I sent my lens to Canon, for adjustment. They said that the AF focusing mechanism was not functioning properly (don't remember this being a problem, and I've never mishandled this lens).

They sent it back. I did a family photo shot, and the back row of people was out of focus.

The shot was taken in manual mode, using a couple of speedlights. f/8, 45 mm, 1/200 sec. I used a 5DII. Distance from the front row was 11 feet. I used the center focus point, and focused on the person's face, sitting in the center of the front row. (disconnected the shutter button and used the back AF button for this purpose). Everyone was in a straight line in both rows.

The second row of people were about 1ft to 18 inches (max) further back from the front row.

The image showed the back row out of focus. I didn't notice this on the LCD, of course.

Based on the depth of field, both rows should have been in sharp focus.

A year earlier, I did a similar shot, and everyone was in very sharp focus (13 ft from the lens).

Also, I redid a test with two people, using my 24-70, under the same conditions, and both people were in sharp focus.

Canon supposedly adjusted the lens, but it's worse than when I sent it in.

Is this a calibration error on their part? Or, do I need to do an adjustment myself? Or, should I just send it back to them for correction?