No human eye can accurately set a stylus perfectly at 90 degrees inside a record groove. You can get close visually, but there will always be microscopic deviation. And that tiny deviation matters far more than many people realize.
One of the posters here correctly mentioned that azimuth error does not simply shift channel balance. The real issue is microscopic crosstalk between channels. Information from the left channel slightly bleeds into the right, and vice versa. You usually cannot “fix” this with amplifier balance control because the problem is not balance — it is loss of focus.
The easiest way to hear it is on drums and rhythm sections. When azimuth is precisely adjusted, drum hits become sharply focused, bass articulation improves, and the entire rhythmic structure gains clarity and precision. Even a tiny stylus deviation can soften this focus.
The best analogy is camera focus. If you take a DSLR and achieve perfect focus, then slightly rotate the focus ring even by a tiny amount, the entire image immediately loses sharpness. Vinyl playback behaves in a very similar way.
The problem is that visual alignment is not enough. First, the eye simply cannot accurately detect such tiny angular deviations. Second, even expensive cartridges often do not have the stylus mounted perfectly relative to the cartridge body itself. The body may appear visually straight while the stylus is microscopically off-axis. And don't forget: the important thing is not to place the stylus properly, but to place the needle in the groove.
This becomes especially interesting on tonearms without azimuth adjustment. Even famous high-end tonearms such as the SME V assume ideal manufacturing tolerances: a perfectly mounted cartridge, a perfectly machined tonearm, and perfect geometry. But in real-world conditions, microscopic deviations still exist.
I personally had a case where a Lyra Kleos cartridge measured noticeably off during AnalogMagik calibration. The correction ended up requiring an extremely thin plastic shim under one side of the cartridge mounting screws — something that looks almost ridiculous on a serious high-end setup. Yet the improvement in focus and articulation was dramatic enough that the client was genuinely shocked.
That is why tools such as Fozgometer and programs like AnalogMagik are so valuable. Today, AnalogMagik has largely replaced older Fozgometers, but the principle remains the same: proper azimuth calibration cannot be done reliably by eye alone.
And BTW - Fozgometer itself needs calibration too.
I have a dedicated thread on this forum where I discuss analog setup techniques, cartridge alignment, turntable calibration, and various topics in vinyl playback optimization. Feel free to ask questions there.