At work today, I lent my camera for some employee photos. I was using my trusty Sigma 10-20 and a tripod, but a few photos came out looking like CRAP. I was shooting around F/5.6 @ 20mm, ONE SHOT Focus, ISO 100-400, but the photos came out dark.... and people couldn't stand worth a crap... BUT the next group I shot later on in the day was F/11, ISO 800, ONE SHOT which turned out better... I'll post some photos later, I'm at work still... any advice to give?