Centre filter overcompensation?

[Filmboy]

A question about the Centre Filter:

You apparently need it to compensate for light fall-off around the edges of the image when shooting on a 45mm lens, at small apertures, especially on transparency film.

But if you just leave it on the lens (I seem to remember somebody jokingly advocating superglue) won't that result in overcompensation when shooting, say, negative film at F4? So you'll get the same problem in reverse? (Darkened centre, rather than the—often quite attractive—vignetting towards the edge?)

Just wondering... not sure I want to spend, what, £170 on something that's going to compensate for a problem so much that it causes a new problem!

 

[David Goldfarb]

No, a center filter wouldn't cause overcompensation in that way. A center filter will be optimized for about 2-3 stops from wide open usually, but that doesn't mean that it overcompensates when wide open. In fact wide open, you would need more compensation, not less.

Most center filters don't actually compensate fully for falloff, because they would require more compensation than most photographers would tolerate. If you had a filter that required 2 or 3 stops compensation at f:22, would you use it? Sometimes perhaps, but Bob Salomon from HP marketing mentioned a while back on the LF forum that when Rodenstock offered them, they didn't sell well.

 

[Filmboy]

Thanks, I think I get it. The official literature gives the impression that the goal is a perfectly balanced exposure across the negative, but you're saying that the centre filter is more of a subtle cheat than a precise correction. And I guess that it makes more sense, given that the amount of compensation doesn't change, that the filter would be balanced so that its effect would appear across all apertures—but at such a level that the difference between one extreme and the other would be barely noticeable.

Or possibly I've been at work for too long and I should go home and stop rambling! 😀 Anyway, thanks for the reply.