Here is the quick summary: the firmware update for the Canon mount Sigma 18-35mm f/1.8 (2014-8-22) did more than add support for the EOS C100, as claimed by Sigma. It also changed the lens ID from 137 to 150, and seems to have improved auto-focus accuracy, although not precision. Update: The auto-focus is as good as a Canon EF 85mm f/1.8.
I got one of Sigma’s 18-35mm f/1.8 lenses to go with a new Canon EOS 70D over a year ago. At a New Year’s event with a local band that includes a neighbor of mine, I took a bunch of pictures with this combination. The result seemed to work pretty well in spite of the low light at the outdoor venue. However, I have found that many images I have taken with the lens since then have not been in focus when using the optical viewfinder. This is a fairly common problem with Sigma lenses, although it doesn’t account for its early good performance. I’m guessing that at the New Year’s event most of the pictures had a deeper depth of field from focusing far enough away.
To deal with the problem, I got one of Sigma’s dock gizmos. I printed out a focus test page and made a table that I filled out with the adjustments needed. Every time I used the dock, the Sigma software asked about updating the lens firmware. I always refused because Sigma claims they just added support for a camera I don’t have. I don’t like to update things unless the update is actually beneficial. It is a way of limiting the chances of dealing with an update that breaks something.
The attempt at improving auto-focus didn’t go well. I got very contradictory results from two attempts, each starting from no adjustments, and neither improved the results. I figured the paper at a 45 degree angle was to blame, so I built a focus target out of Lego. When I cleared the adjustments on the lens before testing, I decided to update the lens firmware just to keep the software from incessantly asking about it. Every test I did with the Lego target suggested the lens was fine. Some tests with more common subjects suggest the accuracy is decent, but the precision still isn’t as good as with Canon lenses, so it is still a good idea to take several pictures and review them.
The software I use for keeping track of my photos, Digikam, did not at first identify this lens. Instead it called it lens 137; it has since been updated. I think the Canon protocol uses an 8-bit unsigned integer to identify the lens model, although now additional information like the focal length range is needed to identify a specific lens model. Since I updated the firmware, Digikam identifies the new images as being taken with lens 150.
I don’t know if this change from 137 to 150 is needed to make the lens work with the EOS C100. It is possible that the change will affect how the lens and camera work together. From what I’m seeing, it has a favorable effect on auto-focus performance with my EOS 70D. I don’t know why Sigma wouldn’t mention this, and I really don’t like the short list of changes common in the photographic industry for such updates. I have suspected that some changes are omitted from the public list of changes, and my experience with Sigma’s 18-35mm f/1.8 lens deepens that suspicion.