Schneider Center Filter IIIa & IIIb - difference?

[Steve Goldstein]

I found the two documents that mention these two center filters on
the Schneider web site. It seems that both IIIa and IIIb are for the
90/8 Super Angulon (among a few others). The thread sizes seem the
same, too. Does anyone know why there are apparently two different
versions of the same thing?

Steve

[Dave_B]

I don't see the IIIa listed in my brochure but typically Schneider makes two versions of each size filter, a 2 stop one and a 1.5 stop one. For example the two stop version has two stops of difference in intensity between the outside and the center of the filter. For the lenses I have, they say one can use either filter, the 2 or the 1.5 version. The half stop difference seldom matters.
Cheers,
Dave B.

[Steve Goldstein]

The IIIa appears in a scanned "Archiv" pdf on Schneider's web site. The original
appears to have a date code of April 1984. The IIIb appears in a much newer pdf,
also on their web site. There's no date code I can see, but it does refer to the SSXLs
and lists an email address so it can't be too terribly old. Both filters have the same
threads fore and aft, and both are given as 3x. Maybe they slightly changed the
gradient to make the one filter match better to the superwides...

Always curious,
Steve

[David A. Goldfarb]

I've compared the III to the IIIb and the difference is 1/4 stop. I use the III on my 90/8 Super-Angulon, 75/4.5 Grandagon-N and 55/4.5 Apo-Grandagon. There are times when I'd like a stronger center filter, but it's something of a subjective thing, since even the dedicated filters don't correct 100% for falloff. If they did, they would have to be darker than most people would tolerate.

[eric black]

I have been using a III for my 58mm/5.6 XL and even though it is not the Schneider recommended filter the chromes I shoot all look excellent with no apparent light drop off- I would agree with the 1/4 stop difference cited above and IMHO the difference is a non-issue.

[Michael T. Murphy]

I've compared the III to the IIIb and the difference is 1/4 stop.

Do you know what the rating is on the III? I didn't see it listed on teh Schneider web site.

I just got one last week and have been testing with Polaroid.

Thanks!

Michael

[David A. Goldfarb]

I treat the III as 1-1/4 stop. I think that's the same as the IIIa.

[eric black]

testing with my light meter says that its about 1 1/3 stop

[Michael T. Murphy]

Thanks! Never thought of the light meter technique - duh! :p

Polaroid sucks for fine gradations.

[Michael T. Murphy]

Using my Sekonic digital meter I get 1.4 stops. So I changed the ISO by 1 1/3 (+1.33) of a stop to compensate. Less than .1 stop difference between the two, and I am giving +1/3 stop exposure for negative film, so I am safe. ISO for Prtra 160NC is 50 :)

Thanks!!
Michael