Re graduated neutral density filters (post #1), I asked Lee Filters about hiring someone to make a 112mm adapter ring for its SW150 Holder. If I decide to go this route at some point, I'd engage S.K. Grimes to do the work given what Lee's reply says. This is from Lee's U.S. office:
"So i just heard back from the UK and they let me know that the performance of the holder on a custom ring would really depend on the design of the ring and the expectations you have for the holder.
"In theory you could have a 112mm thread with an extension out in front to mount the holder, but you likely won't achieve the wide angle performance of the LEE Adaptor Rings. This is due to relying on the ability to position the holder over the lens, which puts the first filter as close to the lens element as possible. Based off the info provided you wouldn't be able to mount the holder in the optimal position, which may cause the holder to be visible in the image.
"Ultimately it could work but we cannot guarantee it due to it not being a 3rd party product."
S.K. Grimes has made filter holders in the past, and it might make sense to ask it to make one of its own design that will work with 150mm wide filters.
There are some discussions on the internet that have convinced me to also try hand-holding a graduated ND filter, which appears to be a more common practice than I would have thought.
I think that it will be useful to spend some time learning how sensitive the lens is to vignetting when the centre filter is in place. With the lens by itself having a filter diameter of 95mm (3.74"), the centre filter adds a further 17mm (0.67").
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