Professional photographer Dan Carr recently had a chance to borrow a set of ND filters by Aurora Aperture (edit: the filters were provided by B&H Photo, not Aurora Aperture) designed for the Canon EF 11-24mm f/4L and other Canon ultra-wide angle lenses. These are rear mount filters.
The set of 4 ND filters from Aurora Aperture includes a 3-stop, 6-stop, 12-stop and 16-stop filters. Dan had a level of excitement for the filters, because it seemed like an easy solution to add ND to the EF 11-24mm f/4L. While it doesn't solve the circular polarizer challenges, it seemed to easily solve the ND issues in a nice and compact package.
However, Dan found softness issues shooting with the 12-stop and 16-stop filters. You cannot use AF or Liveview focus with ND filters this dark. It's standard practice to focus your shot and then add the filter and shoot. The 3-stop and 6-stop filters will allow enough light through to focus the show with them equipped.
The 12-stop and 16-stop issue as explained by Dan:
It was immediately noticeable that the shot was soft. So soft that I didn’t need to zoom in on the image to see it. At first I thought that maybe I had nudged the focus ring during the lens removal procedure, but after 4 or 5 more attempts at the same routine, I couldn’t get a sharp shot. For these tests I was shooting the mountains in my local valley, with the focus being set on extremely distant features, basically at infinity.
I also tested the 16-stop filter and had the same problem, so that ruled out a single faulty filter. Read the full story
If anyone else has used these filters and experienced the same problem, please sound off in the forum.
Update: Aurora Aperture has acknowledged that some photographers are experiencing this issue, and they are investigating the cause. We'll update if we hear more.