you can effect a gradual darkening toward the center of an image in Photoshop, p erhaps replacing a center filter for most applications. The steps are: select gr adient tool, go into its options, select 'radial, foreground to transparent', an d play with the grad tool. you can select different colors, and opacities. I've evened out a number of half to full stop differences with no problem, and I've r eally only been working with it for a few hours - with more experience, I'd be h ard pressed to see the need for a center filter.
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