|Not long ago, we wrote about our first experience with the Sigma USB-dock; in my eyes one of the most original innovations in lenses. You may have already read the enthusiastic Sigma Optimization Pro and USB Dock article from Roger Cicala. Now we’re publishing our test results and experience with the USB-dock.|
What can you do with this USB-dock? With the Sigma USB-dock, you can connect a Sigma Art, Contemporary or Sports lens to your computer. With the free Sigma Optimization Pro software, you can install a firmware update for all Sigma A, S or C lenses and fine-tune the AF. With S lenses, you can refine the image stabilization, focus limits and AF speed—lens customization, in other words. For just 55 euro. We tried out all the options.
AF fine-tuning Sigma 120-300 mm f/2.8 Sports
|Not every photo is focused at the right spot. Often, a photographer has aimed wrong. I speak from experience. For multiple AF fields, the camera can make a different choice than you want. That’s why I prefer to use one AF field. With SLR cameras it can happen, even using one AF field, that the camera focuses just in front of the subject (front focus) or just behind the subject (back focus).|
In modern high-end SLR cameras, you can fine tune the autofocus, by introducing a correction factor for individual lenses on the camera. That way, you can lessen front focus and back focus. Even so, installing one correction factor for a lens is not ideal. Back focus and front focus depend on the focal length and on the distance that is in focus. Which value do you choose for a zoom lens if you can only introduce one correction factor?
|With the Sigma USB-dock, you correct for back focus by adding a negative correction factor. With a positive correction factor, you correct for front focus. In our Sigma 17-70mm Contemporary review is the first positive experience from one of our readers with the USB dock and the Sigma 17-70 mm Contemporary. We have tested the AF fine-tuning with the Sigma 18-35 mm Art and the recently released Sigma 120-300 mm 2.8 Sports lens.|
|The use of the USB-dock is self-explanatory. As you can see in the picture above, with the Sigma USB-dock, you can enter an autofocus fine-tuning for 4 different distances. For zoom lenses, that can be for 4 different focal lengths. When you connect the lens to the USB dock, the software indicates which focal lengths and distances you used for the test shots. The reported distance is the distance from the sensor (with the associated symbol shown on the top of the camera body) to the subject of the test shot. It’s handy that the Sigma Art and Sports lenses have a distance scale (shown below right), where you can read the set distance. Even so, for certainty we measured shorter distances with a ruler.|
|For the fine-tuning of the auto focus, you have to be very precise. For the test shots, you can use a tool such as Lensalign. But you can also use a ruler or a ready-to-print scale, such as Jeffrey’s Autofocus test chart or as described in the Focus testing article from Bob Atkins. You set the camera on a tripod and set the lens to the largest possible aperture. The smaller the focal distance, the more quickly you can see a difference.|
If you can measure the resolution, then you can also measure the performance of the AF with shots of a test card. As the first experiment, we photographed a test card from a distance of 5 meters (as a substitute for ∞) with the Sigma 18-35 mm f/1.8 Art lens at a focal length of 35 mm. We input different correction factors in the Sigma Optimization Pro and then took a new shot each time. From the graph, it’s clear to see the effect that inputting a correction factor has.
Because it’s a first examination, we entered the measurements individually. To get the most from the graph, I needed to take more shots. From the graph, it appears that a correction factor of around -2 delivers a higher resolution than without correction. Here, it’s about subtle differences. In the graph, you see the smallest differences, but by eye, many photographers perceive no difference between images made with a correction factor anywhere between -1 and -5. In practice, you fortunately are not photographing test cards, and you aren’t making a graph of the sharpness of your shots in practice.
Front focus or back focus is not just a property of the lens. It is the result of a less-than-ideal interaction between the camera and lens while focusing. It is possible that a lens could be used with different camera bodies. Then you will probably want to input for that single lens a correction factor that depends on the camera body used. That won’t come up often, but it is certainly worth considering. It’s probably the best choice to reset the AF-correction before using a customized lens on another camera.
AF fine tuning Sigma 120-300 mm f/2.8 Sports
Sigma 120-300mm f/2.8 Sports: Move your mouse over this image for the effect of the AF-fine-tuning.
|Above is a Dutch scene, where the focus is on a point that corresponds to a mark of 45 on the ruler that we laid down. The shots were made with the Sigma 120-300 mm lens from a distance of 2 meters with a 200 mm focal length. On the left is the shot where we focused as precisely as possible on the basis of the sensor signal, with help from Liveview. In the right-hand side of the image above, the focus was done by the AF of the SLR camera. With different combinations of focal length and the distance focused, there is no visible difference between the manual focus and AF. If I have to input a single AF correction factor for this lens, then I would input 0. Even so, with a focal length of 200 mm from a distance of 2 meters, we noticed that the AF focused somewhat too far forward (“front focus”); the marking of 40 is sharper on the right than the left.|
AF customization Sigma 18-35 mm f/1.8 Art
|We also made a number of test shots with the Sigma 18-35 mm 1.8 Art lens. Sigma Optimization Pro fits the autofocus of the Sigma 18-35 mm f/1.8 Art lens on the basis of correction factors that you can input for four different focal lengths (18 mm, 24 mm, 28 mm and 35 mm) and four different distances (infinite, 50 cm, 35 cm and 28 cm). The inputting of the correction factors also leads to more precise focusing of the AF, as can be seen in the shots below. Such small differences, as in the example below, show up with many lenses, regardless of kind or brand. Many photographers don’t notice the small differences, or don’t bother themselves with them. If you don’t use a tripod, the influence of your own breathing on the focus point is even greater.|
Move your mouse over the image.
Image stabilization customization
For image stabilization, the Sigma Optimizer Pro offers three options:
With the USB-dock, I downloaded the Moderate view settings in Custom 1 and the Dynamic view settings in Custom 2, but I didn’t tell the other testers. I found it difficult to make out from the instructions what the differences are between the three settings. It is also difficult in practice to determine the difference between these three possibilities. None of us was sure that there was a difference between standard, Custom 1 and Custom 2. Remember that the testers are not photographers who work professionally with telephoto lenses daily. It’s possible that a paparazzi would notice a difference. Or if you compare the sharpness of a large number of test shots with each other, you may discover that there are differences. But if we try taking pictures by hand with the different image stabilization settings, then we don’t notice any difference.
Limiting the focus range
|For wide-angle lenses, cameras change focus lightning-fast from infinity to the shortest distance setting. For telephoto lenses with a long focal length, focusing goes more slowly. A great deal of the focus change is used for focusing between the shortest distance to a couple meters’ distance. With a 300 m telephoto lens, 30% of the focus change is used to change focus between 10 meters and infinity. For this reason there is often on a telephoto lens a focus limiter, where the shortest distance is limited to a fixed distance of a couple meters. This means that the AF-motor has less distance to travel, meaning that the camera focuses sooner.|
|For Sigma A, C or S lenses with a focus-limiter switch, right now that is only the newest Sigma 120-300 mm f/2.8, the Sigma Optimizer Pro offers the ability to set the focus range to your own preferences. You can set both the shortest and the longest distance, which can be handy if for example you’re photographing a tennis match where your subject is in a small area and you don’t want to photograph from closer or from further away. You can choose two different limits and save these in Custom 1 or Custom 2. The distance limits work. You have to realize that you have two Custom functions where multiple parameters are set at the same time: image stabilization, AF speed and the limits of the focus range.|
For the AF-speed, you have three options:
Here too, I programmed the Custom button without telling the other two testers what the settings for the buttons were. One tester didn’t notice any difference; the other noticed that the Motor’s drive was faster. None of us discovered any difference between Standard and Focus accuracy.
Conclusion Sigma USB dock
|My compliments to Sigma, that they succeeded in coming up with an innovative product that is very simple to use and with which you can achieve a very good result. With a one-time investment of 55 euro, you can update your Sigma Art, Sports and Contemporary lenses with the latest firmware. The fine-tuning of the auto focus on four distances, for zoom lenses with four focal lengths, is easy to do yourself and leads to the complete elimination of front focus and back focus. Someone who is satisfied with the photos that he or she is making has no need to use the USB-dock. But for any serious photographer who wants to get the most from an SLR camera with a high resolution sensor, the USB-dock is an absolute advantage.|
Personally, I won’t use the options to customize the AF speed or the image stabilization on a Sports lens. The AF of the Sigma 120-300 mm f/2.8 Sports is fast enough for me without customization. The standard image stabilization also works satisfactorily. For professional photographers where a millisecond’s difference makes the difference between a failed or successful shot, that may be different. We’re curious about your experience with the Sigma USB dock.