How it gets information in and out of the lens
Sigma sells a USB dock (it looks like a hockey puck) tailored to your lens mount type. It’s cheap enough that you have no excuse not to buy it. The software is free from Sigma. It retrieves the connected lens model, mount type, serial number, firmware version, and customization settings using the Optimization Pro program. It sends firmware updates and any customization settings into non-volatile memory inside the lens.
One dock will work with all of your Sigma lenses that support the USB dock (unless you have a mix of Nikon/Canon/Sigma/Pentax/Sony bodies). Older Sigma lenses don’t support this.
Optimization Pro Features
Update the algorithms for anti-vibration, focus speed, manual focus ‘throw’, and even future added features that haven’t been invented yet (such as focus fine-tune for different apertures on fast lenses? Are you listening, Sigma?) It lets you get any firmware bug fixes without having to send your lens to the factory, and it enables Sigma to alter any communications interface changes, in case Nikon or Canon get cute and try to make a compatibility-breaking change in the future. Tammy (sorry, I mean Tamron) and Tokina users might be out of luck if the lens communications requirements get changed in the future.
The lens firmware version is checked when the program gets executed while the lens/dock is connected, so you don’t have to remember to request update checks.
Focus Fine Tune
For zoom lenses, it can calibrate focus at 4 distances for each of 4 focal lengths (16 total settings). The distances are (1) closest focus (2) near range (3) far range (4) infinity.
For prime lenses, you can calibrate focus at 4 distances only.
Now, even multiple copies of a lens can be custom-calibrated. Nikon, for instance, can’t tell the difference between two copies of the same lens model, and only knows a single fine tune number to apply. It’s very annoying when you have two camera bodies and you have to mark which lens to assign to which body (I paste the fine-tune value inside each lens cap, along with the lens serial number). Sigma can read its lens serial number electronically, so every lens is unique. Nikon and Canon et al. need to pay attention to details like this. Also note that lower-end cameras with no focus fine-tune can still get tuned lenses if they’re Sigma and use the dock.
I think all AF lenses could benefit from this kind of focus fine-tuning intelligence. Real-world zoom optics need different fine-tuning at different focal lengths and different distances, but so far only Sigma seems to acknowledge this problem even exists.
The 16 fine-tune settings for my 150-600mm Contemporary zoom
Notice the different characteristics of each focal length shown in the “Focus Setting” fine-tune screen. The pathetic single AF shift value that my Nikon D7000 offers would leave the lens functioning very poorly indeed. The available shift range is plus/minus 20. Now I can finally leave my camera AF tune setting on “0”. I suppose a second camera body using this same lens would need a non-zero AF fine-tune value, but at least that would shift all 16 in-lens settings.
Note the “Rewriting” button in the dialog box above that should say “Save” instead. Don’t be one of those people that get caught staring at this screen waiting for “Rewriting” to finish.
I personally use MTF Mapper software and special focus test charts to evaluate critical focus. MTF Mapper is free; the author is Frans van den Bergh (this guy is absolutely brilliant). His program is available here: . This software lets me get really accurate information to enable me to enter good offset data into the Optimization Pro Focus Setting screen. MTF Mapper can also be used to evaluate lens sharpness, but that’s a topic for another day. It’s an iterative process to estimate the focus offset values to use, reprogram the lens with Optimization Pro, re-photograph the focus test charts, and then re-analyze the photos to see how you did. MTF Mapper uses “dcraw.exe” to decode RAW files, which is frequently updated for new cameras from most vendors.
Focus chart analyzed and annotated by MTF Mapper program
The above (unsharpened, RAW) photo shows one of the iterations to evaluate focus accuracy. This particular focal length and distance shows a focus error which will require the fine tune value to be decreased to pull the focus more toward the camera.
The MTF Mapper web site also provides graphic files that have the test chart images for printing (both focus charts and resolution charts). I mostly use A0 size prints (about 33” X 47”) mounted onto a board; the bigger the chart, the better the results. I also print onto heavy glossy paper. Regular uncoated, lightweight paper sucks for trying to mount it and keep it totally flat. The more diffuse illumination you use when photographing the chart, the better. Also, you want bright light to give your camera focus system the most help (EV 10 or better for most cameras).
Charts are of course useless for the “infinity” test shots. With long focal lengths, heat shimmer is a real problem. Maybe something like craters on the moon in cool weather would work best; I have used really distant tree branches or pine needles (600 – 900 feet) with the long focal lengths.
Detail from the annotated focus chart created by MTF_Mapper_Gui program
Profile view of the focus chart from MTF Mapper program
The focus chart large vertical edge is depicted as the vertical blue line above. The smoothed plot of each measured square is depicted in green. Each individual square edge reading is plotted in red (units of MTF50 line pairs/mm). Focus is properly calibrated when the blue line aligns with the green peak.
The above plot is from a test of the Sigma 150-600mm Contemporary lens at 600mm at f/6.3 using a Nikon D7000 camera. Note that a few edges actually measure over 40 line pairs per millimeter (and this is considered worst-case sharpness for this lens)!
Focus testing was done using AF-C mode (with ‘focus’ button), and it is actually pretty repeatable. This is how this lens is mostly going to be used, so I tested it using the same focus mode. Test-to-test variation was pretty small. I shoot 10 shots at each focal length and distance, and then choose the average focus distance to decide how it is calibrated. By the way, the tested lens (150-600 Contemporary) had zero focus chatter in AF-C. The focus locked and stayed there on the target. This is very un-like my 70-300 Nikkor that chatters constantly, even in good light on high-contrast targets. It’s extremely annoying, and a deal-breaker for me. I read that the new Nikkor 80-400 AF-S VR has the same annoying focus chatter (see the review at photographylife.com). I used to think the chatter was a camera body problem, but now I realize it’s a lens problem. Chatter equals missed focus about half of the time, and focus fine-tune is an exercise in futility. But I digress.
Full view of focus chart at 600mm at 19 meters (with annotation text).
Note that the focus chart large ‘rectangle’ shown above is actually a trapezoid, to help counteract perspective. The large black ‘rectangle’ edge nearest the photo edge is actually much taller than the large edge in the center of the photo. The little squares are also farther apart vertically at the left edge than the right edge.
Use high shutter speeds to eliminate any motion blur side effects; high ISO values have minimal effect on MTF measurements compared to motion blur, so don’t be afraid to push it to get blur-free pictures. I also use my infrared remote unit with mirror lock-up to rid vibrations, in addition to a big-honking tripod.
Just remember: “garbage in, garbage out”. It’s worth your while to do careful focus calibration. If you read reviews of a Sigma lens that supports the USB dock but the calibration wasn’t done, then don’t put too much stock in the review in regards to focus accuracy. In this case, the saying goes “nothing in, garbage out”.
I don’t see any fundamental reason why Sigma can’t do all of this focus calibration at their factory, but talk is cheap here. Hey, at least they’re giving us the tools to help ourselves. No other manufacturer is even close to this level of sophistication (yet).
There a bunch of videos here from a Canadian company that go into detail about using the Sigma software and Sigma topics in general.