PhotoVision’s One Shot Digital Calibration Targets; White Balance And Exposure, Right In The Camera Page 3
How To Use The One Shot Digital Calibration Target
My experience is based on using the target with Canon D-SLRs (EOS 5D and 20D). Imaging sensor sensitivity patterns and Custom WB procedures vary with manufacturer and possibly with camera model, so follow instructions outlined in your camera's Owners Manual. I've sketched out a broad-based approach, which may need to be tailored to fit your circumstances. Whatever camera you use, you will need to display a histogram during playback/review to determine the best image to use for Custom WB and exposure settings.
1. Positioning the One Shot Digital Calibration Target. Place it so that the white patch is on the same side as the main light, making sure the target faces the camera but angled ever so slightly in the direction of the key light, keeping the target upright. This ensures an accurate white point reading, which we'll need at the next stage. If skylight or ceiling bounce light is the main light, position the white patch at the top. With portraits, have the subject hold the target in front of their face. With other subjects, place the target as close to the subject as possible.
2. Determining exposure. You can make the initial exposure with the camera's meter or an incident meter. When adding the calibration target, fill the frame as much as possible with the target, aiming the lens at the target's crosshairs on the gray patch, and make the exposure.
a. When using an incident meter (outdoors or with studio lighting), take a reading from the subject position to determine the optimum exposure and white balance. Set the camera to Manual Shooting mode and, using the exposure derived from the incident meter reading, take a picture of the target. Check the histogram on the camera to evaluate the exposure (see Step 3). (Alternatively, you can take a handheld reflectance meter reading off the gray patch. Then follow the steps outlined.)
b. When using the camera's TTL metering in any Autoexposure mode, first reset exposure compensation to the null point (centered on the scale). Next set the camera's TTL metering pattern to multi-pattern (or evaluative or matrix) metering and read the exposure off the target. If necessary, hold the target at arm's length, provided subject and target are under the same light, again filling the frame and focusing on the crosshairs. Bracket in 1¼3 or 1¼2 stops above and below the metered exposure up to +/- 1 stop (or thereabouts). Once the optimum exposure has been determined, check the histogram (see Step 3), set the camera to Manual mode; set the f/stop and shutter speed based on that initial reading, keeping flash sync in mind where applicable.
c. When using TTL or auto-thyristor flash, set the flash to Manual mode (for consistent output) before making any test shots with the calibration target.
3. Setting Custom WB. This procedure may vary with camera model. With my Canon EOS 5D, I begin by setting white balance to Auto WB (or you can start in Custom WB). After making the initial exposure, which targets the gray patch, I read the histogram on the camera. I check that the black and white points are fairly the same distance from the edge of the frame, with gray peaking in the middle. I then set Custom WB keyed to the exposure that appears optimum for this situation. (If you've bracketed, you may have to review the images to find the optimum exposure.) You now have not only the white balance tailored to the situation, but, thanks to the black (shadow) and white (highlight) points, you also have an optimum tonal range for the subsequent exposures. Don't worry if the histograms for the actual subject look nothing like the calibration histograms. The histogram for a low-key image would lean more to the left, the shadow side, and a high-key histogram would exhibit a highlight bias on the right.
a. Note: following the Expose to the Right rule, it might benefit the shadow values if you push the histogram slightly to the right by 1/3 or 1/2 stop--in other words, overexposing just a bit, provided this step does not blow out the highlights. During raw conversion or when editing the image, bring the exposure back down to the optimum level. This practically eliminates digital noise in the darker regions of the picture.
4. Shoot a second reference image of the target at the established exposure and Custom WB settings (optional, but I like to take this step just to confirm that I nailed it). Now turn your attention to the subject, leaving all the guesswork behind.
Availability And Info
The One Shot Digital Calibration Targets are available from PhotoVision online at www.photovisionvideo.com, or by calling (888) 880-1441. The website also provides online tutorials, although a comprehensive tutorial DVD comes with each target, with tips on using Adobe's Lightroom.
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