Aperture (A)
Much like the iris in your eye, the aperture (a mechanical diaphragm) controls the amount of light passing through a camera lens. The opening of the aperture can vary from wide open - which allows maximum light through to the film or digital sensor - to fully "stopped down", or nearly closed. Very bright scenes, such as direct sunlight, might need a small aperture to reduce transmitted light for proper exposure, while a dimly lit scene would need a wide open aperture.

The relative size of the aperture is expressed as a number called the "f-stop", which is a ratio of the lens focal length over the optical diameter of the lens. Lower numbers, such as "f2.8", represent a larger aperture opening, while higher numbers (f16) have smaller openings (admitting less light). Typical f-stops marked on a lens might follow this sequence: 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, and 22. With each larger number (from 1.4 to 2.0 for example), the aperture admits one-half the light of the previous f-stop.

Besides controlling the amount of light used for exposure, the aperture also affects depth of field. This is the range of the scene (near to far) that is in focus. Higher f-stops (smaller aperture openings) have greater DOF, meaning more of the scene is in focus. Photographers might use a wide aperture (lower f-stop) to intentionally minimize the depth of field so that only the subject is in focus, leaving the background blurred. Or, they might use a small aperture (high f-stop number) in a landscape shot to keep everything from near to far in focus.

    Shutter Speed (S)
The shutter in your camera is like a window-shade; open it to let the light in, close it to block the light. Shutter speed affects the exposure of a photograph by controlling how long the film or digital sensor receives light. Shutter speeds are usually expressed on a camera as fractions of second - "500" would indicate 1/500 second.

Typical shutter speeds are 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, and 2000, although speeds in between these numbers are common in modern cameras. Note that each number is twice the one before (or half the duration since we are representing fractions). So, a setting of 1000 (1/1000 second) exposes half the amount of light as a setting of 500 (1/500 second). Long exposures (with shutter speeds over 1 second) give the photographer creative control over dimly lit or night scenes.

Shutter speed has an important side effect. Since it controls how long light from the subject is exposed, it can effectively freeze motion. Say you're shooting a running athlete and you use a relatively long shutter speed (1/60). The runner will be blurred since he has moved a significant distance during that 1/60 second. If you choose a much shorter shutter speed (1/1000), you'll "freeze" his motion and get a much sharper photo. Higher shutter speeds can also freeze camera movement and help a photographer overcome unsteady hand-holding of the camera, which is further magnified when using telephoto lenses.

    Film Speed (ISO or ASA)
To allow for proper exposure over a wide range of lighting conditions, photographic films were created with different levels of sensitivity to light. A film's speed rating number, originally called ASA (from American Standards Association) and later called ISO (International Organization for Standardization), typically ranges from 25 to 800 or higher.

Film with an ISO rating of 25 would be suitable for brightly-lit scenes such as direct daylight, while ISO 800 would have the much higher light sensitivity needed for dim interiors (or where very high shutter speeds might be required). Special film "push processing" could extend ISO up to 3200 or even 6400.

In digital cameras, the same ISO standard is used to set the sensor's light sensitivity. Basic "Point and Shoot" (P&S) cameras might have a variable ISO from 100 to 400 automatically set depending on ambient light conditions, while higher end DSLRs (Digital Single Lens Reflex cameras) may have ISO speeds above and below this range, selectable by the photographer.

Choosing an appropriate ISO is important since higher light sensitivity comes at a price - reduced image quality. With film, higher ISO needs larger light-sensitive particles in the emulsion, which causes visible grain to appear in photographs. Digital cameras have a similar problem which manifests itself as noise, resulting from higher amplification of the light sensors. The photographer has to therefore walk a fine line between higher image quality (lack of excessive grain or noise) and higher light sensitivity when selecting ISO.

    Histogram
Digital cameras capture an image as a collection of "pixels" or dots, each having a specific color and brightness. This can be seen by greatly enlarging a digital photo until the individual pixels appear as blocks of color (see illustration below).

A histogram shows the distribution of pixels from dark (black) to light (white) contained in a photo. Typically, a black pixel is assigned a value of "0" while a white pixel is "255" (for 8-bit color depth), so there are 256 lightness values on the graph. Note that pixels also contain color information for the amount of RED (R), GREEN (G), and BLUE (B), so lightness is actually a combination of the three color levels.

The histogram can be very helpful in determining if a photo is under or over-exposed. Many digital cameras include the option of viewing a histogram overlaid on the LCD image just taken, allowing the photographer to make adjustments in exposure if necessary.

The sample histograms below illustrate what you might get with an under-exposed and over-exposed photograph. Notice in the under-exposure, brighter pixels are missing at the right end of the histogram. In the over-exposure, dark pixels are not present (left end of the histogram).