Different Centerfilters for Super Angulon XL 90mm

[cyron123]

Hello, at the internet site at Schneider Kreuznach are two Centerfilter for the super Angulon XL 90mm. One with 3 stops and one with 4 stops. Called IV A and IV B.... What is the difference? What is the better one..? Thank you.
Best regards Cyron

[Dan Fromm]

Go here http://www.bhphotovideo.com/c/search?atclk=Brand_Schneider&sts=ma&N=4291231926&Ntt=nd+center+filter and read carefully.

[cyron123]

Thank you Dan.
I have talked to Schneider by phone: both filters are possible. The harder filter have more correction in the corners than the lighter one. The lighter has already some black corners. If anybody is interested...

[Old-N-Feeble]

Keep in mind what other lenses might use the same filter. If one requires a 3-stop filter and another is 'best' with a 4-stop filter and you want to share the filter for both lenses then I suggest getting the 3-stop filter since, IMO, a bit of undercorrection is better than overcorrection.

[Corran]

The 4a is for the 90mm XL while the 4b is for the 72mm XL, according to Schneider. I am confused if Schneider implied the 3x one would not fully correct the 90XL, when that's the lens it was designed for, rather than the 4x. By the way, don't confuse the filter factor with the # of stops for correction.

There is a similar connection between the 47XL and 58XL. I have both but use a 3x CF for both. It doesn't quite correct the 47XL but that's not a huge issue (half a stop). I will probably get a 4a someday for my 72XL and 90XL.