There are three ways to approach this.
The first, as Richard says, is to use sequence settings that match your clip settings.
However, if you are purposefully using a sequence that is sized differently than your clips, you may right-click and choose “Scale to frame size” to make the clip fit the sequence. Premiere will resize the clip without adjusting its scale property, making the clip match the sequence when scaled to 100%.
Starting in CC 2014, you may right-click and choose “Set to frame size” to have Premiere scale it to your frame size by adjusting its Scale value to something other than 100%.
In the first example, “Scale to frame size,” a 1920×1080 clip with “Scale to frame size” in a 1280×720 timeline will fit the timeline perfectly at 100% — but the scaling is done and rasterized down to 1280×720 in the render pipeline before any other effects or properties are processed, so adjusting the scale up to push in will immediately soften the image because it doesn’t have access to the original 1920×1080 raster.
In the second example, “Set to frame size,” a 1920×1080 clip with “Set to frame size” in a 1280×720 timeline will also fit the program monitor perfectly, but its scale property will be adjusted to 66.67%. This allows you to push in without softening, and the original 1920×1080 raster is preserved into the rendering pipeline.
In the preferences on the General page, there is an option for “Default scale to frame size” which will perform the above operation for you by default when you put a clip into a sequence of mismatched size.
Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
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