Question for Schneider 75mm Super-Angulon + Center Filter Users

[nlambrecht]

Recently I came across a Schneider Center Filter IIIA at a very good price so I bought it to try out on my 75mm f5.6 Super Angulon (I know this filter is for the 90mm f8, but I gave it a shot). After doing some tests I found that the fall off has been eliminated on the lens, but my image circle has been reduced. Schneider says at f22 the lens has an image circle of 198mm, with the IIIA filter attached it has become a 178mm image circle.

What I would like to know is if any of y'all out there have similar results with the Center Filter III, or I know some of you use the IIIb on the 75mm Super-Angulon. Does your center filter cause a reduced image circle?

If your image circle have similar results with the III or IIIb, I'll just keep the one I got. Otherwise I may be convinced to return it and wait for someone to sell the III or IIIb.

Thanks for the help.

-Nathan

[nlambrecht]

Anyone know someone who I can direct message about this topic? I know this is a rather specific question, but any help would still be greatly appreciated.

-Nathan

[George Hart]

Nathan, I have this lens and the CF III, and there's no impact on image circle. Given that the CF should be much larger than the front element of the lens, it's difficult to see how this could occur with the recommended filter. Can I ask what the front mount thread size is for your filter? The CF IIIA is not listed in my Schneider CF catalogue. One possible explanation is that the CF IIIA may have a smaller front thread size, capable of vignetting with this lens. The CF III has a front thread diameter of 86 mm.

George

[StoneNYC]

I want to thank you for something.

I never realized the CF went on the front of the lens... I always assumed that a CENTER filter went in the center... Between the lens cells and the shutter...

So thanks for posting the question as I was confused apparently for years!

[nlambrecht]

Nathan, I have this lens and the CF III, and there's no impact on image circle. Given that the CF should be much larger than the front element of the lens, it's difficult to see how this could occur with the recommended filter. Can I ask what the front mount thread size is for your filter? The CF IIIA is not listed in my Schneider CF catalogue. One possible explanation is that the CF IIIA may have a smaller front thread size, capable of vignetting with this lens. The CF III has a front thread diameter of 86 mm.

George,

Thanks for the reply. The CF IIIA has a 67mm rear thread and a 86mm front thread, just like the III. The front thread is in a recessed channel if you want to add additional filters to it. The raw specs seemed the same, so I gave it a shot since I got it on sale from KEH. When I got the filter it seemed thicker than what I have seen in photos of the III or IIIb. I have attached a few photos, including the thickness.

After doing some test exposures focused at infinity with heavy shift and a known point marking where optical center is I could measure that the circle had been reduced to 178mm, rather than 198mm that the 75mm Super Angulon has. In angle of view terms the coverage was reduced to about 93 degrees from 105. This cuts the capable movements of the lens in half.

The good news was gradient of the center filter was dead on. I even did what others tell you not to do and photograph an even toned wall and there was no noticeable color or tone shift.

However, since I like to use a lot of movements this filter is going back Monday. I will just have to wait until someone here sells one at a reasonable price.

Once again, thank you for the reply. If your CF III created a loss of movement similar to mine then I would have thought about keeping this one. I have been back and forth about it for two weeks. In this case if it sounded too good to be true, it is.

-Nathan

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[George Hart]

Nathan, the photos are most helpful and I suspect that you have correctly identified the problem as being the thickness of the IIIA filter, though I would need to do measurements to be sure. Your Fig. 3 gives the thickness of the mount, excluding the screw thread, as ~11 mm, whereas the equivalent measurement on my III filter is 9 mm. I would guess that this could be enough to account for the vignetting you have seen with the IIIA on the 75/5.6 lens.

George