Aperture - the size of the opening that the light can get through. The picture shows (1) a large aperture and (2) a small one. There are several important practical things to note about this:
- The higher the f-number, the smaller the aperture.
- The size of the aperture determines how much light gets in to the 'film' (in digital cameras, a CCD array), so in low light a larger aperture might be helpful
- The size of the aperture also determines the way the image is focused. A small aperture will only let light 'rays' come through straight, resulting in focus over a relatively large range. This is what you will hear referred to as "depth of field" - a small aperture allows a larger depth of field. Conversely, a large aperture does not limit the rays as much and so only light coming from one plane of focus will be well focused. This image will be well focused, though. The diagram below (from http://www.bobatkins.com/) shows the light rays from an image on the left focusing on the sensor on the right. The black lines show something that would be in focus. As you can see, the smaller aperture brings objects at all three distances into a much tighter focus than does the larger aperture. Of course there are other factors that influence the depth of focus, such as distance away from the object, but if you start playing around with this you will get the idea and see what a difference it makes.
Shutter Speed - This is what it sounds like: how fast does the shutter stay open? When the shutter is open, light hits the film or CCD. For example, on a bright sunny day if you leave the shutter open for a long time your film (or CCD) will be "overexposed" and you will probably just get a white (or nearly white) picture. In bright lighting you want a faster shutter speed. Conversely, in dim light, you will need a slower shutter speed. Another factor to consider when setting the shutter speed is blurriness: if the object you are photographing (or your hand holding the camera) is moving faster than the shutter, it will move during the time it is open and you will get a blurry image. Sometimes this is desired, but it is important to be able to control this.
ISO - This stands for International Organization for Standardization, the people who came up with the system used for quantifying the speed of film - how sensitive is it to light: A higher number means higher sensitivity. The film sensitivity must be carefully paired with the shutter speed (and the aperture) A helpful rule of thumb is the "sunny 16 rule" which goes something like: 'On a sunny day set aperture to f/16 and shutter speed to the reciprocal of the ISO film speed'. For example, if the ISO is 200, you set the shutter speed to 1/200 of a second. If you had more sensitive film, say an ISO of 400, you would have to expose it for an even shorter time, say 1/400 second. One more thing to watch out for is this: if your ISO is high (i.e. very sensitive film or sensor) you will also get more noise, resulting in a more grainy image. Sometimes this is the only option in low light situations, and occasionally you may want this look, but in general you should avoid using high ISO to avoid this problem.
Finally, a few examples of what I've been talking about:
|An example of an overexposed image on a sunny day - a faster shutter speed, a lower ISO, or a higher f number would have helped fix this.|
|shutter speed too slow (trying to compensate for low light): my hand moved and made the picture blurry|
|Use of a large aperture to create selective focusing|
|Use of a larger aperture in portraiture.|
I hope this was helpful! There are lots of other great tutorials etc out there if you are more interested.