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My understanding is stadiometric optical rangefinders till 74, then laser after that. T-55A gets 'Newa' TKI laser rangefinder in 74.
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Naturally stadia lines were used for ranging with the sights, but this is not comparable to a full blown coincidence/stereoscopic rangefinder as employed on the early
T-64/T-72 and 60's western MBTs.
With stadia lines you are essentially guessing range on the basis of the apparent size of the target: if it looks big it means it is close, if small it's far. One problem is, the stadia marks used for measuring have to be calculated and drawn for a predetermined target; if the target differs from that, because of profile, actual size etc. accuracy is off. This is not an issue with the latter types.
Now,
how much this is an issue in real life battles I don't know, but adding a relatively maintenance intensive piece of kit must have been felt to be an advantage, at least for the type engagements planned (same applies to stabilizers on russian
T-XX).
It has been reported that there was an istance of iraqi T-55s engaging USMC AMTRACs and missing with all shots falling short. Supposedly because they were bigger than the standard size target, so they appeared closer than actually were and therefore insufficient superelevation was applied. But such stories have to be taken with pinch of salt.