Audio University breaks down how to read a microphone polar pattern graph, going beyond the basic question of "where does the mic pick up sound". Using the Shure SM58 as an example, Kyle Mathias demonstrates that directional mics don't behave the same way at every frequency. Explaining what the frequency-specific response lines in the graph actually mean.
The key concept here is off-axis coloration: a directional mic doesn't have a uniform frequency response in all directions, so a sound coming from the side will be colored differently than one coming straight on. By the way: Low frequencies tend to be less directional, but that's usually manageable with a high-pass filter.
Here's the practical implication: when choosing mic placement, check the off-axis response in the frequency range of your source, before making a decision. And if a mic naturally darkens off-axis sources, that's not always a problem - it can be used as a creative tool for shaping tone without reaching for an EQ.