first some background so I do not get accused of product bashing. I have owned and used the ExpoDisc since the canon D30, so about 4 or 5 years. While I found the ExpoDisc useful to some degree, it was not satisfying my need for a White balance reference. That is why after searching for an existing solution, I developed the WhiBal.
The downsides of ExpoDisc in my work (mostly street photography for myself and now some event photography for clients):
Expensive if broken or lost
using it requires taking off lens hood
To gain proper WB reference I must change my shooting position so that I am where the subject is and I must shoot into the light. Not convenient or even possible in most of my work
if you shoot from camera position aimed at subject an inaccurate WB ref will occur as ED is now measuring the subject as well as the light. Does not work, no matter what anyone says. Simply try it.
When working in the RAW converter, the ED ref shot is a gray blob with no context.So I have no way to know if the ref shot is for the previous series or the next series or how I shot it (toward light, subject, etcetera. Absolutely no context.
unless special exposure compensation is made, the reference is too dark for optimized use with any raw converter.
What situations I use ED in even today where Whibal will not work:
Wide angle room shot with mixed lighting.I take from camera position and get an average WB ref that will serve as a good starting point. WhiBal cannot do this.
I think that the virtues of WhiBal from my mouth are well known, but in simple terms the features over ED in 95% of my shooting:
VERY hard to damage
provides faster and easier usage in my work, wither with or without models involved
Each WhiBal reference shot shows the context of the conditions and placements of the capture
provides Black and White ref points
Light gray card optimized for use with Raw converters
Please let me know if you need more details about any of the above. The point is simple. If the ExpoDisc served my White Balance needs, I never would have created the WhiBal. it was not created as a business venture. it was created as a tool for me, and being the entrepreneur that I am I figured that if this is useful for my needs (which are not unique) then others would also find it useful. This has come to be true, and I thank everyone for their support.Expo
Here is something that I posted on DPR...
I am not here to knock Expodisc. I own a few, and have found them useful at times. But they are 2 different animals. The Expodisc has to be used at the subject position so that the light source is shining into it. Why.....Because if you aim it at your subject and the subject is wearing a red shirt, the expodisc will think that the light is more red which it is not. So the reality is that the Expodisc is incident and must be used at the subject position pointing back to where the camera will be, and the WhiBal or other gray card is refelctive and the subject holds it and the camera reads exactly the reflected light that is hitting the subject and the WhiBal.
So if you can use the incident Expodisc, that is fine, but if you, like me find that inconvenient, a Gray card is accurate and the Expodisc is not because it is influenced by the color of the subject. But at times it can be useful, like taking a picture of a Ballroom with many light sources...the Expodisc will average them while a gray card in a specific spot will only give you the light in that spot.
So both good tools if they are used properly.
And as pointed out I did not put it into my comparison because it cannot be measured by a spectrophotometer.