Aperture, ISO, and shutter speed are the three  factors which determine exposure or the amount of light which enters your camera.

Photography is all about light, and without an ample amount of light entering your camera, you have nothing but a dark worthless picture.

Learning how to determine the right combination of Aperture, ISO, and shutter speed settings can be a tedious task, but understanding what these settings do will make it much easier.

Shutter Speed

Shutter speed is simply how long your camera's shutter stays open when you take a picture. These speeds can range from thousandths of a second to 30 or more seconds. The longer your shutter stays open, the more light your camera lets in. A shutter speed of 1 second lets in 4 times the light of a shutter speed of 1/4 second. The shutter speed can also determine the clarity of a picture. A longer shutter speed will blur the shot, and create trails from even the slightest bit of movement in your picture, whereas a shorter shutter speed will 'freeze' any action and create a sharp picture in which time appears to be stopped. For an example, take a picture of a constant drip of water using both a fast and a slow shutter speed. The shot taken with the slow shutter speed will create a soft blur of water, whereas the shot taken with the faster shutter speed will catch every individual drop in mid-air.

A fast shutter speed can also help eliminate blur due to camera shake when not using a tripod.

Aperture (f/stop)

Aperture (also known as f/stop) is how large the iris (or eye) of your lens opens up. A larger aperture means a larger opening in your lens for light to pass through. When referring to aperture, a smaller number is always a larger opening. For example, an aperture of f/5.6 is a larger opening, and therefore lets more light in, than an aperture of f/11. Each unit of measurement in aperture is called a 'stop' one stop up would be making the lens opening larger, and one stop down would be making it smaller. A single stop down of aperture lets half the light in that the previous stop did.

Comparison of the diameter of different f/stops.

Adjusting aperture also changes your Depth of Field. Depth of field is how much of the area, measuring away from your camera, is in focus. If you are tightly focused on an object which is relatively flat, you have short depth of field. If you are focused on a group of people standing at varying distances, you would need a long (or large) depth of field. Basically, a short depth of field (which would be caused by a large aperture) will be clearly focused on a relatively shallow area. The item you focus on may be sharp and clear, but any objects in the foreground or background may be blurred. A smaller aperture would create a larger depth of field, and bring all objects into perfect focus.

Depth of Field Comparison

Film Speed (ISO)

Film speed (or ISO) is a measurement of how sensitive your camera's sensor (or in the case of a film camera, your camera's film) is to light. The larger the ISO (higher number), the more sensitive it is to light. The smaller the ISO (smaller number), the less sensitive it is to light. Each step up in ISO doubles the amount of light sensitivity (ISO 400 is 2x as sensitive to light as ISO 200). Using a higher ISO, you can sometimes get shots in low light that would have required a longer shutter speed or a larger aperture if you were using a lower ISO. However, this does not come without its setbacks. The higher the ISO is set, the grainier your picture will appear. At higher ISOs, you will notice some extremely substantial grain. ISO noise is much less noticable in DSLR and other large sensor cameras than it is in point and shoot cameras.

ISO Comparison

Below are some general ISO guidelines that you can follow.

• 100 ISO - Less grainy, good for shots with plenty of light.

• 200 ISO - Still not very grainy, don't need as much light as ISO 100. Grain will be more noticable when printed in larger formats.

• 400 ISO - Mainly used for shooting lower-light outdoors or indoors without a flash, but with an ample amount of light. Slightly more grainy than ISO 200, but not by much.

• 800 ISO - Very grainy, but will give 8x the light sensitivity of ISO 100.

[Photography 101]