Created almost six decades ago by George Wallace (a student of Ansel Adams, Minor White and Edward Weston), the ExpoDisc is destined for the measurement of exposure and lately for monitoring and correcting the balance of white.
Presently the White Disc has become a very useful accessory in manually determining the white balance in a shooting location. This is helpful especially in the very difficult situations in which several sources of light, with different temperatures of color, contribute to enlighten the scene.
The ExpoDisc is made of two glass or plastic blades. The blade which looks towards the subject is made of a series of prisms which overtake the light from a certain solid angle, on its back being a second opaque, white blade.
The idea is simple: the light or the lights falling on the subject are mixed and after that offered to the integrated expometer in the camera for the global determination of the white balance.
With the ExpoDisc mounted, your digital camera is transformed into an expometer and colorimeter of the incident light. This means that you will have to measure the light that is falling on the subject and not the light reflected by it.
Here are the major steps to insure a right exposure of white:
1. After you have chosen your subject you wish to photograph, identify the sources of light and the directions these are coming from.
2. Install the ExpoDisc on the one of the objective; the installation is done on the port filter ring through pushing – the ExpoDisc is equipped with three stoppers that keep it in the position – and set the focalization on manual.
3. Set the white balance of the camera through the “custom” mode (the setting modality depends on one pattern or another, but in general the WB button must be pushed and through the selection wheel you can navigate in the Custom mode).
4. Situate yourself in the immediate vicinity of the subject and focus the objective towards the sources of light. The orientation of the optical axis is essential in the process of evaluating the white balance.
The optical axis oriented to low or two high leads to the modification of the proportions between the sources of light and in conclusion it can alter the evaluation.
Most of the times it is enough to visualize the place where you will be in the moment you will take the photo. Now you can take the shot. You have obtained a frame which some cameras need as a witness for the white balance.
The frame used to adjust the white balance must not be exposed; it is good to verify the image histogram which must not reach the extremes. Establishing a personalized balance of white may differ; depending on the model of the used camera (see your camera documentation).
5. Select the last image as a witness for the white balance.
6. Detach the ExpoDisc of the objective. Switch to auto focus if you need to work in this mode and start shooting the desired subject.