According to the above discussion, it seems Nikon allows users to reverse the firmware update on their camera bodies.
I have no idea how you did it.
This is not always possible with some other brands.
In at least one other brand, once you have updated the camera to new firmware, you cannot revert to the old firmware (except maybe at the factory).
I thought updating firmware involves making a permanent change to the programmable ROM chip.
Manufacturers warn users to use fresh battery so that it does not die during the update process, which may make the body unusable.
But if Sigma ART lens is such a hassle, then why pay money to suffer?
You pay and pay again.
First you pay money to buy the lens.
Then you pay money to buy the USB dock, so that you can make the lens focus properly.
The USB dock costs money about USD$60.
Maybe the USB dock should be FREE, bundled with the Sigma ART lens
Including an apology by Sigma for all the trouble caused to the customer.
Then you have to suffer the laborious process of calibrating.
If you have a Sigma 18 to 35 ART lens, you have to adjust the lens calibration (maybe a total of 16 times).
At 18mm, 24mm, 28mm, 35mm focal lengths at 4 focus distances 0.28M, 0.35M, 0.5M and infinity.
And what if you have another camera body to "calibrate" with the lens all over again. Ouch.
When you pay a lot of $$$ for a lens, you expect it to work properly first time, every time.