Re: Best Grey Card / Panel for luminance and color accuracy?
jzucker wrote:
the problem with evaluating pictures on a screen is that it\'s subjective IMO. The only accurate method has to involve exposing a known grey color and then testing with the eyedropper in photoshop. If the card isn\'t a true 128,128,128 grey then you\'re screwed I guess. From an engineering perspective, the problem with the method you describe is that the white detail method is subjective and dependent on the output device calibration and how much light source it\'s exposed to as well as your own perception of detail.
You are confusing what I suggest for EXPOSURE CALIBRATION with gray balance. Two separate tools for two different tasks.
Gray Balance Reference:
Yes you need a known R=G=B target for setting custom WB and \"snapping\" color to neutral in Photoshop as a starting baseline. But there is no such thing as a \"128,128,128\" gray card. Seting custom WB off the card will result in R=B=G. That the actual eye dropper readings will wind up will depend on how you expose the shot containing the card with ISO/shutter/aperture of camera. That\'s exactly why the gray card is of limited use for calibrating your EXPOSURE meter TO YOUR CAMERA ISO.
Exposure Meter Compensation:
What you are \"calibrating\" is actually a compensation factor between you incident EXPOSURE meter\'s ISO setting per factory calibration at ISO 100 and what your camera\'s sensor sensitivity is when set at ISO 100.
If you can meter a scene containing a white object and the white object (and skintones) are perfectly exposed in your images VISUALLY and per detail and per Photoshop eye dropper readings then no compensation is needed. For example a white card in the scene when correctly exposued should have eyedropper readings of around 250 with only specular reflections off shiny objects at 255. Logicailly to see that distinction between 255 specular and 250 solid white you\'d want a white target that is both solid and specular, not a gray card.
If the EXPOSURE meter out of the box doesn\'t create a 250 value on a white card that tells you the meter ISO and actual camera sensitivity are not in sync. You can\'t adjust the camera sensitivity so you change the meter reading to match the cameara using the complensation factor or adjust the ISO speed of the meter (if it does not have a compensation feature) until the meter reading does accurately render the highlight values of solid white objects at 250.
If you try what I suggest it will become obvious why I suggest doing it that way.
Mar 23, 2012 at 06:43 AM
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