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The Exposure Trilogy

The exposure trilogy is about coordinating three of the most important camera settings so you can capture properly exposed images. Those three important settings are the ISO, the aperture, and the shutter speed.


The first camera setting you will address is your camera’s ISO number. You’ll choose this number based on the light intensity of your scene. But you should also balance this decision with the quality you expect to see in the final images. Generally, you want to use the lowest ISO number you can use in order to retain the highest amount of quality.


If you don’t know where to adjust your camera’s ISO, then check your camera’s manual or look on-line to find the location of the ISO setting. Most times, the camera comes with the ISO setting on automatic.


The ISO setting controls an instrument inside your camera called a light meter. The light meter is the tool that will help you balance and coordinate all three aspects of the exposure trilogy. The light meter measures the average light intensity of any scene in front of your lens and helps you choose the appropriate aperture and shutter speed.


Seeing through the lens, the light meter evaluates all the lights and darks in a scene and makes exposure recommendations based on the average of the tonal range it sees. The more experienced a photographer becomes, the more they will be able to use this average to make their own choices about aperture and shutter speed. In the beginning, the light meter’s recommendation is a good place to start. To activate the light meter, simply push the shutter release half way down. You should see a light meter scale pop up in your viewfinder.


If your camera is set on manual, then you decide whether you want to start with aperture or shutter speed. If your subject matter involves motion in any way, you will probably choose to start with the shutter speed that will portray motion in the way you want.


After choosing the shutter speed, your light meter will recommend the aperture you will need for a properly exposed image. Light meters are all function the same way, but can look slightly from camera to camera.  You will want to find out what your light meter looks like and how it operates in your particular camera.


So, it takes a particular amount of light on the CCD to get a properly exposed image. The light meter will use the ISO setting to find the correct combination of shutter speed and aperture needed to yield an image that isn’t too light or too dark.


It’s kind of like making brownies. You need the right combination of time and temperature in the oven to make brownies that aren’t too moist or too dry. In the camera, the right combination of light intensity and exposure duration makes an image that isn’t too dark or too light.


You might also compare film or a CCD to your skin, which is a light sensitive media. If you get too much sunlight, you burn. If you get too little sunlight, nothing happens. If you get just the right amount your skin is able to adapt and you tan. Like the ISO of film or a CCD, different skin types have different sensitivities to light and will react accordingly.


If your subject is in motion, you will begin choosing your camera settings by looking at your shutter speed. Look through the lens or at your view-screen, and adjust the aperture. On your light meter you will notice a changing series of bars somewhere on your screen. In the center of that linear scale is where you want the moving bar to end up. When the moving bar aligns with the center of the scale, the exposure is correct.


After you get your exposure right, its time to focus your lens on your subject, compose the image and push the shutter release.


It seems like a lot of adjustments just to get a photo, but the more you practice, the faster and more precise you’ll become.


If your subject matter is stationary you are probably more concerned with depth of field. I this case you’ll want to start with the aperture setting that best expresses how much of the image’s foreground and background you want in focus. Once you choose an aperture setting, look through your viewfinder or look at the view screen on the back of your camera, and change the shutter speeds to zero out the meter.


Other than the camera’s manual setting, there are two other viable options for balancing the exposure trilogy. The first is called the aperture priority setting. This setting allows you to choose the aperture and the camera will automatically choose the proper shutter speed.


The second setting is called the shutter speed priority. This allows you to choose the shutter speed you want and the camera chooses the appropriate aperture setting. Shutter priority is used a lot by sports photographers because they know what shutter speeds they need to capture action and it’s happening, they don’t have time to decide on an aperture.


 Based on the intensity of light in the scene your camera may not let you choose the settings you want. There may be too little light, based on the ISO, or chip sensitivity, you chose. Or there may be too much light in relationship to your ISO. You will have to either increase or decrease the sensitivity of your CCD by changing the ISO setting. You could also make different choices for aperture or shutter speed to restrict or let in more light.


Once you have a correct combination of ISO, aperture, and shutter speed there is a trick you should know about using the exposure trilogy. Once you get the information for a correct exposure you can figure out, in your head, other correct exposures.


For example, if your correct exposure is f-5.6 at 1/60th of a second at ISO 100 you know that this combination is exposing the CCD with a specific amount of light. If we stop-down the aperture to f-8 (which is a smaller aperture opening) and lengthen our exposure to 1/30th to accommodate for less light on the CCD, the result will be the exact same amount of light. So, f-8 at 1/30th is the same exposure with ISO 100.


If we open our aperture to f-4 and increase our shutter speed to compensate for more light through the aperture, then 1/125th of a second will be the same exposure. Take a look at this exposure scale. Once you know one correct exposure, you know them all for that photo. Though each combination will produce different visual results, all of these combinations will make the same exposures.


There is one more thing I would like to mention about getting perfect exposures and it’s a technique called bracketing. Bracketing means making an exposure based on your light meter’s recommendation, then making 1 or more incrementally over exposed images and 1 or more incrementally underexposed images by changing you aperture or shutter speed settings. Sometimes the over or underexposed images come out better than what you thought was the correct exposure. As an experiment, I recommend you try bracketing in one-stop increments. If the images look unusable, you can delete them immediately. But sometimes you get surprising results.



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