[xmprt]

My experience with small QR code readers like this on ESP32 boards is that without a viewfinder, they lose a lot of utility. The feedback loop that a viewfinder gives with positioning and knowing when the QR code is detected is invaluable.

[baby_souffle]

All of the cheap camera based two-dimensional barcode modules that I see on AliExpress tend to ship with a white LED as well as a red LED. The white LED is for general flood illumination and the red LED has some sort of pattern and lens array that keeps it centered in the cameras field of view and appears sharp and focused when the barcode is at the optimal distance.

Camera detects motion, turns white LED and red LED on. Paper or screen with the QR code is now illuminated and humans can move up and down until the red Target appears in the center and in focus. Approximately a quarter of a second after that happens, you hear the beep noise indicating a successful read and decode

[Liftyee]

In places where QR payment is commonplace, readers without viewfinders often get round this issue with a pyramidal frame in front of the camera marking out its FOV, the rectangular opening indicating where the QR code should be aligned. From experience, the code often reads before you even finish approaching the reader.

[nemomarx]

I wonder if you could project some red LEDs instead, like barcode scanners?

[therein]

You can, and it still results in a subpar experience. There are products out there that do exactly that.