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Saturday, December 1, 2007

Digital Exposure


What does good exposure give you? And can you trust the histogram?

Monarch Butterfly Catepillar
This image of a monarch caterpillar on butterflyweed was exposed to gain the most from the sensor; the histogram was used to interpret the exposure.

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Auto-exposure systems built into cameras today are very good. Camera manufacturers have done an outstanding job in creating complete systems that give excellent results. The computing power inside a camera equals powerful stand-alone computers of not that long ago. Multiple metering points are measured at the instant of exposure, evaluated, compared to a database of what a good exposure should be for the conditions and an exposure is computed and sent to the camera controls to execute in terms of ƒ-stop and shutter speed—all within that same instant.

I still find that nothing beats a handheld spot meter for the most accurate readings, but it requires understanding how to deal with such readings, and it’s not good practice to use a single reading. There’s a learning curve and it takes time to do.

That said, you need to understand exposure and how the metering system works to get the best digital exposures whether you shoot RAW or JPEG. Over- and underexposure can cause problems that increase workflow and reduce image quality. Here are some things to consider:

Overexposure washes out highlights so no detail is left and weakens color in the bright areas (the chroma or color quality of the color is reduced).

Underexposure increases the incidence of noise and weakens color in the dark areas (the chroma is reduced).

Over- and underexposure can affect the total range of tones available to work with so that the image picks up unneeded contrast in certain areas (especially the midtones). That may or may not hurt the image (it depends on your needs).

Both over- and underexposure usually increase the time needed to process an image in the computer. In nature photography, problems with exposure typically come from these areas:

-A bright scene the camera underexposes.
-A dark scene the camera overexposes.
-A scene with a very bright area of light that causes the rest of the photo to be underexposed (common in backlight).
-A photo deliberately underexposed to avoid washing out highlights (generally not a good idea).
-A photo deliberately overexposed to expose to the right of the histogram because RAW files can handle the exposure (this can be problematic).


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