Sigma USB Dock quick review
Sigma Optimization Pro Software (version 1.0.0)
The USB dock doesn't do anything on its own - it's merely the interface for programming the lens using Sigma Optimization Pro. This software is for the most part pretty straightforward and intuitive (in marked contrast to the 'Sigma Photo Pro' raw converter), although it does have some 'version 1' oddities. It enables firmware updates and detailed AF microadjustment with all supported lenses, and further tweaking of focusing and image stabilisation behaviour with the 120-300mm F2.8.
|This is the opening screen when you attach a lens to the dock. The software remembers all of the lenses that you've used with it, including their serial numbers and current firmware version. You get two options - Firmware Update and Customization.|
Sigma provides a fairly comprehensive software manual in the form of a fully-indexed PDF (accessible from the 'Help' menu), which explains what all the buttons do and how to change your lens's settings. But, as tends to be the case, if offers little practical guidance about why you might want to change the settings. For example, there's no attempt to explain how to identify whether a lens is really misfocusing, and the process of setting and verifying an AF microadjustment value to compensate.
|Click the Firmware Update button and the software looks for a newer version, which you can then download and install on your lens.|
This function allows you to update your lens's firmware, something which users of mirrorless systems have got used to, but up until now hasn't been an option for owners of the most popular DSLRs which use film-era lens mounts. In principle it means Sigma could release firmware to enhance AF or OS performance, and provides a degree of 'future proofing' against future technology developments (or changes in camera manufacturers' communication protocols).
In a way the 'Firmware Update' button is actually redundant - Sigma Optimization Pro goes and look for the latest lens firmware every single time you attach a lens to the dock (as well as the latest dock firmware at the start of every session). If you're swapping the lens between dock and camera to set and check AF microadjustment, for example, this quickly becomes tedious - we'd prefer it only checked once per software session (at most).
Click the Customization button and you get another screen with two buttons. The first, 'Focus Setting', is available for all lenses and allows detailed AF microadjustments. The second, 'Customization Mode Settings', is currently only available with the latest 120-300mm F2.8, and allows further tweaking of AF and OS behaviour.
If we have a reservation about this, it would be that the process is rather long-winded. You have to establish whether your lens tends to focus in front of, or behind the subject, then plug the lens into the dock, enter a correction value, and test whether it's better on the camera. You'll probably need multiple cycles of trial and error to get it all right. With in-camera adjustment, the process is a bit easier - just as long as your SLR has the option.
One other less-obvious point can be seen from the screenshot above - the various correction distances are preset, and for the 18-35mm, rather grouped-together in the close focus region (0.28, 0.35, and 0.5m). Presumably Sigma has good reason for doing this, but if you find your lens has a specific problem shooting environmental portraits at 2m (for example), it's not clear what you'd do about it.
The 'Rewriting' button
One quirk of Optimization Pro - likely the result of a dodgy Japanese to English translation - is that the button used to record new settings to the lens is called 'Rewriting'. If you haven't made any changes it's greyed out, but the moment you enter a microadjust value, the text turns white. At this point the software looks like it's rewriting the firmware automatically, so you'll be terrified that you can't remove the lens from the dock without breaking it. The problem being, it never says it's finished.
In reality the software isn't rewriting the lens's firmware at all, and will only do so if you actually click the button. At this point the software behaves quite sensibly again.