Using the USB Sigma Dock with FoCal to perfectly calibrate your lens
You may have read my previous articles and other articles detailing how with modern high resolution dSLRs it’s needed that you fine tune lenses to get the best out of them. You’ll see that Canon and Nikon both off this facility with Canon allowing calibration at the wide and telephoto ends of zooms lenses, whereas nikon only allow a single calibration value across all lenses. These are all done however for one target/subject distance. Typically canon and nikon recommend between 25x and 50x the focal length of the lense being calibrated ( there’s a handy tool here to work out the best distance ). However sometimes you will find that the value may be slightly different at extremely close distances for example. This is where the Sigma Dock comes into it’s own.
I’ve been using FoCal for many years now and always FoCal my lenses when they arrive so that I know they’re working well and if they need any AF fine tune adjustment. So when I had an awesome Sigma 20mm ART lens to use I thought I’d use FoCal and the USB dock to see how easy it was to calibrate more accurately than using the camera alone (remember a prime will only allow one setting).
So the procedure was the same as usual using FoCal to calibrate the lens using the camera at the different target distances as shown by the Sigma Dock Software (shown below).
The procedure is as follows:
1. Take the 4 distances from the dock software
2. Calibrate the lens at all 4 distances using FoCal and make a note of all 4 results. In this case it was -5, -5, -7 and -14 corresponding to 0.276, 0.35, 0.52, infinity (i used the focus markings on the af scale to get these as per the sigma software
3. Put the lens on the sigma dock and dial in the numbers as indicated for the different distances
4. Run the calibration again at all 4 distances using FoCal and make a note of all 4 results. In this case they all came out as zero, so I knew i was done. If i’d have non-zero results i’d have made changes as per 3 and re-run until all come out at 0.
Once I’d got values for the different distances I then dialled in the numbers FoCal had come back with into the Sigma Dock software. I then re-ran the calibration to ensure that all values were at zero. This means that the lens has been calibrated correctly for all distances. In this case the mapping was a direct 1:1 meaning that a setting of 1 on the Canon 5d3 mapped directly to a value of 1 in the sigma dock software. I wouldn’t be surprised however if this 1:1 mapping would change with different lenses, so although the process would be the same then the precise numbers may vary from lens to lens.
I am hoping to verify the exact numbers for this with a few more sigma art lenses, but i’m pleased that using the dock and focal I can quickly calibrate at different focus distances and have the benefit FoCal offers alongside the benefits of Sigma Art lenses and the USB dock.
If you’ve used FoCal and your sigma art lens and USB dock on other lenses I’d love to hear from you to see if you have found a different ratio than 1:1 for that lens.
I hope you found this little article helpful.