Although the photographs are the same size when I preview the slideshow, when I export the slideshow they jump to different sizes within the frame.
Here's an example: Sarah and Al photofilm on Vimeo
The images seem to jump where the original photograph has been cropped to a different size (primarily from 8:12 to 8:10) - I've tried with 'zoom to fill frame' turned both on and off, but the images still jumped to different crops rather than filling the frame.
I'm using LR6 and the same thing happened in LR5.
I've tried using the widescreen export template but the same thing happened.
These are the settings I have selected:
I'm exporting it with 'high quality' and at 1080p.
The videos are to embed on Vimeo if that has any bearing.