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May 20, 2008 - Dave Hecei
Whether you are shooting film or digital, proper exposure is key. If the photo is under exposed (not enough light) or over exposed (too much light) you will not get a quality image. Proper exposure allows you to get every bit of detail in a scene, or allows you to express a proper mood. You may be able to fix it in Photoshop afterwards, but with the proper exposure, there will be less fixing and more shooting.
The built-in metering system in a modern DSLR can take light readings in multiple areas in a scene. It can then calculate how much light needs to hit the sensor (or film). The aperture of a lens controls how much light comes through the lens, while the shutter speed controls how long that light hits the sensor. The combination of the two gives an exposure.
As sophisticated as they are, DSLR metering systems are not perfect. They look at the entire scene and try to choose the appropriate shutter speed and aperture. But all meters are designed to give an average exposure. If you were to take a photo of a blank white wall, the image will actually reproduce as gray. If you were to do the same with a black, or near black, wall, it will also come out gray. This is because the meter is trying to create that average exposure, it doesn’t know from white or black. But you do.
When you are shooting a scene, whether you are shooting a landscape or a portrait, you are the one in control. There are a couple of different ways for you to take control of the exposure on a DSLR. Most all DSLRs have a manual mode. This is where you can set both the shutter speed and aperture yourself. First, take the exposure reading the camera thinks is proper. By stopping down the lens, going to a higher F-stop number, you will let less light in. By opening the lens, going to a lower F-stop, you will let in more light. Changing the shutter speed can do the same thing. Setting a faster shutter speed lets in less light, while a slower shutter speed lets in more light.
Another way is to use something called Exposure Compensation. Check your camera’s operations manual on how to activate this function. It is usually shown as a scale with zero in the middle. You can then adjust the setting to the plus side, to add more light to a scene, or the minus side, to reduce the light. Exposure compensation is used while still in the automatic mode.
Here’s an example. You are taking a portrait of someone wearing dark clothing against a dark background. You know now that the camera will read this wrong and let in too much light, making the dark image gray. The camera thinks it needs to shoot at 1/60th second at f/4. Set the camera to manual mode and either shoot at 1/250th at f/4, or 1/60th at f/8 (these settings will give the exact same exposure – the first one has a faster shutter speed at the same aperture, while the second one has the same shutter speed at a smaller aperture).
The same effect can be done with the EV compensation function. In the example before, we need to let in less light. This is done by dialing in an EV of –1 or –2. The –1 setting lets in 1 stop less light, the –2 is 2 stops. [A stop is measured by one full step of aperture or shutter speed. Going from 1/60 to 1/125 on the shutter speed dial is one stop less light. Going from f/8 to f/11 on the lens is also one stop less light.]
This is where digital beats out film. With film, you don’t know if you’ve got the right exposure until you get the film back from processing. With a DSLR, after you take a photo you can instantly review the photo. If the exposure is off, just retake the photo.
On most DSLRs you can see a histogram of the scene showing you light values for the whole scene. On some mid to upper-level DSLRs, there is even an exposure warning system where it will show you the areas in the photo that are beyond the ability of the camera to record. It will show these areas in the preview screen as red outlines or flash them in red fills.
These features, and knowing how the camera meter works, can help you get that proper exposure.
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