centre-graduated filter

Definition: Neutral density filter used to even out light fall-off of wide-angle lenses. * The centre of the filter is darkest and the edge is transparent, with the gradient equal and opposite to that of the light fall-off of the lens, for which it is tailor-made. * Generally needed only at wider apertures.
Expensive bit of dirt
Centre-graduated filter (or centre-grads) are terribly expensive, merited only for terribly expensive lenses but, all the same, they just look like a rather grubby bit of glass, being unevenly sooty and grey. The filters also swallow up at least an entire stop of aperture. Strictly for super-wide-angle freaks for whom e.g. 47mm on 5x4" is standard.

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