The Sigma USB Dock ($59 direct) is an inexpensive accessory that allows you to take full control over the performance of your Sigma lenses. It's compatible with recent glass that's part of the company's Global Vision Line; you'll know that your lens is part of this generation by its Contemporary, Art, or Sports designation. It's available for lenses that use Canon, Nikon, or Sigma mounts. At this time that only includes five lenses; and only one of them, the 120-300mm F2.8 DG OS HSM is compatible with the full breadth of customization options.
Other currently compatible lenses include the 35mm F1.4DG HSM, 17-70mm F2.8-4 DC Macro OS HSM, 30mm F1.4 DC HSM, and 18-35mm F1.8 DC HSM. The software is compatible with OS X and Windows. All of the lenses support firmware updates and focus adjustment. Firmware updates are pretty self-explanatory, but focus adjustments take some understanding of how a camera's autofocus system works.
Essentially, light is bounced from the mirror to a dedicated autofocus sensor. In an ideal world, every camera from a specific manufacturer would be perfectly calibrated, but the world isn't ideal. Users who demand critical performance from lenses often encounter back-focus or front-focus, where a lens is focusing slightly in front of or behind the subject. If you've got a focus issue with your lens and body combination you can adjust the lens to correct it. Some higher-end bodies support in-camera focus adjustments, but they don't go as far as the software that works with the dock. It can adjust at different focal lengths and focus distances.
The rest of the functions are currently only compatible with the 120-300mm at this time. There's an OS setting that lets you adjust the way the lens stabilization system works. There are three modes: Dynamic View, Standard, and Moderate View. Dynamic View shows the effects of the stabilization system in the viewfinder, moving elements around as quickly as possible to capture fast action. Moderate View slows down the stabilization system, and is less jarring when you're taking your time to compose a scene. The Standard mode is somewhere in the middle, and represents the lens's default behavior.
From the AF Speed menu you'll be able to prioritize speed over accuracy, accuracy over speed, or revert to the default setting, which is somewhere in the middle. Finally there's a Focus Limiter setting that lets you limit the focus range over any distance. If you know that you're going to shoot a sporting event and you know that you're only going to be shooting action from a distance you can set the limit from 50 feet to infinity and speed the autofocus system. This function will only be enabled on lenses that have a toggle switch to enable or disable the focus limitation system, so setting this in the software can't prevent you from focusing across the entire range of the lens when in the field.
The Sigma USB Dock isn't an accessory that every photographer needs or will use. Its functionality is limited based on the lens you're using. If you've got a lens that's not quite focusing correctly, it's an inexpensive way to calibrate it for use with your camera. The more advanced features are most useful with the 120-300mm telephoto zoom, and future long telephoto and macro lenses are sure to benefit from the focus limiter and autofocus speed settings. It's easy to use, though some care should be taken when adjusting focus calibration. At $60, it's not an expensive accessory in the camera world, and it's a good tool to have if you own a compatible Sigma lens.
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