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Monday, October 1, 2007

Best Techniques For Digital Exposures

Setting everything on full auto isn't always the ideal solution. Try these tips to get your best shots every time.

 Best Techniques For Digital Exposures
Many photographers use a technique that double-processes the RAW files to get a perfect image. You process the file once for the highlights and once for the shadows.
Manual Exposure Mode & BDE

Sometimes I don’t use an exposure meter. I frequently photograph birds flying (or swimming) across a lake that has bright areas reflecting the sky and dark areas reflecting shaded foliage. In AE modes, the meter will underexpose the bird as it moves across the bright areas and overexpose it as it moves across the dark areas. There’s no way I can keep adjusting exposure comp quickly enough to compensate. So I just switch to manual mode, set the Basic Daylight Exposure (BDE) (an exposure of 1/ISO at ƒ/16, or equivalent) and fire away. The bird is always correctly exposed, no matter what the background brightness. Because the subjects are moving, I shoot with the lens wide open to get the fastest possible shutter speed: At ISO 200, the BDE for bright sun is 1/200 sec. at ƒ/16, or 1/400 sec. at ƒ/11, 1/800 sec. at ƒ/8, 1/1600 sec. at ƒ/5.6 and 1/3200 sec. at ƒ/4 (which is what I use with my 300mm ƒ/4 lens).

BDE is useful on clear sunny days, but not as useful on partly cloudy days, when the sun comes out, ducks behind a cloud and comes out again. When the light is rapidly changing, I use multi-segment metering and aperture-priority AE (and sometimes AEB with continuous drive).

RAW Format

Another RAW advantage is that nothing is done to the pixels until you save the image as a JPEG or TIFF after processing it. When you shoot a JPEG image, the camera processes it using default or user-selected parameters (white balance, sharpening, contrast, saturation, tone curve and more) and saves the image as an 8-bit lossy-compressed file. When you then open this image in your editing software to work on it, the missing pixels must be reconstructed, then pixels are again thrown out when the image is recompressed when you resave it. Thus, there’s a loss of image quality every time you resave a JPEG file.

With a RAW file, nothing is done to the pixels until you save the image as a TIFF or JPEG file after applying all your changes—and even then, your original RAW image remains safely unaltered.

Double-Processing RAW Images

Because RAW images contain more and better data than JPEGs, you can double-process RAW images to improve shots of contrasty scenes. Process the RAW image so the dark areas look right and save it; process the same image so the bright areas look right and save that. Then, simply combine the two perfectly registered images in your image-editing program to get the best of both in a single shot. Photoshop’s layer mask feature is a big help here.


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