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You can adjust preview aspect ratio from the preview screen as it seen below. There are four options for this as “No change”, 1:1, 4:3 and 16:9. “No change” means; show preview screen as default which depends on capture card's settings. Other options will adjust the preview screen according to chosen ratio. These changes do not affect recordings, they are only for previewing.
The The Preview Screen can be set as four different formats. These are free, v. tile, h. tile and best fit.
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