Necessary high ISO

This is the altar of the Siena Cathedral in Siena, Italy. The area was roped off during my first Tuscany tour, but when I took my second group here a couple of days ago it was open. I shot this with the Sigma 14mm f/1.8 lens. Notice that the columns are perfectly vertical, and they are parallel with the left and right sides of the frame. Usually, wide angle lenses cause keystoning where vertical lines angle inward. The reason that didn't happen here -- and no, I didn't correct anything in Photoshop -- is because I made the back of the camera (i.e. the plane of the digital sensor) exactly perpendicular to the floor and parallel with the columns. In other words, I shot straight without angling the camera upward. When you do that, there is no keystoning at all, even with an extreme wide angle lens. In order to maintain complete depth of field, I moved back a few feet from the immediate foreground -- the incredible design on the floor -- as well as used an f/8 lens aperture. It was very dark in the cathedral, and this forced the ISO to be 20,000. No tripods were allowed, and I wanted to do a 3-f/stop incremental HDR using three frames, thus the extremely high ISO. I used Neat Image software to minimize the noise.