James Stephens

This post details a brief review of the ExpoDisc Digital White Balance Filter, a prismatic device designed to assist with white balance calibration.

The Expodisc comes in different sizes to suit numerous lens thread sizes and simply pushes onto the front of the lens. Once in place it collects and averages light from a wide radius. Thus, it works via transmitted light, rather than reflected light like a gray card does.

There are 2 ways to obtain a white balance with the Expodisc.

Custom White(or Gray) Balance

1. Capture an image with in-camera white balance set to Auto.
2. Set the image using the ‘Custom White(or Gray) Balance’ feature of your camera.
3. Snap away with the white balance taken care of in-camera.

Gray Frame Reference

1. Capture an image with in-camera white balance set to the dominant light source.
2. Snap away.
3. Balance the reference image in Adobe Camera RAW (ACR) with the White Balance Tool and then synchronize the resulting white balance across other images.

   Note - If you shoot in Jpeg, instructions are provided with the product for post-process image correction in Adobe Photoshop.

There is obviously a question as to which method of these two methods is best. The ExpoDisc product manual doesn’t answer this question, but a representative at Expodisc expressed this opinion:


We recommend that users do a CUSTOM gray/white balance, "pre-process," in their camera right before taking pictures in any given scene/lighting condition. This is the best, most accurate and efficient way to use this tool. We realize that is not always possible with certain cameras or when time doesn't allow. If that is the case, then you can still do it in "post process" in the computer by photographing one frame with the Expo/Disc at any time in that particular lighting condition. You can set your camera to "auto exposure" for this, but NOT auto white balance.

(Read the full text of this message “here”).

In order to test the Expodisc I took a number of test shots of a Gretag Macbeth ColorChecker Chart chart under different lighting conditions. For each lighting condition I captured a Gray Reference image, an image of the MacBeth chart with camera white balance set to Auto (to adjust according to the Gray reference), a Custom White Balance, and lastly an image of the MacBeth chart with camera white balance set to Custom. I had an unambiguous way of testing that white balance was adjusted correctly, because if it was then the CC20 patch (2nd from bottom left) on the Chart shouldn’t cause a white balance change in the image when selected with Adobe Camera RAW’s white balance tool.

My recommendation from these tests is also to use the Custom White Balance over the Gray Frame Reference. Neither method is perfect, but it seems to me that there are additional inaccuracies in the Gray Frame Reference method which arise from having to set white balance correctly to the scene in-camera as opposed to leaving it set to Auto for Custom White Balance.

Conclusion

If you shoot Jpeg I would say that the ExpoDisc is a useful tool. Bear in mind though that in order to work most effectively one must capture reference images while pointing at the source of the light that falls on the image, and this often means standing where the intended subject is and facing back towards the intended shot position (something there often isn’t time or opportunity to do).

At some point since purchasing this item I made a transition to shooting RAW exclusively and am perfectly happy adjusting white balance by eye in Adobe Camera RAW, synchronizing betwen images where necessary.

I do use the ExpoDisc for one thing though, and that is to take a reference image and check for dust on the sensor once in a while.