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


I now needed to create a custom rendering setting in Lightroom for ETTR. Since images exposed to the right initially looked overexposed, I needed to alter the rendering controls to produce an acceptable color and tone appearance and create a new default preset that I called “normalize.” Since Camera Raw and Lightroom utilize the identical processing pipeline, I can use this custom normalize default setting in either product. This is especially useful in Lightroom because I can select a custom preset upon importing my images (Figure 3). I never see the ETTR images in Lightroom as overexposed. If you use another RAW converter, you should still be able to produce a custom setting that compensates for the initial “overexposed”-appearing rendering. Alter the necessary tone controls in the converter to produce a white with just a small amount of tone, then save that as a new default. The key is exposing such that white in the target is as close to, but not clipping, 255/255/255.

In Lightroom, two stops over the base exposure blew out the highlights, and no amount of exposure compensation could rectify this. The sensor was pushed to full saturation, a situation that should be avoided. The 1½-plus exposure bracket could be normalized by setting the Lightroom Exposure slider to -1.47. No other settings would be adjusted. I attempted to set the gray background of the Sekonic target to match the “normal” exposure while keeping the all-white targets below 100 percent pure clipping value.

For the ISO 400 image, I had to move the exposure slider in Lightroom to -1.68 to get the overall gray of the target background to read approximately the same as the normal version, while avoiding highlights clipping. I noticed that the overall contrast of the various images looked slightly different from each other. This indicates that boosting ISO may affect other areas of the tone curve.

For the ISO 800 image, I was able to use the normalize exposure preset used for ISO 400 to produce a close match to the base exposure. With this information, I was able to extrapolate the actual-sensor ISO for exposing images to place as much data in the first stop of highlight data without blowing out the highlights.

To better match all exposure versions visually, white balancing was necessary. I used the Lightroom WB tool on the neutral gray found on the WhiBal card in the same location for all images. The result produced only minor alterations throughout the image, while not all grays in all images produce the same numeric values. The various grays visually appear slightly different even though the values are within +/-2 to 3 percent as indicated by the Lightroom info readout.


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