Shutter speed & Aperture settings

If your interest in photography has grown beyond simply taking happy snaps, then you may be interested in learning a little more about how your camera operates. Thus, allowing more control over the pictures you create.
Before you get started, however, it is important that you have a good grasp and understanding of the wording of your camera's instruction manual, because, as they say, "When all else fails, read the instructions!"
Then, you need to understand the three main factors needed to bring about a correct exposure (photo); the shutter speed, the aperture (diaphragm) setting and the correct ISO rating (International Standards Organisation, formerly ASA, American Standards Association), or the digital sensor's (film's) sensitivity to light.
To have a correct exposure all three of these factors must work in equilibrium with each other.
Nowadays, most modern cameras will have user options. Whereby, they can be operated fully automatically, semi- automatically or even fully manually. With different mode settings such as Close-up, Landscape, Portrait, Sport, Auto or (P) Program and of course with the added option of auto or manual Focus or auto or manual Flash.
If for instance, you totally disregard any of these mode settings and you operate the camera fully automatically, as a surprising amount of users do, then you will be using it simply as a "point-and-shoot" camera. Whereby, the camera automatically determines, what it considers to be the right exposure, based on the readings it receives from its in-built light sensors for the scene being recorded. The results of which are generally fairly good under most normal lighting conditions.
Being a little more adventurous, you may wish to use the semi-manual settings known as Shutter Priority or Aperture Priority.
In Shutter Priority, you set the desired shutter speed and the camera automatically sets the correct aperture for that setting. Or vice-versa, you set the aperture you want and the camera sets the correct shutter speed. Hence, you have gained some control over your picture taking.
However, in order for you to use your camera fully manually, you will need a sound knowledge and understanding of these settings and what they mean.
So, we will start by looking at shutter speed more closely and for the benefit of those who are completely in the dark, so to speak, we will go right back to the very beginning.

The shutter speed is the ratio of the speed at which light is permitted to pass through your camera's lens to the digital sensor (or film). In other words, when you depress the shutter button you open the shutter, the speed of which, relates to the duration of time it takes before it closes again.

The range of times of the camera's shutter speed is measured in seconds or fractions of a second. For example, from a fast 1/2000 sec to a slow 30 sec. That is 1/2000 sec, 1/1000 sec, 1/500 sec, 1/250 sec, 1/125 sec, 1/90 sec, 1/60 sec, 1/30 sec, 1/15 sec and so on. The "B", or Bulb setting, sometimes used for long exposures, such as photographing star trails, etc., will allow your camera's shutter to be open for hours at a time. It all depends on your camera's capabilities.
Let's say we have a bright sunny day and we are shooting a broad landscape shot. ISO rating is 400. On this sort of day, we might use a Shutter Priority setting of 1/500 sec and the camera might set the aperture at f16. A fairly standard exposure and all should be in focus from foreground to background.
However, on a slightly overcast or dark day, to get a similar landscape shot you might have to set the shutter speed at 1/30 sec because of the lack of light. Also at this low speed, you would surely need some sort of camera support.
I for one, whenever possible, use a good sturdy tripod. At least to help against camera shake. Not only at low speeds but also for larger lenses. Basically, the rule of thumb is if you are using a lens with a focal length of around 100 mm, you can safely hand-hold your camera at speeds at or above 1/125 sec or if your lens is around 200 mm, 1/350 sec, 300 mm, 1/500 sec. And the larger the lens, the more support required.
Now, if say you wanted to photograph your daughter doing a back flip into the backyard pool and you wanted to freeze the motion and capture her in mid-air, anything above 1/125 sec would probably do that for you. However, you would do better on the safe side, to choose a speed as high as the light conditions will allow.
On the other hand, you may want to shoot a waterfall and create that soft, moody effect of movement in the water, just like the pros. For that, you will need the opposite. A very slow speed from say 1/4 sec and possibly longer. Again, it all depends on the light conditions and your ISO rating. If you can't come to a compromisingly slow enough speed for those conditions, there are other ways and means of getting a correct exposure by the use of Neutral Density filters (NDs) or "bracketing", etc.

For an explanation of "bracketing", see "Altering The Camera's Exposure Settings".

The Shutter Speed, as mentioned, controls the time it takes for the permitted amount of light to pass through the camera's lens and onto the film, or digital sensor, whilst the aperture controls the amount of light coming through the lens.
The shutter is situated inside the camera, just ahead of the digital sensor (or film), whilst the aperture is actually in the lens itself. It is an iris type mechanism also known as the diaphragm and is calibrated mathematically in steps called "f-stops". They are calculated by dividing the lens focal length by the effective aperture. That is the beam of light passing through the lens. I'll try not to over-blind you with too much science, yet!
F-stop numbers start at 1, then increase logarithmically. For example, f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32. 
It may also be worth mentioning here, that although f1 is the smallest number, it is in fact, the largest aperture opening. For instance, f32, in this example, is the largest number but is the smallest opening.

As with shutter speeds, each step or stop of "increased" aperture doubles or allows twice as much light through than that of the previous stop. Or, alternatively, each "decreasing" stop permits half as much as the previous stop. That being the case, when you release or press the shutter, you are allowing light to pass through the aperture, but you need to have both an aperture and shutter, because the "Depth of Field" in an image is governed by the aperture as well as contributing to the bringing about of a correct exposure. And for the benefit of those who are scratching their heads saying, "What's depth of field?" The depth of field relates to that area of any image which is in clear focus, as opposed to the remainder of the image that is not.
In other words, if you look at a portrait shot of say, a flower head, the background area surrounding the subject, we assume, will be out of focus. Whereas the main subject, the flower head, should be sharp and clear.
Shutter and aperture working together:
If your ISO rating is at 400 and your camera's in-built light meter shows a reading of 1/250 sec, whilst the aperture setting is at f16. You can bring about the same exposure by increasing, (or slowing) the shutter speed by one stop and by opening up (or enlarging) the aperture by one stop, or by speeding up the shutter speed by one stop and opening up the aperture by a corresponding stop. So each time you increase, (or slow down) the shutter speed, you will need to open up (enlarge) the aperture.
What is the reason for all this, you ask? Well, if you want to use your camera's manual settings to better your photography, you need to know what they are all about and how they work.
For instance, it's a sunny day and you are shooting a landscape scene and you want everything in the image to be sharp and clear from the immediate foreground to the distant background.
Using aperture priority, you will need to set your aperture to f16 or smaller, but f16 is a good starting point. Because it is a sunny day, you might find your shutter speed is set at 1/250 sec. We'll assume it is. If you turn the dial from f16 to f22 the shutter speed should adjust accordingly to about 1/125 sec to arrive at the same exposure.
If taking a head and shoulders shot and you only wish to have your sitter in clear focus and the background out of focus, as you would, you might then set your aperture at around f2.8, thereby turning the surrounding area out of focus to get the desired effect. So that's the correlation between shutter and aperture and when you set either aperture or shutter priority, for everything to be working happily and in equilibrium, either will determine the setting of the chosen ISO rating or film speed to set the correct exposure. A bit of a love triangle you might say.