Her first official portrait, which took Emsley four months to paint, was roundly criticized when it was unveiled last week.
Among the barbs detractors offered were that it made the pretty, 31-year-old duchess look older and less attractive than she is.
But Middleton, who sat for the artist twice last summer, has publicly called the portrait "brilliant."
"Some of the words written about it were so personal. I'd be inhuman if I said it didn't affect me," Emsley told Hello! about the backlash the painting received. "When you take on commissions like this, it is hazardous and you expect a bit of flak, but I expected nothing like the criticism I have received. I didn't expect it to go to the levels it did."
Emsley, 65, compared the media firestorm to a "witch hunt" and "circus."
"At first, the attacks were so vicious that there was a point where I myself doubted that the portrait of the duchess was any good. But now I've had time to reflect, I am still happy with it and am getting on with my life. There is nothing I would have changed," he said, adding the portrait may not photograph well, so people should see it for themselves at the National Portrait Gallery in London.