Why does the camera say Subject is too Dark and refuse to click the shutter

Question

I was trying to take some nice pictures of the Toronto city skyline from the shores of Lake Ontario under a full moon night. Now I put my nikon d5200 on a tripod and set f/4 and shutter = 30 sec. I am using 18 mm focal length. But from time to time, it tells me that 'Subject is too Dark' and the shutter won't even press all the way down! I am in 'M' mode.

The lens keeps rotating clockwise and counterclockwise to try and focus and gives up.

How to get around this? Do I need to experiment with manual focus? The funny thing is I point the camera directly at the bright full moon and still it says 'Subject too Dark'.

Now the funny thing is that when I look at exposure meter in view finder , it shows that the image is over exposed by 3 bars. And I expect this while looking at a full moon for 30 sec with a wide aperture.

How can it be overexposed and still be Too Dark ?

Answer 1

I suspect that you have the camera set on auto focus single shot which by default will only fire if it can acquire focus. Since getting AF to work in the dark is not easy (but not always impossible) I think what happened is the camera could not get a AF lock and would not trigger.

To take pictures like that switch to manual focus and set the lens to infinity (or focus via the view finder). Then you will have no troubles taking a picture.

Answer 2

How can it be overexposed and still be Too Dark ?

The autofocus system and the light meter are telling you two different things. All the AF system cares about is whether there's enough light falling on its sensors at any moment, with the lens wide open, to make an instantaneous decision about whether the selected point is in focus. The meter, on the other hand, looks at the brightness in the metered area and then calculates what the final exposure will be at the selected aperture, shutter speed, and ISO setting. You can change the exposure that the meter reports just by adjusting any of those three parameters, but using a slower shutter speed or a larger aperture doesn't make the actual scene any brighter, so it doesn't help the AF system at all.

On many cameras, the center AF point is the most sensitive, so it can help to focus using that center point and then reframing to get the image you want. You may also need to focus on the brightest part of the scene in order to get enough light to make autofocus work. Fortunately, when you're shooting something as big and presumably far away as a city skyline, especially with a wide angle lens, if you can get one part of the skyline in focus you'll probably have the whole thing in focus.