• Aperature - Volume of Light
• Shutter Speed - Exposure Length
• ISO - Sensor Sensitivity

If you look through a lens that is not mounted on your camera, you will probably see the diaphragm, It's a series of metal leafs that change angles as you change the aperature settings causing the hole through which light travels to your camera's sensor to become larger or smaller. f22 represents a smaller hole while f3.5 represents a larger hole. Lenses vary in their largest and smallest aperatures. Genearally, lenses cost more as the maximum aperature (smaller number) increases. Fast lenses, as those with large maximum aperatures are called, are desirable since they allow for shooting in lower light levels. It's inportant to remember though that as you open the aperature in your lens, you decrease the field of focus. Field of focus is the range behind and in front of your subject that is in focus. For landscapes you want a deep field of focus whereas for portraits a very small depth of field is better.

The number that you see on the aperature ring of your lens (some DSLR lense won't have the aperature ring as the aperature is set through the cameara) represents a ratio. The ratio is derived by dividing the focal length or distance from the rear lens element to the sensor (or film) of the camera (f) by the diameter of the aperature (D). number = f/D.

It's probably quite easy to understand now how the aperature affects the amount of light, and thus the exposure of your image. A larger aperature means more light is coming through the lens to the camera

The shutter in your camera is normally in front of the sensor blocking any light from reaching it. When you press the shutter button on your camera to take a photo, the shutter moves out of the way and allows light to reach the sensor creating an image that gets stored in your camera. We've seen that the aperature controls the volume of light coming in, but that is only part of the story. It's not just the volume, it's the length of time that the volume is allowed through the gate. The longer the shutter is open, the longer the light is allowed to come through the lens opening. So a large amount of light coming in for a short period can create the same exposure as a small amount of light coming in for a longer period. The longer you have your shutter open, the more susceptible your images are to being blurred from camera movement if you are holding your camera in your hands.

The goal is to create a well focused, well exposed image with the correct depth of field, good sharpness, and great composition. As a general rule, most of us cannot hold a camera steady enough to use a shutter speed longer than 1/60 second. So, unless you have very steady hands, you should try to use an aperature that allows you to shoot at 1/60 second or faster to avoid blurry images. You can also use a tripod if light is limited and you want to use smaller aperatures and longer shutter speeds.

Generally speaking, the overall exposure of an image is the result of the aperature and shutter speed combination you choose. You can shoot in Aperature Priority (you select the aperature, the camera selects the shutter speed) or Shutter Priority (you select the shutter speed, the camera selects the aperature) or choose manual and you select both settings.

ISO refers to the sensitivity of your camera's sensor to light. Generally ISO ranges of 100 - 1600 are available on most DSLRs. Some cameras offer greater ranges. If lighting is limited and you have no flash, you can increase your camera's ISO so you can use faster shutter speeds. The drawback is that higher ISOs generally create higher levels of digital noise in images. Cameras are gettting much better at creating clean images at higher ISOs however. Read your camera's manual if you are not sure how to change this setting - and don't forget to set if back if you use a high setting to shoot images at night. For well lit day shots, you should try to use lower ISO settings like 100-200.

Hopefully, you have at least a general idea of how exposure is determined when you take a photo. You will probably learn more by experimenting with your camera now than if you continue to read theory. If you are beginning your journey into photography, I would advise you to keep a record of your shots:

• Conditions - sunny, cloudy, windy
• Shutter speed
• Aperature
• Lens used
• Any other pertinent info about your photos

Much of the info about your image is stored by your camera in the image file. Depending on the software you use to edit your images, you should be able to access this info easily. The important thing is to pay attention to it and to learn how settings affect your final image. I'll be writng more about this in future articles.