Do report on your experience with the glass center filters.
Of course. I definitely want to put back into this forum, given how much I've gotten from it.
I don't understand the point of the centre filter with a large image circle lens like the 43XL. Straight shots with no movements no problem but as soon as you move the lens away from the centre of the sensor the filter doesn't match the fall off pattern and are you not just back to digital correction?
The fall-off is a function of the lens. Since the filter sits on the lens and moves with it as it's shifted, then the filter by default compensates for the fall-off pattern. Yes, the fall-off gets stronger as you shift because you are by definition reaching the outer edges of the IC and also because of increasingly oblique light angles exiting the lens (i.e. potentially leading to mechanical vignetting and compounded by photons having to squeeze into pixel wells at these steep angles). But the general
shape of the fall-off remains the same. With shift, the filter becomes less efficient at compensating for the
amount of fall-off, but it's still better than relying solely on software for pushing exposure in dark corners. The filters are not perfect but they begin to bring the edges and centre of the IC closer in exposure values.
The filters are optimised exposure-wise for zero movements as a default base value, but this does not negate their usefulness. Software can help get us the rest of the way towards a more evenly exposed frame.
The alternative would be to fix the filter in place, say at the sensor plane. But as you shift the lens then the filter would be useless to compensate for the fall-off pattern because the IC is moving relative to the fixed plane of the filter. It will in fact make matters worse because you will end up with the dark edges of the filter overlapping with the dark edges of the IC (imagine two donuts on a parallel plane being shifted relative to each other).
Centre filters are a fact of life when using super-wide lenses with large IC's, whether we are talking about digital MF or 4x5 film. They are notoriously difficult to produce, having to be image neutral, colour neutral and match the fall-off pattern of a particular lens. I'm guessing Schneider would probably prefer not to have to produce centre filters anymore given their insistence that everyone make use of their PS plugin. Fortunately, enough people yelled loud enough about the 43mm XL. I'm sure there some people insisting on a filter for the 28mm Super-Digitar (I would do the same if I owned it).