Why did my Canon 80D recommend such a fast shutter speed in near-total darkness?

Question

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.

---

Bonus details!

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.

Answer 1

It sounds like the scene your were framing was outside the operating range of your 80D's meter.
It's rated for EV 1-20.

At ISO 100, EV 1 is equal to f/1 for 1/2 second.¹

The maximum ISO for the 80D is 25,600. That's 8 stops higher than ISO 100.

f/8 is six stops darker than f/1.

You report that 30 seconds was a good exposure time for ISO 25,600 and f/8.

30 seconds² is 5 stops brighter than 1 second.

Compared to EV 0:

• ISO 100 to ISO 25,600 = 8 stops brighter exposure (for an 8 stops darker scene)
• f/1 to f/8 = 6 stops darker exposure (for a 6 stops brighter scene)
• 1 second to 30 seconds = 5 stops brighter exposure (for a 5 stops darker scene)

The net difference is 7 stops brighter exposure than EV 0 which means your scene is 7 stops darker than "EV 0" and 8 stops darker than "EV 1".

Thus, the scene was well outside the rated range for the light meter in your camera.

_{¹ The "baseline" for Exposure Values is EV 0 which is f/1 for 1 second at ISO 100.
² For exposure times, 30 seconds is really 32 seconds. Set your camera to 30 seconds exposure and use a stopwatch to time it. The sequence is 1, 2, 4, 16, and 32 seconds. Many of the numbers we use in photography are rounded from powers of 2 to easy to remember base 10 numbers.}