There are plenty of places on the internet to learn about the basics of photography, but for the sake of completeness and just because I want to explain it my own way, I’ll throw in my own two cents.
There are three main in camera processes that change the amount of light that creates the final image. The first is known by a few names, in digital cameras it is known as the ISO, and in film cameras it is known as film speed or ASA. Put simply this is either the film or the digital sensors’ sensitivity to light. When the ISO/film speed is low, such as 100, it is not very sensitive to light, and when it is high, such as 1600 it is very sensitive. Lower ISO/ film speeds provide images with less grain/pixilation than higher ISO/film speeds, so if with your given light conditions you can use a lower ISO/speed, you should.
A high ISO allowed by a fairly bright subject ensured a non-pixilated sky and snow. It is even more important in night shots as shown previously.
Next is aperture, which is measured in f-stops (a subject unto itself, but I’ll keep it simple here). There are blades in a camera lens that form a hole that can be open or closed to allow more or less light through to the film or sensor. The aperture f-stop number is how open the blades are. The lower the f-stop number the more open the blades are, the higher the more closed they are. A rather low/open f-stop is f1.4 and a rather high/closed f-stop is f16. Different lenses allow different ranges of f-stops. When an f-stop is very low the resulting image will receive more light, but will have a shallow depth of field, and when it is very high there will be low light transmission, but a great depth of field.
This simple shot was take with a low/open aperture causing only the one flower bud to be in focus.
Third to consider is shutter speed, simply how long the camera shutter is open to allow light to reach the film or sensor. Use fast shutter speeds to freeze action, and slow ones for dark and unmoving subjects. Also to consider is camera shake, or how still you can hold your camera. I generally do not use shutter speeds slower than 1/8 of a second without a tripod. Shutter speeds around 1/500 or 1/1000 or a second or even faster are best for freezing action such as in sports.
A fast shutter speed helped to freeze this hydroplane and the “rooster tail” behind it.
