Film format
Switching format applies the recommended layout: side by side for half-frame, stacked for full-frame. When you switch to a single image, the second image is hidden for now; switch back to two to restore it.
Canvas ratio
Adjust the export dimensions automatically from the image ratio and current spacing.
Canvas size
Calculated automatically from the image size and outer margins: a single image keeps its original pixel size, and with two images the short edge follows the images' long edge. Switching to manual keeps these values.
Outer margin
Adds film grain to the background and perforation bands, and gives the perforations softer, less uniform edges with natural light and shade. Photos are left untouched.