Among many other things that are fading away with film is the viewing filter. The Kodak Wratten #90 has long been the standard, though as a discontinued item, it is getting rare and expensive. The Zone VI mounted filter is long gone. If you get moving, you can still pick up the Tiffen Viewing Filter #1 ($40), which is a Wratten #90 laminated in glass and mounted in a phenomenally nice metal holder made in the U.S.A. (you cannot say so much for the velcro pouch). It is also only marginally more expensive than an unmounted #90.
If you read the casual descriptions, a “viewing filter” is something that “converts scenes to black and white.” That’s not exactly true; such a filter uses a dark color so overwhelming that your eye cannot easily discriminate the colors in a scene. The #1 filter, designed for black and white photography, is a very dark brown. It purportedly shows you a “normal” film response, which is something arbitrary (the look of a film really depends on your film and developer).Viewing filters come in other varieties and filter colors: they are (or were) also made for low- and high-speed cinema films and chroma key work.
But at a minimum, the device does show you where certain dark tones get muddy and where the highlights are. This in itself makes such a filter worthwhile – at least as a warning device. You can stick your black-and-white contrast filters in front of it (for example, a green filter to correct incandescent light), but it only works to a point – objects of complementary colors do indeed darken, but your eye quickly adjusts to acquire whatever color information it can, however weak.
As to the ready-made unit vs. unmounted gel issue, you might want the unmounted gel if your goal is to implant this filter into an existing accessory viewfinder. A Wratten gel is optically insignificant in terms of distortion, and because it is moisture-sensitive, it benefits from being inside a viewfinder unit (rather than the outside). A ready-made unit will be more durable and resistant to abuse, though it is just another thing to haul around (though you could attach it to the strap for your light meter).
Are alternatives available? Of course. You could go through a $2 Roscolux swatch book until you found something with a similar effect (though it might be a different color). Or you could find a set of old-school, bottle-brown sunglasses – that though not quite as dark as a #90, are quite helpful for visualizing black and white. And if you want to be truly perverse, you could set your iPhone to its black-and-white filter and use that as a visualizer.
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