What is aspect ratio?
Aspect ratio is the shape of the frame: its width divided by its height. 16:9 is the widescreen shape almost all video uses. 9:16 is the same shape turned on its side, which is what phones shoot and what vertical feeds expect. 1:1 is square, 4:3 is the old television shape, and cinema uses wider ratios still such as 2.39:1.
The number itself is simple. The consequences of changing it are where the difficulty lives.
Something has to give
A 16:9 frame cannot become a 9:16 frame without losing something. You have three options and they are all trade-offs.
Crop. Cut the sides off. You keep full resolution and lose about three quarters of the width. If your subject is centred this works well. If two people are talking on opposite sides of the frame, one of them is now gone.
Letterbox or pillarbox. Add bars to pad the frame to the new shape. Nothing is lost, the composition survives intact, and the picture is smaller on screen. Perfectly respectable, and often the right answer for something that was carefully framed.
Fill with a blurred background. Scale the original to fit the height, then fill the space either side with a blurred, enlarged copy of the same frame. It reads as deliberate rather than accidental, keeps the whole composition, and has become the default look on vertical feeds for good reason.
Reframing is a different thing entirely
Cropping picks one region and keeps it for the whole clip. Reframing follows the subject, moving the crop window as they move, so a person walking across a wide shot stays in the vertical frame throughout.
That is much closer to what an editor would do by hand, and it is the difference between a vertical version that works and one where the subject drifts out of shot halfway through.
The mistakes worth avoiding
Stretching. Squashing a 16:9 frame into a 9:16 box makes everyone look wrong in a way viewers notice without being able to name. Never do this. Crop, pad, or fill.
Cropping before you have finished editing. Once the sides are gone they are gone, and any later decision to reframe has nothing left to work with. Keep the full-frame master.
Baking bars into the master. Letterbox for a specific delivery, not in your archive copy, or a future crop will be cropping your own black bars.
Ignoring safe areas. Vertical feeds overlay captions, usernames and buttons over roughly the bottom fifth of the screen. Anything important down there will be covered.
Doing it on a Mac
Crisp handles all three approaches on-device and will pick a sensible default rather than making you specify one. Ask for a vertical version in plain English and you get the fill-and-blur treatment unless you say otherwise, because it is the option that loses nothing and reads as intentional.
Aspect ratio questions
Why does my video have black bars on the sides?
The player is pillarboxing a narrower frame into a wider window rather than distorting it. That is correct behaviour. Bars top and bottom are the same thing in the other direction, usually a cinema-ratio frame in a 16:9 window.
Is cropping or blur-fill better for vertical?
Crop when the subject is centred and there is nothing important at the edges, because you keep full resolution. Blur-fill when the composition matters or several things are happening across the width, because nothing is lost.
Does converting to vertical lose quality?
Cropping keeps the pixels it keeps at full quality, so the result is lower resolution overall but not degraded. Blur-fill keeps the whole picture and scales it down slightly. Neither reduces quality the way a re-encode does.
What resolution should a vertical video be?
1080 by 1920 covers essentially every vertical platform. Going higher rarely helps, since the platform will re-encode to its own ladder anyway, and it makes the upload slower for no visible gain.
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