AI, which is not guaranteed 100% accurate, offers this...
"NCH PhotoStage does not natively support exporting a slideshow where each individual image maintains its unique aspect ratio automatically when exporting to a standard video file. When you export with the "match content to fit" setting, PhotoStage scales all photos and videos to fill the chosen output resolution, which inherently stretches or crops anything that doesn’t match that resolution’s aspect ratio
. This is why you’re seeing distortion: all content is forced to fit the export frame.
Current capabilities and limitations:
The preview shows your images as they are, but exporting compiles everything into a single, fixed aspect ratio video. There is no “per-slide aspect ratio” feature in the standard export process.
Even the official manual and export tutorials indicate that the export format (e.g., MP4, AVI) must use a single, fixed resolution and aspect ratio for the entire slideshow.
While you can choose a “Custom” resolution and optionally check “Keep aspect ratio” in the export dialog, this only locks the overall slideshow’s ratio—not individual slide images.
Possible workarounds:
Add letterboxing or pillarboxing yourself: Manually edit your images (before importing into PhotoStage) to add black borders (top/bottom or left/right as needed), so each image matches the intended video aspect ratio (e.g., 16:9). That way, when exported, the original image content is preserved within the frame, surrounded by black bars instead of being stretched."
Videopad might prove a more flexible tool for the purpose, and can be test driven for free during a limited trial period. It has aspect ratio changes that can be applied to one or all clips.