Monday, October 8, 2007
Exposing For RAW
There are special considerations to take into account if you're shooting RAW and you want to be sure that you're getting a proper exposure
You can control the lighting ratio to fit this range, or expose to record either highlights or shadows, resulting in loss on the opposite end of the tone scale. The opposite extreme is actual overexposure, whereby the sensor can't record additional data in the highlights past the point of full-sensor saturation. Here, exposing for digital or film is the same: avoid overexposure that blows out highlight detail you hope to reproduce.
When you edit the RAW file in a converter, the software is operating on this linear-encoded data, which provides some useful advantages, such as powerful control over adjusting the tones in the first stop of highlight data. Some RAW converters can recover highlight data that would be difficult or impossible to alter in a nonlinear and gamma-corrected image. When film records a scene, it's necessary to follow the old practice “expose for the shadows, develop for the highlights.”
This brings us to the concept of Exposing To The Right (ETTR), that is, exposing for highlights and developing for the highlights. This ETTR recommendation came about on the Luminous Landscape Website several years ago, based on an interview with Thomas Knoll, one of the original authors of Adobe Photoshop and Adobe Camera Raw (www.luminous-landscape.com/tutorials/expose-right.shtml). Expose to place as much data within this linear-encoded RAW image without losing highlight values you wish to reproduce. Capture the most image data the sensor can record.
Is What You See What You Get?
ETTR presents a few problems, one being that the LCD camera preview, including the histogram and clipping indicators, isn't based on the linear RAW data. Instead, this preview is based on the rendered gamma-corrected JPEG your camera is set to produce, even if you don't save that JPEG and only shoot a RAW file! If your goal is to produce the best possible exposure for RAW, using the ETTR technique, the feedback on the LCD could steer you in the wrong direction. The camera uses its on-board processors to render color and tone from RAW to create a JPEG, which may be vastly different from what you hope to render from the RAW using a stand-alone converter. This is primarily why photographers shoot RAW. You want to control the color and tone rendering instead of accepting how the camera processes the RAW into a rendered JPEG. Yet you have no direct feedback about this RAW data when shooting.