Lack of discipline, on my part. Where I went off track is starting to shoot in my U2 setting, because that is set to record only jpegs. But it is primarily configured for the pop-up to in Commander mode, with flash WB.
So I guess that the EC crept in. Then, somehow I switched to S, when my intention was M mode so I could sample at extreme shutter, and then extreme and mid f-stops. Boy, was I off.
It was an overcast day outside, and inside the store was lit by sodium vapor lighting.
The Tamron seemed sharp enough, though. Yet the Sigma equivalent, reviewed by Thom Hogan is superior in a few ways. It has SWM focus, instead of screwdriver. Though people say that Sigma products vary in quality. So it would be chancy to buy one off eBay. Perhaps new is the best route, after taking test shots and jotting down the serial number for purchase after.
I really should be adhering the checklist I created before pressing the shutter. I keep one in my camera bag for the camera, and one for configuring flash. Didn't have my bag with me.