I have sausage fingers and often shoot in cold and wind. I have a 77mm circular polariser and rings that I leave on my lenses that step them up to 77mm. I find screwing and unscrewing the polariser an absolute nightmare. Sometimes it sticks. Sometimes my sausage fingers are frozen stiff. Sometimes the adapter ring comes with it etc.
I've been looking at filter systems, and the Lee system looks nice, but really I have no interest in nds, nd grads or any other rectangular filters, and the cost is not insignificant when I factor in the holder, the piece to mount the polariser and adapters for my lenses. Plus I lose the use of my lens hoods that have saved my lenses on countless occasions, with the alternative to pay another whack for a compatible Lee lens hood. This seems like a long way round to make swapping polarisers quick.
There has to be a better way. Is there a simple way of quickly swapping a circular polariser between lenses (of different filter diameters)?
There is a solution that involves using magnetic adapters, but this only works with sharing filters between lenses of the same diameter. And I'm not sure how sensible it is putting powerful magnets next to a lens's circuitry or how safe they are.
Is there a simple way of quickly swapping a circular polariser between lenses (of different filter diameters)?
The simple solution is to have a polarizer for each lens so you don't have to swap them in difficult conditions.
The cost-effective solution is to do as you are currently doing and use step down rings.
Magnetic filter holders and lens adapters, such as those offered by Xume, would be a little more cost effective. But they would also present many of the same problems that you are currently experiencing. You would still need to use step-up rings on your lenses smaller than 77mm. In the adverse conditions described in the question, they may be too difficult to properly align and your expensive Singh-Ray would probably be more prone to accidental damage from unintended separation.
You have to choose one or the other. You can't have convenience and economy at the same time when using filters on lenses of different sizes.
I have used the Xume adapters. Awhile back I standardized on 77mm filters, and also found myself swapping them around all the time. I thought the Xume would be a perfect solution. When I bought them, they did not have a dedicated Xume-compatible lens cap, so I "made my own" with old-school screw-on/off caps and dedicated Xume rings for them. I also put Xume rings on both the front and back of my filters, because I prefer the screw-on/off filter stack caps to the plastic filter boxes, and I definitely don't like the "padded" cloth filter pouches.
While I loved the idea of the Xume adapters, because I added a "lens ring" to the front of my filters so they would accept a magnetic cap, I had noticeable vignetting even at 35mm with just one polarizer (a relatively thin one at that). Plus, my stack of 3 filters (with stack caps on both ends) went from just under 1" thick to almost about 1.75" thick.
I never worried about magnets near a lens's circuitry. Static magnetic fields near circuits is not a problem. Magnetic fields near magnetic storage media (like hard drives) is another matter, and I don't recommend it. But honestly, the magnets in the Xume adapters are not so strong that I would personally worry having the Xumes near an external hard drive in my camera bag.
Regarding the Xume adapters, here were my biggest worries:
Knocking them off. The Xume magnets are quite nice, strong enough to feel solid, yet not so strong that you have problems getting the filter off the lens. But I use a sling strap to carry my camera, with the lens pointing down. And a couple filters I own are quite heavy. I was always concerned about just the right impact on my camera knocking the filter off. It was probably an unlikely event, but it always made me nervous.
Dirt. You don't realize it, but there's a lot of iron in most dirt. The last thing I want is to set my camera down and attract dirt to the filter (or worse, lens). And once iron-laden dirt is stuck to the magnet, it is very tedious removing the particles. They can't be simply wiped away - you have to be deliberate to scrape, tweeze, or otherwise pick the particles from the magnet. I used to have some Zen Magnets / Buckyball desk magnet balls (size of BB's) on my desk at work. After dropping them enough times on the carpet, little bits of dirt were stuck to most of them. It was almost impossible to get all the dirt off.
Buy a filter for each lens. If your budget is tight put the best quality you can afford on the longest lens. Cheap shows up most under magnification.
I know what you mean about sausage fingers. And in cold / wet you are more likely to get finger prints on both sides of the filter as well as rain between the filter and the lens.