B&W filter preview
See your scene the way B&W film records it through a yellow, orange, red, green or blue filter.
Where to find it
Viewfinder B&W icon (Quick Actions) + filter ring or Filters picker
Summary
With the B&W preview on, the viewfinder, the frozen still after metering and the photo saved to the shot log show the scene the way panchromatic film would record it through the mounted contrast filter (Yellow #8, Orange #21, Red #25, Green #11, Blue #47). The filter comes from the filter ring or the Filters picker, the same one the exposure uses, so the meter already includes its factor; the B&W button itself only turns the grey preview on and off. It is a simulation built from Kodak's filter and film data, not a film scan. The B&W preview also follows the film: load a black and white stock (in the wizard, the film picker or a rig swipe) and it turns on, load a colour one and it turns off; it only switches when the film changes between B&W and colour, so turning it on to judge tones on colour film sticks.
How it works
Turn on the B&W preview with the half-filled circle in the Quick Actions and the viewfinder goes grey. Pick a yellow, orange, red, green or blue contrast filter on the filter ring or in the Filters picker and the grey changes with it: the preview shows the scene the way panchromatic black-and-white film would record it through that filter. A red filter turns a deep blue sky dark and makes red brick pale; a green one lifts foliage a little and darkens reds. The frozen still after you meter, and the photo saved to the shot log, use the same rendering.
Switching filters
Change the filter on the filter ring or in the Filters picker and the live preview follows at once: Yellow #8, Orange #21, Red #25, Green #11 or Blue #47. If a metered still is frozen on screen it changes too, so you can compare filters on the same frame. The B&W button only turns the grey preview on and off; it never picks a filter for you.
The exposure follows
There is no separate preview-only filter. The preview shows the filter you mounted for the exposure, so the meter has already added that filter's factor: put the red #25 on and the exposure opens up by its three stops. Previewing red means shooting red. When you take the filter off the lens, set None so the meter stops compensating. In the B&W Filter Advisor, Preview in viewfinder does the same with the filter it suggests and turns the B&W preview on for you.
ND, polarizer and custom filters
An ND or a polarizer has no colour to preview, and a custom filter only carries its stop count, so with any of them mounted the preview stays the plain grey you get with no filter. The app works with one filter at a time: picking a colour filter replaces an ND or polarizer you had selected. If you stack a colour filter with an ND, add the pair as a custom filter to get the combined factor, and expect the neutral preview.
How it's worked out
Each colour channel of the camera picture stands for a band of the spectrum. From Kodak's published transmission tables for the Wratten 8, 21, 25, 11 and 47, the spectral sensitivity of Kodak T-MAX 100 and Tri-X 400, and daylight, the app works out how much of each band the filter still lets through to the film and reweights the grey conversion to match. A grey card stays the same grey, because the meter has already paid for the light the filter takes. What you see is tones moving against each other, not a darker picture.
It's a simulation
A phone sensor is not film. The preview gets the direction and rough size of each shift right, which is what you need to choose a filter, but it cannot bring back colour the camera clipped, and very saturated colours can move a little more or less than they will on the negative. Yellow and green change the picture least: on panchromatic film they mostly pull tones back toward what your eye sees, and the no-filter preview already starts there. Use it to judge the effect, then trust the meter for the exposure.