Here's a quick comparison demonstrating the effectiveness of the ExpoDisk for setting custom white balance. The ExpoDisk slips over the end of your lens, you aim in the general direction of the source of lighting in the scene, put your camera in custom WB preset mode, and press the shutter button to meter the light through the ExpoDisk. It is a filter that contains a bunch of prisms on the front to pick up light from the scene from multiple directions and get an average light temp value of the surroundings. It is certainly more accurate than relying on your camera's Auto White Balance mode, and also more accurate than calibrating WB using a gray/white card, because light reflecting off the card can vary depending on the angle to the lens and the light hitting the card.
I set an egg on the couch cushion in my living room and took 3 photos: the first while using my camera's Auto White Balance mode, the second after setting custom white balance using an 18% gray card, and the third after setting custom white balance using the ExpoDisk.
There are incandescent lights above and to the left, along with a window with filtered light further to the left of the subject position. All 3 photos were shot in RAW and no post-processing was done whatsoever. I only imported the RAW files into Lightroom, cropped the images, and exported to JPEG. Again, no adjustments to color, contrast, sharpening, etc. were made; the images are exactly as they came out of the camera. All 3 used the same camera settings: manual mode, f/2, ISO 400, 1/20 shutter speed, no flash, using my Nikon D800 with Voigtlander 58mm f/1.4 lens.
The first image was shot using the camera's Auto White Balance mode. As you can see, it is noticeably warmer than the other 2 images, with the egg having a yellowish tint. AWB is supposed to correct for the lighting, and this isn't complicated mixed lighting, yet it still got the WB wrong:
The next photo was taken after calibrating WB by aiming the camera at an 18% neutral gray white balance card in the same lighting conditions. As you can see, it has a much cooler WB, and is closer to reality, although the cushion color isn't quite correct, being a bit too "cool."
The last photo was taken after calibrating a custom WB using the ExpoDisk. As you can see, it's slightly warmer than the previous photo. As best I can tell, it's spot on. You'll have to trust me that the cushion color is more correct.
Once I started using the ExpoDisk, I will never go back to any other way of setting WB in-camera. Yes, if you're shooting in RAW, you can adjust WB any way you wish non-destructively. Still, having WB closer to reality is a big help with reducing post-processing time for photos where accurate color is critical, such as portraits, where you want accurate skin tones. Or, where you're shooting in weird mixed lighting. After you use it a few times, it's fairly quick and easy to set WB for the particular scene. At $50, it isn't very expensive. To me, it's worth every penny.
Give it a try!
------------- Ted
Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle.
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