I have a genuine Kodak Series VI Wratten 23A filter, which is redish-orange. And I have a Harrison Series VI YL3 (K2) filter, which is yellow. Well, I just told you everything I know about filters. So I don't know what the effect of these filters would be - as to their effect on B&W and the number of stops they drop.

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A color contrast filter should work just fine. Yellow -1f orange -1.5f green -2f. I personal rate Tri-X at 250 ASA.

I have the retaining ring, and a thin black threaded ring which the manual suggests may be the "adapter ring insert". That threads into the end of either the lens or the retaining ring. It appears the filters are not threaded, but are just dropped in and are held in place by one or more of those rings.

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Since the fastest shutter speed on the Medalist is 1/400 second, I would like to find a 2-stop ND filter that would let me use Tri-X as though it were ASA 100, and open the aperture a bit even in bright sunlight. I think there was actually a ND4 Series VI filter made back in the day, but I've had no luck finding one. Anyone know where I might find one?

Alternatively, if anyone has used some other brand of filter successfully, it would be helpful to know which one fits.

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Sherman, you're understanding is correct regarding how the series filters attach to the Medalist - you can just drop in a filter and screw on the retaining ring to hold it in place. There is also another type of retaining ring that you may not have, which has additional threads and space for another filter on the other side, and these allow you to stack multiple filters and/or add a lens hood.

There definitely are Series-VI ND filters out there. This for example is a 2 stop ND by Tiffen that I picked up somewhere:

Lens calculator

The ND and other type series filters can often be found on eBay (try both "series-VI" and "series-6" in your search), and certainly at camera shows and swap meets if you are lucky enough to have one in your area. Being old and un-glamorous, series filters often go unnoticed in the bottoms of bins with other old camera "junk" since a lot of people don't know what they are or how they are used. And, sometimes you can get them quite inexpensively because of this

If I can't find a Series VI ND filter, I thought I might be able to use a threaded ND4 filter of another brand - somewhere between 41mm and 43mm. My 23A is 41mm, but it looks like a somewhat larger size would still drop in. 43mm seems to be a common size for modern cameras. 41mm - not so much.

Another online source that comes to mind for series filters, adapters, retaining rings, and so forth (although certainly not at a discount price), is Songofsnow.com. Their series VI page is here: http://www.songofsnow.com/Series-VI-6-s/65.htm, although there are no series ND filters there at the moment.