Odd underexposure with 7D

The facts: I was taking photos of a Heron with my 7D and 100-400, with the lens set at 250mm. The camera was set on AV, with f/6.3, ISO 800 and EV +2/3. Partial metering was selected, and I was shooting RAW.

I took a few shots of the bird from my car, which turned out to be perfectly exposed with a shutter speed of 1/400 sec. I then moved ahead a few yards and took three more shots from a slightly different angle. No camera settings were changed.

For these latter shots the camera selected a shutter speed of 1/6400. Indeed, these images were four stops underexposed, based on the exposure increase in LR necessary to make it match the earlier shots.

I was not taking these shots in any burst mode. There were four seconds between these exposures, which means that I was re-metering each time. Subsequent shots, of other subjects, were properly exposed.

I have never seen this happen before. Any ideas what might be going on? Thanks.

You probably should post the good and bad images so we can see the area around your main subject. One thing that comes to mind since a heron is around water, is a slight position change caused sun or a reflection to enter the field of view and affect the metering. Some lens flare could also cause an issue with just a slight change in position.

John,

Thanks for the suggestion. I had considered that, but there isn't much difference between the scenes in terms of reflections on the water. The stream was in shadow, so there is no glare from the Sun, and no lens flare in either image. Nothing in the underexposed images approaches being properly exposed, including any water reflections that might be suspected of messing up the metering.

Having said that, I could not come up with any other explanation, given that the meter in this camera has been very reliable. If it was a slight shift in a water reflection, I am surprised (and happy) that this hasn't happened to me before.