Aperture/f-stop - the aperture of a lens is the diameter of the lens opening which determines the amount of light that enters the camera. Aperture is expressed as an f-stop. The aperture size is related to the focal length of the lens. An aperture diameter of 50mm on a 100mm lens results in a aperture size of 1/2 (50 divded by 100). The aperture size is represented as a fraction, f/2, F2, or f:1.2, and is referred to as an f-stop or f-number.

Each successive aperture decreases the incoming light by half by reducing the aperture diameter by a factor of 1.4. So, the next f-stop after f/2 is f/2.8 (2x1.4=2.8). The smaller the aperture number is, the larger the opening is. Smaller numbers mean more light enters the camera, larger numbers less light. Stopping down a lens by one f-stop, ie f/4 to f/5.6, means you will decrease the incoming light by half.

Aperture Priority - a shooting mode on a camera where the photographer is able to choose which aperture is used. The camera automatically selects the shutter speed to determine the correct exposure. Selecting the aperture is useful when the photographer wants to control the depth of field.

Barrel Distortion - a lens effect where the horizontal and vertical lines of an image bend outward towards the edges. This effect usually occurs at the short end of a zoom lens.

Bokeh - the quality of the blur in the out of focus area of an image, usually behind the intended subject. Bokeh is considered good or bad depending on if it's smooth or coarse and if it's distracting to the main subject.

Chromatic Aberation - color distortion in an image caused by the inability of a lens to create identical focal planes for different light rays. CA appears as fringes of color along the edges of objects.

Depth of Field - the depth of field (DOF) is the area in front and behind the main focus area that is also in focus. See an example here. The butterfly is the main focus area. The flower the butterfly is resting on is included in the depth of field. The flowers in the background are not, they are out of focus.

Factors that affect depth of field include aperture, focal length, and distance to the subject. A large aperture (small f-stop, ie f/1.8), results in a shallow or small depth of field. A small aperture (large f-stop, ie f/22), will result in a greater depth of field and more of the image will be in focus. A lens with a shorter focal length will have a greater depth of field at the same aperture as compared to a lens with a longer focal length. Moving closer to a subject will decrease the depth of field, while moving away from a subject will increase it.

Diffraction Limit - the point at which stopping down the aperture of a lens results in a softer image.

Exposure - the amount of light that reaches the camera sensor. This amount is determined by the aperture and shutter speed and is affected by the ISO (sensitivity).

Fast Lens - a lens with a large maximum aperture, ie f/1.8, is considered a fast lens. The larger the aperture, the more light that enters the camera. This allows for the use of a higher or faster shutter speed, hence, a fast lens.

Focal Length - the distance measured in mm from the center of a lens to the focal point of the sensor or film when the subject is focused at infinity. A shorter lens, with a smaller focal length, ie 24-35mm, is considered a wide angle lens because there is a greater field of view. A longer lens, with a larger focal length, ie 80-300mm is considered a tele lens because the field of view becomes narrower as you zoom in.

ISO (Sensitivity) - the number which indicates the sensitivity of a camera's sensor to light. A lower ISO number requires more light than a higher ISO. The higher the ISO number is, the more visible noise will be present (grainy appearance).

Metering - the system by which a digital camera measures the available light of a scene and determines the correct exposure. There are several different kinds of metering:

Matrix - the camera divides the scene up into several areas or zones. Each zone is evaluated separately. The measurements are then compiled and compared to determine the correct exposure. This method usually provides the best exposure.

Spot - only a small area of the scene is metered. This is ideal for a brightly lit subject such as the moon.

Center-weighted - an average of the entire scene is taken with extra emphasis on the center portion.

Pincushion Distortion - a lens effect where the horizontal and vertical lines of an image bend inward towards the center. This effect usually occurs at the long end of a zoom lens.

Purple Fringing - a purple/blue outline which appears around dark edges in a photograph, which can be caused by chromatic aberration, overexposure, or lens flare.

Shutter Priority - a shooting mode on a camera where the photographer is able to choose which shutter speed is used. The camera automatically selects the aperture to determine the correct exposure. Selecting the shutter speed is useful if you want to stop the action, ie sports, or to slow down the action to create special effects with blurring.

Shutter Speed - the length of time light is allowed to enter the camera. Shutter speed is expressed in fractions of seconds, ie 1/200, which would be 1/200th of a second. The faster the shutter speed the less chance there is of blur due to camera shake.

Vignetting - a reduction in the brightness of an image along the edges when compared with the center of the image.