• Aperture-priority auto exposure (AE)
  You select the aperture, then the camera automatically sets the optimum shutter speed according to subject brightness.
• Shutter-priority AE
  You select shutter speed, then the camera sets the aperture according to subject brightness.
• Manual exposure

Check your camera instruction manual if you're not sure which system your camera uses.

The essence of exposure
Film must be exposed to light in order to imprint an image on film. And the degree to which film is exposed to light in a camera depends on two camera settings:

• Aperture (size of lens opening), and
• Shutter speed (duration of shutter opening).

A photographer can control these settings to adjust exposure after considering the desired effect and available light conditions.

Let's take a closer look at what these settings involve.

Even apertures have values
In general, interchangeable lenses used with single-lens reflex cameras feature built-in aperture diaphragm mechanisms comprised of multiple vane-shaped plates (except in such special lenses as the Reflex Nikkor reflective telescopic lens). If your camera's body lacks built-in aperture control, you can change the aperture by rotating the lens aperture ring.

As mentioned above, aperture diameter can be controlled to determine the amount of light entering the camera. Aperture diameter also determines depth of field.

Depth of field indicates how much of the image appears in focus. The deeper the depth of field, the more an image is captured in focus in the direction moving away from the camera; the shallower the depth of field, the less an image is captured in focus in the direction moving away from the camera.

Now focus on this to keep your views in proper perspective:

• The smaller the aperture value, the larger the aperture opening, and the shallower the depth of field. Open up the aperture for portraits, flowers, and other shots in which a foreground subject is the main focal point of the scene.
• The larger the aperture value, the smaller the aperture opening, and the deeper the depth of field. Stop down the aperture for landscapes, document reproduction, and other shots where you want to keep the overall picture in clear focus.

Shallow depth of field

Deep depth of field

Shutter speed captures the moment
A camera shutter acts something like your eyelid. But it can open and shut at speeds within a range that is wider than the 1/125-1/60-of-a-second range in which it takes a human eye to blink.

Imagine seeing movement through rapidly blinking eyelids. Your perception of the same action through unblinking eyes would certainly be different.

Likewise, shutter speed affects the way movement is captured on film, as you can see in the comparison below of high and low shutter speed shooting.

High shutter speed

Low shutter speed

• High shutter speed "freezes" fast-moving action on film. That makes it great for focusing clearly on details of passing cars on a racetrack, for example.
• Low shutter speed blurs motion on film. Great for expressing the flow of water, cars, or other objects moving by.

Shutter exposure exposed
Inside a typical SLR, focal plane shutters employing front and rear shutter curtains are positioned close to the lens focal plane, just in front of the film.

As it takes a shot, a single-lens reflex camera's two shutter curtains regulate the amount of light exposing the film by regulating shutter speed (see below).

Pressing the shutter release button releases the focal plane shutter. Then, the front curtain begins to travel just in front of the film inside the aperture, and after a preset period of time, the rear curtain follows. Light passes through the gap or slit between the two shutter curtains to expose the film. Adjusting the shutter speed setting adjusts the width of the slit, and determines degree of film exposure.

In other words, the shutter speed (a measure of exposure time) determines the gap (or slit) width between the front curtain's trailing edge and the rear curtain's leading edge, as well as the curtains' travelling speed. The time that elapses before the second shutter begins to travel (time gap) is especially important when shooting at slow shutter speeds.

• Exposure decreases as the slit narrows and/or the travelling speed of the shutter curtains increases.
• Exposure increases as the travelling speed of the shutter curtains decreases and/or the rear curtain traveling delay increases.

Combining aperture value and shutter speed
The table below shows that for a given exposure, opening up the aperture value one stop, for example, decreases shutter speed by one step. The lighting of the final photograph depends on exposure and shutter speed.




In more ways then one, different combinations of aperture value and shutter speed allow you to see the same subject in a different light.

When selecting a large aperture value:
Increasing depth of field brings more of an image into focus; reducing shutter speed is good at capturing the flow of motion, but makes camera shake a greater potential problem.

When selecting a small aperture value:
Decreasing depth of field narrows focal range; high shutter speed is good at stopping even high-speed action in its tracks, while minimizing the effect of inadvertent camera shake.

Which single-lens reflex camera mode do you use to take pictures--aperture-priority AE, shutter-priority AE, or manual exposure? It's always an eye-opener to see life in a different light. Think of this greater sense of control as a new way of looking at the world. Have fun experimenting!