I will try to give a more intuitive answer here. Have you ever seen this effect? It is related to your question.
For simplicity, I will take easy numbers. The camera rate (fs) is, lets say, 1 frames per second: every second it takes a snapshot of the wheel. Now, assume that the wheel is spinning also at 1 turn per second (fw). Result? you see a static wheel, the car is going fast (the landscape is passing by...), but the wheels just seem not to spin at all, but you know they are moving. In this case, $fw = fs$, and you see how $2 \cdot \pi$ [rads] maps to $0 \cdot \pi$ [rads], you lose information in the sampling process.
Now, if $fw$ is 0.9 ([Hz]), the wheels seem to spin counterclockwise very slowly. But the question is, did it turn counterclockwise slowly or clockwise but much faster?. We have two options:
- Assume they are always spinning clockwise (or counterclockwise), i.e. it spun at 0.9 Hz clockwise. So possible frequencies span [0, N) samples or [0, 1) Hz.
- Assume that wheels reached that state through the shortest pat, i.e. it spun counterclockwise at 0.1 Hz. So possible frequencies span [-N/2,N/2) samples or [-0.5, 0.5) Hz.
In the signal domain, a wheel spinning clockwise would be sine and a counterclockwise spinning wheel would be a negative sine ($sin(- \omega \cdot t) = -sin(\omega \cdot t)$), and so on for cosine. Since negative sine is common we better go with the second approach.
In short, negative frequencies equal higher positive frequencies, in the digital domain you can not tell the difference.