Thursday, May 31, 2007
Calibrate Your Camera
To get complete predictability, consistency and control, it's important to calibrate your entire system. We used to do it with film, now we do it a little differently with digital gear.
Use the same lighting you'd use for your most common picture-making situation: outdoors/daylight, indoors/tungsten or studio/flash. At the minimum, you should test for the two major light color temperatures: daylight (5000 K) and tungsten (3400 K). It's almost impossible to fully test for fluorescent lighting because of the uneven spectral characteristics, but if you frequently shoot under a specific fluorescent light, you can test for that. In any case, make sure that you illuminate the test target evenly and that you avoid placing the subject near a colored reflective surface.
The idea is to shoot a bracket around the expected best exposure to determine the actual best exposure.
You can use these techniques with a variety of software packages. Here, Adobe Camera Raw and Adobe Lightroom are illustrated. Each one may work slightly differently or have some unique features, so it's a matter of using what you have and what you're comfortable with. The White Balance tool in Camera Raw is employed to select a point to neutralize the color in the image. In Lightroom, you achieve the same effect by zeroing out the sliders.
Set the camera on manual and change only the ƒ-stops to shoot the bracket. Use the lowest default ISO setting for the camera. It's best to use a handheld light meter; a spot meter would be ideal. I use a Sekonic multimeter that's capable of incident and spot reading in continuous and flash light.
Start shooting at two stops over the measured normal exposure for medium gray. If you use a spot meter, you can measure the fourth patch from the left along the bottom row of the ColorChecker for medium gray. Shoot exposures at 1/3-stop intervals and place a written note in the subject indicating the E.I. for that exposure. In other words, if your expected E.I. is 100 and you expose one stop over, put E.I. 50 on a piece of paper in the shot to indicate that if this shot looks properly exposed, the E.I. would be 50 (one stop less than 100)—1 1/3 stops over would be E.I. 40; a 2/3 stop over would be 64. Shoot enough exposures to get to at least 1 1/2 stops over and under the expected E.I. I find that, more often than not, the typical D-SLR is overrated by one stop, so if the manufacturer says the E.I. is 100, the actual E.I. for the chip is 50.
Evaluating In Adobe Camera Raw
Now you should view the RAW files in the processing software. If you haven't changed the defaults in Adobe Camera Raw (ACR), you'll notice that all the images will appear more or less the same in Bridge. This is because ACR is using auto adjustments to equalize the exposures. We have to turn off auto adjustments and zero out all the sliders before we can get a good idea about the best exposure. Select all the thumbnails; from the File menu, open ACR and uncheck Use Auto Adjustments from the Settings flyout menu.
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