So, what is a B&W viewing filter? In reality it is a Wratten #90 monochromatic filter. The gel version of these filters are available from the usual photographic suppliers. They are a gelatin filter and fragile. Several companies manufacture monochromatic viewing filters. Tiffin has a version for B&W, but we prefer the Zone VI filter. It is rugged, mine is over 15 years old and still going strong. The fragile filter is mounted between glass and encapsulated in a round molded plastic enclosure. The filter comes with a cord so it can hang around your neck ready for use at any time.
So, what does the B&W filter really do? What it does not do is turn a scene to B&W. The filter shows you the approximate tonal relationship between different areas of a scene as recorded by B&W film. It will show you if there are any mergers of tone within a scene. For example, the filter will show you if a building will tonally merge with the sky, thus indicating the use of a filter to darken the sky for separation. It will also give you a good idea of how different colors will look in B&W. I have found that nice little green moss on a tree trunk will merge with the bark and disappear. The viewing filter will show you this with just a glance. Once you get used to using this handy, yet simple little device, you will not want to be without it.
The Zone VI version also serves as a framing device. The 4×5 version has the same proportions as a 4×5 or 8×10 negative. By learning to hold the filter at the correct distance from your eye, you will soon be able to approximate what focal length lens is required on your camera for a specific framing.