Gray cards make excellent targets for one-click (eye-dropper), color-correction in post. For mobile applications, I bought a WhiBal 2″ x 3.5″ ‘G7’ pocket gray card, plus their spring-loaded retractor/spring-clip mount. I used this set-up on a recent trip and it worked great with the spring clip attached to my backpack strap. The cards are both waterproof and scratch-proof (color goes all the way through). For studio/location use, I’m going to buy one of the larger WhiBal gray cards. Again, pricier than competing products, but the manufacturer claims that each card is tested with a spectrophotometer to ensure color accuracy. Well-made, and nicely designed, I think they’re the best of the bunch.
There are actually two discussions here: 1.) Setting a custom, in-camera white balance (e.g., paper or gray card); 2.) Photographing a neutral gray target for “eye-dropper” color-correction in post. Setting a custom white balance in-camera may be attained by framing either a white or neutral gray target. However, if the white paper is overexposed (e.g., all values at 255, 255, 255), all color data will be lost, and this image won’t be useful as a color-correction target later in post.
The best way to photograph a color-correction target (i.e., WhiBal or gray card) is to place it in the scene where only the “preferred” or “dominant” light source is falling on the target. This kind of selective source correction is unattainable from ExpoDisc-type products (which I wouldn’t recommend), which corrects for every light source which happens to be falling onto the on-lens ExpoDisc diffuser. While I prefer the WhiBal gray card, there also are many other, far less expensive gray cards to choose from (e.g., bhphoto.com->photography->general photo accessories->white balancing accessories). I previously bought a set of 8″ x 10″ gray cards for only about $10 (pack of two) from a local camera store.
I also like the Photovision targets, but I read one review where the user reported that the small version didn’t “fold-up” as neatly as expected, so for my “mobile” photographic applications (e.g., hiking), I chose the ‘G7’ WhiBal card with its retractable spring-clip instead (quite a handy accessory–it’s always at the ready, and doesn’t get lost). Note that both the Photovision, and the WhiBal targets include white, black, and gray fields (which I find useful).
Any tool can be misused, over-used, abused, or not used at all. Including a neutral gray target in your scene supplies a known point of reference. Where you choose to “correct” from that point is purely subjective. The most common example is a sunset: white-balancing to its orange hues would render the sunset near-colorless. Contrastingly, “forcing” a tungsten white balance at magic hour shifts the remaining daylight even blue-er (shown below):
That’s why I often find it convenient to introduce one, even in non-studio applications. In my WhiBal example image earlier (beach shot), the camera’s auto-white balance thought it was 5,000°K (I did not perform a custom white balance on-site), whereas later, I found it was actually closer to 5,350°K (this post-correction achieves a similar result as if I were to take a color-temperature meter reading at the location, and then dial-in the indicated Kelvin value in-camera).
My main objective was to introduce a neutral reference in the scene to later correct for the primary source: the sun, and that day’s particular atmospheric conditions (also, the card was tilted up slightly to minimize any bounce from the sand which would’ve shifted the reading). At this point, (and, in this particular case), I wanted the sun’s color temperature to photograph as “neutral,” or “white.” Now, I have my starting point. Any further color-correction or enhancement is purely subjective.
This is where our interpretive skills as “colorists” come into play (which is the point I was attempting to address in my post above). Now, some may say, “Why not just shoot RAW, and adjust later?” In many cases this is fine; however, when using mixed sources (as in the fill-flashed/magic-hour photo of the girl above), you’ll only be able to correct for one or the other, so it’s important to know at the time you’re shooting, which color temperature you’re “favoring.”