The ZoneView is a simple tool to help you visualize the image before you click the shutter!
1. While the ZoneView provides a similar spectral response as the Wratten #90; it does allow a bit more blue light to pass through. (We felt that the Wratten filter eliminates too much blue, resulting in artificially high contrast.)
2. The filter material is not “optically clear". (We don't expect anyone to be photographing through it.)
3. It is a lot cheaper!
Disclaimer: the ZoneView will not instantly make you the world's greatest photographer. As with any tool, it requires practice to master, but once mastered, it can make you a better photographer (at least $9 better).
To keep the cost down, the frame is made of cardboard (with a water resistant coating) and the filter material is plastic. It's designed to last for months, maybe even years, if you're careful with it.
Here's the User's Guide: ZoneView