What's the criteria for what counts as an observer? Geiger counters are, but some photon detectors in a delayed choice quantum eraser experiment[1] aren't?
Basically there is no sharp division, but the more the observation spreads, the harder it is to reverse. For one isolated particle, it's easiest. Anything we can reliably read is basically permanent, like a bit flip in a computer.
So once a photon is detected, like by making a mark on a screen or camera, it's permanent. In the delayed-choice experiment this happens twice, once for the "interference pattern" and again after the choice.
If you accept that, which I think is intuitive, then it makes the DCQE much easier to grasp. If you want to understand the experiment better, I would recommend this thread:
1. https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser