The ExpoDisc (4x5.65") brand_Lensmate The Sennheiser MKE400 is a

The Sennheiser MKE400 is a compact

or 1000 (RIP)

it also has a fast f4 constant aperture and Canon’s premium L-series optics

one megapixel sensor

The ExpoDisc (4x5.65") brand_Lensmate The Sennheiser MKE400 is aThis particular Expodisc is not a disc it's a big 4x5. 65" slab designed to slip into matteboxes but it works the same as the classic. Basically, it's a specialized flat diffuser designed to facilitate setting custom white balance. Rather than hauling around a gray card, or trying to find a plain white surface, or hoping that your camera guesses correctly, you can toss this in front of your lens and lock in an accurate custom white balance in seconds.