Sensitivity (iso rating),
Time (shutter speed)
|four factors of exposure|
There are four major factors which determine how a camera will capture an image with good exposure. These factors are named here as light, aperture, sensitivity, and time. You can remember these four terms together with the acronym LAST.
With a camera, the photographer can attempt to capture an image at any time. However, the optimal photograph is made only if all four of these factors are properly balanced. A change in any of these four factors would require an adjustment to the other factors to compensate, bringing all of the factors back into harmony. Without the proper combination, the resulting image will certainly suffer.
These four aspects can be calculated very simply, and the proper ratios can be achieved with the appropriate numerical values of each of these four components. If all of photography were so simple, everyone would be Ansel Adams. Each of the four factors introduces its own side effect, and it's the artistic understanding of this complex interplay which can distinguish a good photograph from a poor one.
For the purposes of this discussion, we will define an "ideal" image as having the widest true dynamic range with a pleasing distribution of middle shades between the lightest and darkest recorded shades. Without a good dynamic range, many important details in an image are lost and cannot be recovered with darkroom or digital post-processing.
Light (L) refers to the actual illumination from the scene which enters the camera. Our brains are very good for adapting to different lighting conditions, so it may not be intuitive to realize the importance of illumination. A camera must adapt to the incoming light in much the same way, whether through manual adjustments or automatic electronics. The available light is quite different between night-time, indoors, cloudy and sunny conditions. In addition, the illumination which enters the camera is also affected by light-colored or dark-colored subject matter.
On a camera, the trick to recording an image with the best exposure will be to find an appropriate combination of aperture, sensitivity and time to match the available light. Sometimes a photographer can increase the amount of light indirectly by using a flash bulb, or opening a window, or by waiting for more sunlight to be available. A filter can be placed in front of the lens, like sunglasses, to limit the amount of light.
With each new light source added, the overall makeup of that light is changed. Shadows are made deeper or shadows are filled in. The golden glow of your tungsten light bulb will cast across the subject. Sunny blue skies will alter the hue of your image. Use your camera's flashbulbs and the strong mostly-white light will compete with the other illumination. The balance of all these sources will determine the color balance of your subject.
Films are formulated to counteract the color balance of each type of light, such as tungsten and sunlight. Digital cameras often try to determine the "white balance" automatically, to ensure that white objects look white, not too blue from the sky or too warm from the desklamp, but you can often adjust the white balance yourself.
Artistically, you may also use this imbalance to your advantage. You can make shadowing work to your advantage. You can add a filter to your lens to color the light. You can use a different white balance setting on your digicam. You can enhance contrast by spreading out the lights and subject differently.
As discussed, the simplest camera is called the camera obscura, which is also known as the pinhole camera.
One of the drawbacks to the pinhole camera is that of convenience. A tiny pinhole can take a very long time to let enough light through, to properly expose a piece of chemical film or an electronic circuit. The very first camera obscura took a whole day to expose. The sun's overhead movement through all those hours made the shadows kinda silly-looking and indistinct, and nobody would pose for that long.
If you widen the pinhole, more light shines through. A bigger aperture directly equates to less exposure time. If you double the area of the aperture, then twice the light can come through at once.
The size of the film doesn't matter, but the depth of the camera's body does matter, in measuring the effect of the incoming light.
Once the aperture (A) is bigger than a pinhole, though, you'll need a lens. A lens redirects all the light that passes through the hole, so that it comes back towards a coherent image on the film. This page won't go into more details about the lens effect, however.
Film emulsion is made up of tiny crystals that are sensitive to light. Digital sensors are organized into tiny receptors that are also sensitive to light. When you take a picture, these crystals or receptors each react and record a single tiny piece of the whole image.
Film emulsions can be designed to be very sensitive to dim lights, or mostly unaffected by all but the strongest lights. Likewise, digital sensors can be amplified, much like a radio volume knob, to increase weaker light signals.
However, the penalty of using more sensitive films or amplifying the receptor signals is that it can create an image which has many small variations. These variations in light or color in a photograph are called grain (film crystals) or noise (electronic receptors). It's very much like the static you hear on the radio, when the signal is not strong enough to be received accurately.
As we've seen, we can adjust any one of the four factors to counteract any of the others. The shutter time is no different; if you have too much light, you can use a faster shutter speed to maintain your exposure; conversely, a slow shutter can brighten your image beyond what the human eye can perceive.
Shutter time is a very important factor because it means something to the observer, not just to the photographer. If any part of the scene is in motion, then the shutter time you choose is crucial to convey the action to your audience. In fact, in many circumstances, you may want to choose your shutter time first, and then adjust all of the other factors to suit your chosen shutter time.
We now see that exposure is based on four factors: light, aperture, sensitivity, and time. If you halve any of these, you must double another one to maintain an equivalent exposure. Also, besides this simple double/halve behavior, each of the four factors controls some secondary effect. Light controls balance, aperture controls depth, sensitivity controls grain, and time controls action.
Selecting the appropriate combination of light, aperture, sensitivity and time usually comes down to a choice of which secondary effect is most important.
- If low grain is most important, pick a low sensitivity first;
- If stopping the action is most important, set a short shutter time;
- If isolating subject from background, open up the aperture; and
- If the shadows and color are critical, control the incoming light.
As the photographer, the priorities are yours; the camera is a tool.
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