Shutter Speed
Fast shutter speeds freeze movement, while slow shutter speeds portray movement. A shutter speed of 1/250th of a second will freeze most general things, ideal for family portraits where families rarely stay still for very long, if at all.
So, what exactly does 1/250th mean? Basically it means the shutter exposes an image for 250th of a second, hence 1/250. An exposure at half a second, which is extremely slow, is 1/2 is a half a second exposure, useful for capturing movement in subjects.
A shutter speed of 1/50th of a second requires the subject be rather still and not move very much at all, while slower shutter speeds like 1/15th and 1 second require the subject to be completey static or any movement will show up in the final images, 1 second is a long exposure and most selfies are not captured at such slow speeds, most are about 1/100th of a second or higher.
One thing to consider when taking images of people, specially ourselves when doing self portraits, is that we will inevitably move during the shoot, thus we may get blurry images of ourselves without realizing, so to make sure this does not happen, use a minimum shutter speed of 1/100th or higher.
Aperture
Or f/stop, f/number etc, basically a unit of light that is allowed through a lens. A typical lens will have an maximum and minimum f/number, like f/2.8 to f/22 or more.
A small number like f/1.2 lets in far more light than a larger f/22, and with this comes its own bag of tricks, you see, f/2.8 creates a shallow plane of focus, this means that certain parts will be in sharp focus, while the rest will be all blurry and out of focus, whereas f/22 creates more sharpness all round, from foreground to background.
Lets look at that last bit again. A small aperture like f/1.2 or f/2.4 etc, gives a very a shallow depth of field, to give it its technical term, brilliant for portraits, as this really focuses on the eyes for close up shots, and throwing everything nicely out of focus, making the background appear all creamy and silky, this is the bokeh at work.
The Sunny 16 Rule
So now you know about the three single most important camera controls, lets put them all together to try to make sense of it all.
Imagine a stunning bright sunny day, the sun is gorgeous and the sky is blue and the birds are singing sweetly in the trees, a real proper summers day, and you feel like taking some photos.
You want to take photos manually, so set your camera to M mode, this is Manual mode, now, set your ISO to 100. Now lets set our shutter speed to 1/100th too, easy enough so far, ISO 100 and 1/100th of a second exposure, now, guess what f/stop you will use? The clue is there already...
Set the aperture to f/16 and with the above settings, you should get a perfectly exposed image, now, lets use ISO 400, along with that f/16 again, now, guess what shutter speed we will need? Thats right, 1/400th of a second.
It is that easy, and if it is cloudy that day, use an aperture, or f/number of f/3.5 or there abouts. The same is true for very very bright conditions where the sun is reflecting of sand and water, use an f/number of f/22 or there abouts, or maybe a faster shutter speed instead, 1/250th of a second perhaps, or even turn down that ISO we mentioned, to ISO 100 or ISO 50.
Now Try It Yourself
Why not? Digital means we can make deliberate mistakes, and learn from them, it is not like in the old days when it was a waste of 35mm film if mistakes were made, with digital, all you have to do is delete the offending images and no one need know you have been practising all day long to get a correct exposure.
Get out there and try it, it is the only way you will learn.
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