The ExpoDisc (77mm)  type_Off Camera Flash which utilize a square 158mm

which utilize a square 158mm board

though compatibility and results will vary from lens to lens

Nikon Nikkor-W 135mm f/5

with approximately 28k actuations

The ExpoDisc (77mm) &nbsptype_Off Camera Flash which utilize a square 158mmThe ExpoDisc is a clip on (or hold in front of) 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. Bonus: if you actually use this on a lens with a 77mm filter thread, it can replace your lenscap. Yeah, its just a fancy