Introduction

These issues are very simple, in fact, so simple that they confuse beginners who worry about them. In the old days before 1980 you had to worry about them, but today almost all cameras just set these themselves in Program mode so we rarely need to bother ourselves.

Rarely does one need to change apertures and shutter speeds away from what the camera chooses at the Program setting. If you do need to change these, most SLR cameras have a knob that shifts among the various equivalent combinations of aperture and shutter speed.

Explanations

Shutter Speed is how long the camera stays open to expose itself to the image. Most of the time it's just a short fraction of a second. The dimmer the light the longer the camera needs to collect it to make a good looking image. At night outdoors without a flash this can stretch into seconds or minutes.

If you want to change how motion is rendered you can use different speeds. 1/30 of a second looks about natural for running water. 1/500 of a second freezes everything. For sports use the fastest speed you can for most things unless you want deliberate blur. Several full seconds will make waves look like a big, foggy blur.

Aperture is how wide the lens' iris opens. The wider it opens the more light gets in. It's exactly the same thing as the iris of your eye which opens as the light gets darker. The wider it opens for the same subject the shorter the shutter speed will be to get the correct exposure. This is because the camera chooses shutter speed based on how much light gets into the camera. A brighter subject or wider aperture lets in more light.

Big apertures have smaller numbers, like f/4. Smaller apertures have bigger numbers like f/16. These are fractions, so 1/16 is smaller than 1/4. Big apertures like f/4 will tend to have just one thing in focus. A smaller aperture like f/16 will tend to have everything in focus. How much is in focus is called depth of field.

ISO or ASA is how sensitive your film or digital camera is to light. This depends on the the film, and can be changed with special development called pushing or pulling. Digital cameras can be set to almost any ISO. ISO is the same thing as ASA. We used ASA up through the 1980s and have called it ISO since then.

Use the lowest ISO that gives you the apertures and shutter speeds you need. Pump up the ISO up to get smaller apertures and faster shutter speeds. Unlike film, digital interchangeable-lens SLRs usually look great even at ISO 1,600. Don't be bashful: crank it up and it will look great.

For film or point-and-shoot digital cameras a normal ISO is 50 or 100. Faster ISOs are something like ISO 800 or 1,600. Digital SLRs are more sensitive to light than film or fixed-lens digital cameras, so their slowest ISO is often ISO 200.

The reason you want the slowest ISO is for lack of noise or graininess. The faster settings often are more grainy than the slower ones.

Summary

That's it. I have no idea why people make this so complicated. It has very little to do with photography. If you want to spend more time on this any book on basic photography covers it. I prefer the Ansel Adams' book "The Camera" here.