My own test for center filter eveness involves a bit of overkill. I'll expose a completely even mid gray
suface like a gray card, give the sheet of film significant plus development, then contact it on hard
grade 4 paper or max density VC. One thing I discovered this way is that the 82mm center filter I
bought long ago for my 120 Super Angulon has virtually perfect density distribution on my Nikon
90/4.5. But once in awhile I deliberately want a non-spec CF for a little different creative usage.
For commercial architectural shots I always always had the correct CF on hand.
I talked to Dr Mark about the filter and he never uses one. Not all his imagery is portraits and as such he will plant a 4x5 in the center of a room and use natural light and long exposure. I have printed a lot of these negatives and never felt there was an issue on the edges due to fall off.
I would vote not to spend $700 on the filter but buy $700 of Ilford Warmtone Paper.
i'd not use any camera movements and shoot straight on for a while to see if i could get away without the filter
and as a last resort buy it. 700$ is a lot of money ...
There are two kinds of vignetting, mechanical, which is a problem of the lens barrel cutting off the off-axis light at large apertures, and natural vignetting, which is a product of the angle the light strikes the film. Center filters deal with natural vignetting and so whether you shoot at f/11, f/16, or f/22 etc., it does not matter, the center filter would be needed.
I have never used a center filter with a 90mm as I did not find the fall off objectionable nor uncontrollable. A center filter really helps a 72mm lens.
BTW, correcting for fall off in the darkroom is not always idea--the fall off is underexposure and there is so far you can push underexposure in printing.
I would avoid Heliopan center filters as they are really not well made--mine had air bubbles in the glass and the warranty replacement process was a nightmare. I have been very impressed with Schneider center filters.
Centre Filters are things of beauty
To give perfect correction you would need one tailor made to each focal length and as has been noted here, that gets very, very expensive.
Even on B&W Negs using a 90mm you can see the difference – more so as you push towards the edge of the image circle.
The wider the lens, the greater the need for a centre filter.
My experience has shown near enough is good enough and it is worth buying lenses that can use just the one common filter.
Personally, I would buy the lenses that could use just the one Centre Filter and then try them without buying a Centre Filter. If, when you look at the results and you decide you need a CF, then buy one for the lens you consider most needs it (probably the shortest) and then try again, on both lenses.
If, in the end you decide you need an exactly matched Centre Filter to each lens, at least you know you have made the decision yourself and have not be suckered into it by the weasel words of the advertising men.
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I don't think you need to worry about any evil conspiracies from "advertising men" concerning center filters. Not enough of these are sold
in the world in any given year to make anyone rich. These things work.
Do your homework, and do some personal testing to see if your kind of shooting warrants this. But large format film, esp color, ain't cheap either; and a center filter can save a lot of headaches or lost opportunities. Most decent lenses cost more than these things. If you need one, you need one.
Thanks for the follow-up, Bob - $700 does indeed seem even more expensive when you think about the paper and chemistry to can buy instead.
Thanks to everyone who responded. Some really good food for thought. I think, on balance, what I will do is get the lenses and run some tests, make prints etc. Then I'll be in a better position to decide whether I need to spend the extra $ or not.