This morning I went out to shoot the sunrise. I went out well before dawn, and it's one day from the new moon, so it was very dark while I was setting up. I turned the ISO up to the maximum setting for composition and focus purposes with aperture at f/8. The exposure meter was recommending a shutter speed of 1 s, which was way too short. I got a low-key but good exposure with a shutter speed of 30 s. This is a difference of about 5 stops.
I got the same readings from the meter using both evaluative and average metering modes.
At this point I just started ignoring the exposure meter and firing off a bunch of test shots to get everything dialed in (glad I went out so early), but I'm curious why it gave this reading. Once there was enough light for me to make out landscape features by eye, the metering system started giving sane results again.
Why did I get such a wonky metering result in those conditions? Does the camera just give up and fudge a reading in that much darkness? I would have expected it to just flash at the low end of the exposure scale like it does when I've left the lens cap on.
When I first set up, I was still in aperture priority mode from my last session. I was just trying to get a starting point, so I half-pressed in this mode, and it suggested 1 s, which I already figured wouldn't be nearly enough. I assumed that maybe there was some built-in limit so that Av mode wouldn't ever give a shutter speed slower than that, so I switched to manual right away. But even in manual mode, the exposure meter was reading near 0 when the shutter speed was set to 1 s, and the meter was off the bright end of the scale when the shutter speed was set to 30 s, which gave a good picture.
I only used high ISO to get the framing right. I used ISO 400 and lower for the shots I intended to keep, and used shutter speed to control the exposure. And the fun thing about sunrises is that they just keep getting brighter and brighter until eventually it's just daylight, so the ISO got down to 100 pretty quickly, and the shutter speed kept getting faster.