What happens if a DSLR loses power mid-exposure?

Question

I recently purchased a dummy battery for my camera so I could take timelapses over a longer period of time than my battery would last. For now I'm using it in places where AC power is readily available, but I've started thinking about other options, such as an inverter powered from my car battery.

With a battery, I presume that the camera is in frequent communication with the battery about its charge level and doesn't push the battery right to the edge of undervoltage cut-off, allowing the camera to decide for itself when it's time to stop working, and shutting down on its own terms. However, with a dummy battery, the camera just sees "100%" all the time and all is well.

What happens if the AC power is lost during the middle of an exposure? Let's say that you're taking a 10-second exposure and after 5 seconds the AC power goes out. The shutter would be open and the mirror would be up. Obviously you're not gonna find your photo on the SD card. But what about the mechanics of the camera? Are things spring-loaded, and would the loss of power cause things to snap back closed from the springs? Or would the shutter be left open, and the camera have to figure itself out when it gets power back?

Edit: I will mostly be using my Canon Rebel XSi, though I'll also be getting a dummy battery for my Canon Rebel T7 as well.

Answer 1

What happens if a [camera] loses power mid-exposure?

Any parts that are driven by electronic motors will stop moving wherever they are. Old film cameras that are spring driven may continue to operate without power.

Digital cameras will not capture or save the image. Film cameras may over expose, not expose, or properly expose, depending on the state of the shutter when it is stopped.

When power is restored, some cameras will stay off until explicitly turned on. Some turn on upon insertion of the battery. When turned back on, most cameras will return to a ready state. Nikon DLSR and mirrorless cameras show an error that is cleared when the shutter button is pressed.

The above has been confirmed in Canon DSLR, FujiFilm mirrorless, Nikon DSLR and mirrorless, Sony mirrorless.

Answer 2

There are no springs in Canon EOS lightboxes. Everything is powered by electrical servo motors. If power is suddenly interrupted during an exposure the mirror would be left up and the shutter will be left open. There are no springs in Canon EOS lenses, either. Aperture blades are moved in both directions by micro-servos. The lens would be stopped down to whatever aperture it was set, the focus position would be maintained, etc. None of this is harmful to the camera. After all, these things are made to hold those positions for very long exposures.

When the camera is restarted, the camera's startup routine will detect the positions of the mirror, shutter mechanism, aperture diaphragm, focus elements, etc. and return them to the default state for whatever mode the camera is in.

For example, if one was shooting in Live View mode when power was lost and the physical switch on the back of the camera is still in the same position when the camera is repowered, the camera will return to Live View mode.

With STM lenses, and possibly some or all of the few other "focus-by-wire" EOS lenses Canon has made, the focus mechanism will be moved to a default position, even if the lens is currently set to 'M' (manual focus).

What might not be returnable to the default state is(are) the memory card(s) inserted in the camera's memory card slot(s).

In addition to leaving the mirror up, the lens' aperture stopped down, etc., if the memory card is being actively written to when power is lost, the memory card could potentially be corrupted. Of course the exposure in process would not yet be ready to be written to the memory card, but previous exposure(s) might well be in process of transferring from the camera's memory buffer to the memory card at the time of a sudden power loss.

Even if the memory card is not corrupted, any images in the camera's memory buffer that had not yet been transferred to the memory card would also be lost and unrecoverable.

I once had a third party battery grip that wasn't quite as inflexible as it should have been.¹ When flexed in just the right way, power from the grip to the camera would be interrupted for a split second. That interruption was enough to lose all of the frames currently in the camera's memory buffer. I only discovered this was occurring when I couldn't find a burst of images of a game winning touchdown in a triple overtime high school football game! The images weren't on the memory card, but a string of frame numbers (IMG_4567, IMG_4568, IMG_4569... through IMG_4577, IMG_4578) were totally missing from the images that were on the card. It took a bit of experimenting with the camera and grip before I finally figured out what was going on. I ordered a new grip the same day and had it by the next week's game!

_{¹ In all fairness to the maker of the third party grip, the camera had previously taken a pretty hard lick with the grip attached. But the mechanical weakness was not discovered during subsequent testing of the camera, lens, and grip following that collision on the sideline a few games earlier.}