Indecent Exposure – Part 1

by Terry on

bad exposure - licenced under wiki commons

Sorry to get your hopes up but both part 1 and part 2 of this article have nothing to do with streaking. The kind of indecent exposure I’m referring to is directly related to your Digital SLR’s light meter and how it finds the correct exposure. There are a lot of reasons you can have exposure issues; they can be user error, equipment failure or a combination of both. Part 1 explores how your cameras light meter works so when it comes time to troubleshoot you know what to look for.

Meter ModesHow the Light Meter Works:
First you have to understand how it all works. Digital SLR metering systems generally consist of a light sensitive device (light meter) usually located behind the prism just above and in front of the eyepiece. The sensor is precisely adjusted to be centered to the cameras focus screen. The light value reaching this sensor is calibrated with an industry standard calibrated light source at various intensities (light values). The camera circuitry then calculates these values and translates them to the correct shutter speed and aperture settings needed for an even exposure relevant to the ISO sensitivity setting.

Control Settings:
Depending on the model and the manufacturer, the light meter may vary a little. Modern light meters segment out sections in order to sample light values from every corner of the perspective image. Different settings allow you to turn on and off those segments of your light meter. These settings let you to control what parts of the perspective image are properly exposed. Below are the three basic settings that most camera manufacturers use.

Multi-segment Metering: Each portion of the device meters the whole scene to determine the appropriate exposure. In general, it is the best setting for most situations as it evaluates the whole image.

Center Weighted metering: As the name indicates, the light is measured from the center area and gets less sensitive towards the edges. This mode is used to prioritize the middle of the frame where, coincidentally, many place their subject.

Spot metering: The light value is only measured within a small and limited area usually at the center spot of the screen. This is typically used to meter a light value from a very specific area or determine the exposure gradient from a scene to determine the correct exposure.

Click on the link for part 2: exposure issue troubleshooting

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