Why does my dual camera video have black bars when I post it?
Nothing is broken. Black bars on a vertical video almost always mean the video and the box displaying it are both portrait, but not quite the same portrait — one is slightly taller or shorter than the other, and whatever is showing it refuses to stretch or crop to erase the difference. Dual-camera recording is a good place for this to happen, because the canvas shape is something you pick before you start, and it is not always the shape the rest of the internet assumes.
DualCam writes one composited frame per moment of the recording — the exact image in the viewfinder is the exact image saved to the file, nothing is re-rendered afterwards. That canvas is 1080 pixels wide, and its height is one of two values: 1920, for a standard 9:16 frame, or 2340, to fill a modern iPhone screen edge to edge. Both are portrait. Both look, at a glance, like an ordinary vertical video. They are not the same rectangle.
Two portrait shapes that look identical and are not
A 1080×1920 canvas is exactly 9:16 — the ratio nearly every short-form platform, template and video player is built around. A 1080×2340 canvas is taller: same width, more height, sized to match the physical screen of a modern iPhone rather than a round ratio like 9:16. Side by side, they are obviously different rectangles. Looked at one at a time, on a phone, they both just read as “a normal vertical video” — there is no cue in the moment that tells you which one you are recording.
The difference only shows up once the video meets a box that was built for the other shape.
Where the mismatch actually shows up
A Full Screen (2340) recording, viewed somewhere that expects 9:16
Plenty of players, editors, and older or smaller-screened phones are built around a plain 9:16 frame. Drop a taller 2340 video into that shape and it does not stretch to fit — it gets placed inside the frame with the extra height showing as thin black bars above and below, or it gets cropped, depending on how that particular viewer handles a mismatch. Either way, the bars are not damage; they are the gap between two different heights.
A standard 9:16 (1920) recording, viewed full-screen on a taller iPhone display
The reverse happens too. A plain 9:16 video is shorter than the screen of a modern iPhone, so a video player showing it full-screen faces the same choice — leave a sliver of black top and bottom, or crop into the picture to fill the glass. Most players default to the safer option and show the bars rather than cut anything out.
Which one to record, in practice
Pick 9:16 (1920) when the video is headed for a specific platform, a template, or an editor downstream — it is the shape almost everything vertical is built to expect, so it is the one least likely to surprise you later. Pick Full Screen (2340) when the video will mainly be watched on the phone that shot it, or sent straight to someone with a similarly modern iPhone, and filling the display matters more than matching a platform standard.
The choice is made before you press record, not something to fix afterwards. Because the canvas is finalized the moment each frame is composited, turning a Full Screen recording into a strict 9:16 one later means cropping real picture off the top and bottom, not just trimming empty space — there is no extra footage hiding behind the bars to reveal instead.
Common follow-up questions
Which canvas should I use if I do not know where the video is going yet?
9:16 (1080×1920). It is the shape almost every short-form platform, template and editor assumes by default, so it is the one least likely to pick up bars or an unexpected crop somewhere downstream.
Can I convert a Full Screen (2340) video into 9:16 afterwards without losing anything?
Not without a real crop. The canvas is finalized the moment DualCam composites each frame, so fitting a 2340-tall video into a 1920 box afterward means trimming real picture off the top and bottom — there is no hidden extra footage behind the bars to reveal instead.
Do the bars mean the recording is broken or lower quality?
No. They are a side effect of two similar but not identical portrait rectangles meeting each other, not a defect in the file. The video underneath the bars is the full, correctly recorded frame.
Want to just do this?
DualCam records the iPhone front and back cameras at the same time and writes one finished MP4 while you shoot. Free, no account, no ads, nothing leaves the phone.