If you like using the camera app on the iPhone lock screen (because it's so fast to get up and running for quick shots) but would rather use Filmic you might consider this Shortcut. It uses Shortcuts new ability to run automatically when certain triggers occur. In this case as soon as the camera app is opened that's the trigger to straight away get Filmic to open instead!
To try this open the Shortcuts app on your phone and tap on the automation button at the bottom:
- Create a new Personal Automation triggered by Open App (down in the Settings list).
- In the next screen choose Camera as the app to trigger the shortcut.
- Next screen add the action to run when Camera is opened - tap on the Scripting button and choose Open App as the action to run.
- In the Open App action now at the top of the page tap the parameter labeled "app" in blue. Choose Filmic from the list.
- Next screen switch off Ask Before Running if you want Filmic to replace Camera every time, i.e. you always want to shoot video, never stills or other modes. Now tap Done and you're done.
Now whenever you open the Camera app it will instantly be replaced by Filmic even from the lock screen. Note that for it to work from the lock screen the phone has to unlock to open Filmic. So you might have to hold it in portrait orientation with FaceID (iPhone X) or use the fingerprint sensor (earlier models).
Here's a video showing how to do it but bear in mind that I personally don't like watching instructional videos and it turns out I don't like making them either! If only you could scroll through videos at your own pace like you can when you're reading.