I was at my extended family's party this weekend. Posed group photos so nothing fancy. Indoors. I metered off my brother-in-law's face (who has a dark complexion as it is) and started taking photos. External flash (in hotshoe) was in ETTL mode. The initial exposures were 1-2 stops underexposed. I was in Av mode so added +1 EC. Exposure was a bit better. Ended up with +2 EC which was slightly overexposed but I started getting that wraparound effect on peoples' heads from the light from the window behind them. ISO was in auto but never went above 400 for some reason. Instead the camera started dropping shutter speed down (going down to 1/6 for some shots) which meant some shots were blurred. Using 18-135mm lens and generally at 50mm so f/5 aperture. In hindsight, I should have just stuck the camera in manual mode, dialled in the exposure I wanted and taken the photos. But everything started happening very quickly, so I didn't have time for test shots. Is there any reason why my 70D didn't bump up the ISO higher than 400?