Checked 
How to do it
Open Camera and stay in Photo mode. Hold the shutter button as soon as the moment starts: your iPhone records a video immediately, without making you switch to Video mode first.
If the clip should keep recording, slide your finger to the right onto the lock. You can then lift your finger and continue filming. Tap Stop when you are done.
When it helps
QuickTake is perfect for spontaneous moments: a first step, a funny reaction, a quick view out of the window, or anything that may be gone in two seconds. You lose less time swiping between modes.
For planned videos, use the normal Video mode instead. It gives you more deliberate control over framing, timing, and settings.
Small extra trick
If you want burst photos in Photo mode, drag the shutter to the left. Dragging right locks a video. Remembering the direction helps you capture the right format under pressure.
Sources
Selected references used to verify or support this tip.
- Record videos with your iPhone camera Apple Support · official_docs