Sigma USB Dock review

Sigma has been turning heads recently with a number of new lens releases which have been very well received by users and reviewers alike. I’ve been using the Sigma 18-35mm F1.8 A HSM lens on a Nikon D7000 for the last six months, and one of the first things I did was pick up the new Sigma USB Dock. This is what I’ll be looking at in this review, both in design and function.

How it works

The primary purpose of the dock, however, is to improve the focus performance of DSLR lenses when using phase-detect autofocus through your viewfinder – and this is what I’ll be looking at in detail. If you’ve used a fast prime lens wide open before, you may be aware of the need to fine-tune autofocus to optimise the lens to your particular body, due to the natural inaccuracies of phase-detect autofocus. All cameras and lenses have a range of acceptable accuracy, or tolerance value, when they leave the factory, and this variance can cause inaccuracies when shooting wide open. Most enthusiast and professional bodies provide the ability to fine-tune the focus of a lens when mounted to your camera, but there are limitations to this function; in most cases, you are limited to one adjustment at one focus distance, and for zoom lenses, one focal length only.

It is not uncommon to correct for one focus distance or focal length, only to find that this causes inaccuracies at other focus distances. For example, a 70-300mm lens might get focus spot on 70mm at the minimum focus distance, but be slightly out when focusing to infinity at 300mm. Essentially, this means some compromise is often necessary when making these adjustments, such as tailoring your adjustments to your most commonly used focal length/distance combination. Alternatively, you could concentrate on adjustments where depth of field is shallowest.

This dock provides a great deal more flexibility. With prime lenses you are able to adjust for four different preset focus distances, always including minimum focus, infinity and two mid-point distances, and with zoom lenses you can make all of these adjustments at four preset focus distances. In theory, this should result in much more accurate performance without the limitations and compromises described above.

Design and Operation

It’s also a good idea to run a basic AF fine-tune of the lens to make sure everything is as accurate as possible to begin with. When testing focus at Wex, we use the Spyderlenscal which is simple and reliable. For my 18-35mm F1.8 DC HSM I opted to do this at 35mm at around 2m, as this is one of my most commonly used settings. A note on this: to get consistently reliable results you should ensure you’re in a nice bright environment when performing focus testing, getting exposure settings of at least 1/60sec @ F2.8 and ISO 200, or that you are using your autofocus assist on your camera/flashgun to ensure this is accurate. We aren’t torture-testing the autofocus system to see what it can cope with, but we are looking for accurate calibration – so give the camera a helping hand! You should also ensure your test target is perpendicular to the optical path, rather than at an angle, as this will help with accuracy.

Once this is done, your first port of call should be to check the firmware update to make sure you are on the most recent version. Recent firmware versions can provide AF improvements or compatibility with newer camera models among other things.

The Customisation option is what we will be using for focus tuning, and when you go to this page you are met with a grid of options available for adjustments (see the screenshot above). The focal lengths and focus distances are pre-set by Sigma, and this is the one minor complaint I have with this setup. As you can see from the image, the four focus distances that can be adjusted are infinity, approx. 0.5m, approx. 0.35m and 0.28m (the minimum focus distance). There is a large gap from 0.5m to infinity where a lot of people would use the lens, and it may have been better suited having correction around the 1.5-2m mark. In practice, I have actually found my lens to be very accurate at this ‘missing’ focus distance, so I don’t think it’s entirely necessary.

To perform the actual testing, I found it easier to take a pen and paper and take notes of the measurements required from the grid. This will greatly reduce testing time as you can work through multiple measurements in a sitting. You then need to remove the lens from the dock (you are safe to disconnect it as long as you are not actively writing information to the lens). As you can see in the image below, for this type of testing you are working at much shorter distances than the 20x focal length at which you would typically test autofocus, so accuracy is key, given how shallow the depth of field is going to be.

You should also make sure you are measuring from the camera’s sensor plane, rather than the back of the camera or front of the lens; this is indicated by the Φ mark on the body.

You can then proceed to test the focus at the four different focal lengths, using the maximum aperture of the lens, before you estimate how much adjustment will be needed for each of these parameters. As with most optical testing, this can be somewhat tedious, but I found the results well worth it (so stick with it!). I found that each point on the Spyderlenscal test target equated to one point of adjustment through the software, but it’s not absolutely precise and will vary a little with each focal length and distance. You should ensure you take multiple shots for each focus distance and focal length and average your results; phase-detect autofocus is not 100% accurate – I have heard it referred to as a shotgun rather than a rifle in the past, which is fairly apt – and you will find the odd outlier. I find three to five shots is normally enough to make clear where the lens is consistently going to autofocus.

Once you have done this, you attach the lens to the USB Dock once again to make the necessary adjustments. As you can see from the screenshot below, these adjustments may not necessarily follow a specific pattern, it’s just down to your specific lens/camera combination. The results below are from my own personal copy of the Sigma 18-35mm F1.8 A HSM on my Nikon D7000. Once you’ve made any necessary adjustments you simply click on the Rewritting button and that’s it. Another minor gripe: the odd wording here (perhaps lost in translation). I must confess, I did wait a few minutes thinking this was currently updating the lens, when in fact the software was awaiting my instruction. I do wish Sigma would change this to just ‘Apply’ or ‘Re-write’, as a quick search of user comments suggests it may have caught others out too!

Once you have performed the adjustment, go back and re-test the lens on the camera to ensure all is well. I found that only a very small tweak to one point (I forget which) was required to get it absolutely perfect, so the adjustments displayed here are my final ones.

Verdict

The Sigma USB Dock is a unique proposition, so it’s very tricky to make meaningful criticisms or comparisons. Nevertheless, I found it straightforward and reliable in use. The reading and writing of the lens information when connected to a computer is swift, never taking more than a couple of seconds to recognise and read the lens, but this is quite a drawn out process which requires a full hour or so to complete.

I would say for most users – especially those with primes such as the Sigma 35mm F1.4 DG HSM – the built-in AF fine-tune option may be sufficient, unless using particularly high-resolution bodies where focusing errors become more visible.For zoom lenses, however, I have found this incredibly useful, and would go as far as calling it an essential purchase for anyone looking at the very fast 18-35mm F1.8 A HSM lens. While I have been more than delighted with the general image quality of the lens, I am grateful for the opportunity to really optimise the lens to my camera to achieve perfect focus; straight out of the box, it was throwing the odd mis-focused result when used close to minimum focus, but now I am fully confident using this lens wide open.

The lens has held its adjustments very well and hasn’t really drifted in the last six months, so it’s not an item I am going to be using very often, but for a relatively low cost I believe it’s still worth it for anyone looking to really maximise the performance of Sigma’s rather splendid new lens offerings. It’s not for everyone, and some users may find the time and energy required are not worth the extra improvements, but in my experience the results are well worth it; it clearly does work. I’ll be interested to see how long it is before other camera manufacturers look to provide something similar themselves.

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