As a new 4x5 format photographer working on both a financial and energy shoestring, I am a little daunted by the idea of needing to use a center filter for wide angle lenses. I am looking for feedback from anyone who has used John Walker's "pnmctrfilt" digital soulution or who has a position on its efficacy. Here is what he says about the problem and his solution:
"With the wide latitude of present-day film and the colour (or grey- scale) depth of film scanners, it's possible to simulate the effect of a centre filter after the fact by applying an equivalent transform to a raw image taken without the filter. pnmctrfilt applies a centre filter transform to a grey scale or RGB image, allowing you to specify the density, power of the density curve, image circle radius, and optionally shift the centre of the filter to compensate for camera movements."
For those of us working digitally, once we get the chrome or negative this looks to be a real viable solution.
You can find more information at: http://www.fourmilab.ch/netpbm/pnmctrfilt/