameter of the lens opening). Each time you go down from one aperture opening to the next, or stop down, such as from f/4 to f/5.6, the volume of the light entering the lens is cut by half. Likewise, if you go up from f/11 to f/8, the amount of light entering the lens doubles. Each halving or doubling of light is referred to as full stop.
Most cameras today offer not only full stops but also the ability to set the aperture to one-third stops i.e. f/4, f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, and so on.
Depending on the make and model, your camera may offer shutter speed from a blazing fast 1/8000 seconds all the way down to 30 seconds. The shutter speed controls the amount of time that the amount of light coming through the lens is allowed to stay on the digital sensor or film. The same halving and doubling principle that applied to aperture also applies to shutter speed.
Let me explain further here because it might sound tricky. Set the speed control on your camera to 500. This number denotes a fraction representing 1/500 sec. Now change from 500 to 250; again this represents 1/250 sec. From 1/250 sec. you go to 1/125, 1/60, 1/30, 1/15, and so on. Whether you change from 1/30 to 1/60 (decreasing the time the light stays on the digital media or film) or from 1/60 sec. to 1/30 sec. (increasing the time), you've shifted a full stop. Again this is important to note, since many cameras today also offer the ability to set the shutter speed to one-third stops; 1/500, 1/400, 1/320, 1/250 sec. and so on.
The last piece of the puzzle is ISO. Whether you still shoot film or you embarked into the digital SLR world, your choice of ISO has a direct impact on the combination of apertures and shutter speeds you can use. But to better understand the effect of ISO on exposure, think of ISO as a worker bee.
If my camera is set for ISO 100, I have in effect 100 worker bees; and if your camera is set at ISO 200, you have 200 worker bee. The job of these worker bees is to gather the light that comes through the lens and make the image. If both of us set our lenses at the same of f/5.6 - meaning that the same volume of light will be coming through our lenses. You set your ISO to 200 and I set mine to 100, who will record the image the quickest, you or me? You will, since you have twice as many worker bees at ISO 200 that I do at ISO 100.
To understand the concept of the triangle and the impact of one setting over the other two in the triangle, let's do this small exercise. I want you to get your camera as well as a pen and paper. Choose Aperture Priority or Manual mode. Set ISO to 200 and set aperture opening to f/8, and with the camera pointed at something that's well illuminated, adjust your shutter speed until a correct exposure is indicated (that needle points to zero in your viewfinder). Write down the shutter speed provided by your camera light meter. Now change ISO to 400 and point again at the same subject. Observe and write down the provided shutter speed. You will notice eventually that your camera and in order to provide the right exposure ended up with different shutter speeds for different ISO number. Finally, change the ISO to 800 and repeat the steps above. What have you noticed?
When you increase ISO, your shutter speed becomes faster from 1/160 to 1/320 for example.In other words, when you increase the number of worker bees (ISO) from 100 to 200, you cut the time necessary to get the job done in half. Going from 1/125 to 1/250 sec. is half as long an exposure time.
Now if you enjoyed the exercise above, you can do the same just as easily by leaving the shutter speed constant - for instance at 1/125 sec. - and adjust the aperture until a correct exposure is indicated in the viewfinder.
The Exposure Triangle in Practice
Now that we have a basic understanding of the three elements of exposure let's examine how we might use them and see the interaction between them in real life. Let's say you were hired to shoot a car race. First you want to stop the action. It's a bright sunny day so you set your ISO to 100. You then want to stop the action of the leading car so you choose 1/1000 sec. shutter speed - but with that shutter speed according to our camera meter that would give us an aperture of 5.6.
The problem is this might give us a shallow depth of field and there are other cars you want to include within focus. So how could you fix this? You can't change our shutter speed, so you turn to the other part of the triangle; ISO.
You moved ISO up two stops to 400, you can then make out aperture two stops smaller and get the depth of field you need plus the shutter speed you need to freeze the action.
Afterwards, while you are there you spoted a beautiful classic car in the parking lot. You want to isolate it from the ugly background so you decide that you now need a shallow depth of field (DOF). So you open your lens up to f4 on your 200mm lens.
This gives you great separation and DOF but that drives your shutter speed up to 1/6000 sec. This would be OK - you are hand-holding your camera, not stopping any action - but you want this to be a great shot worthy of a very large print. So you lower our ISO two steps to ISO 100. That brings your shutter speed down two stops to a still fast 1/1600 but because you lowered our ISO you will have much less noise in our image to be printed large.