markhamwong

Checking Filter Quality
You can check the flatness of any filter by looking through it with a telescope or one side of a pair of binoculars. The image should look unchanged, but even the most insignificant lack of flatness will look weird ort change the focus when magnified.

Hoya has been the world's largest maker of optical glass for decades. Not that anyone goes on record about this, but as far as I know, everyone, including Nikon, Canon and Leica, use some Hoya glass whenever they need certain types that they can't get elsewhere. Hoya also makes special-order glass when lens makers ask for it. Optical glass making involves a lot of trade secrets, so if Hoya makes something someone needs in a lens design, even if they know exactly what the glass does, they still can't make it themselves.