VMA404 DC/DC step down converter


I want to supply my DSLR PENTAX K70 from an external 12V battery in order to get time-lapse and star-trails, all are long time consuming energy ( may be over 1h).
I imagine to step down the 12V with the VMA404 to 8V : the DSLR original battery is 1050 mAh (7.8 Wh). half the max amperage of the VMA404.
My question is : what happens if for any reason the step down converter fails : is there the input voltage at the output or no ?

Thanks for your response.