Armor in the late fifth century bc.

[Giannis K. Hoplite]

Yes,as long as you carry a hoplite shield,you have a helmet and carry a spear,and most importantly,you join the phalanx,you are heavy infantry.
We only have indications on why this happened,and in fact we are not even sure to what extend it happened.
The first of them to me is in the first book of Thucydides,where he says that the war he is writing about (the Peloponnesian that lasted the last 30 years of the 5th century) is the greatest that ever happened not only in importance or length,but also in the armies that took part. Because,as he says, not only many city states took place taking the part of the one or the other league,but before that war all cities had much fewer hoplites than that those that they employed in this one, and they actually had fewer than they themselves believed!
Now this statement is telling,in my opinion. He is clearly saying that for some reason the hoplite armies grew in one night. And this in my opinion cannot mean that suddently thousands of citizens decided that they can afford a full panoply.
Now,of course tactics also evolved and changed. I am not sure which was the consequence of the other. I guess it's a circular arguement,but the point is that the situations allowed for more flexibility even in hoplite armies,and these often had to perform tasks that very heavily armed troops couldn't do.
Nowhere has an ancient historian stated that the Boeotian hoplites were more heavily armoured than the Laconian ones. Actually,the fashion changed everywhere,at least on how the soldiers are depicted on funerary stelae. For insance,you will see Boeotian soldiers wearing the "laconian" pilos. You will also see Athenian soldiers wearing the "laconian" pilos. And some of them were actually just felt caps and men seem to have gone to battle in them!
On the other hand,there is also indication that some armour was expected by spartan hoplites,and that there would be punishment if they faught "naked" even if they proved to be the most valiant in battle.
The end of the fifth century was also the age when hoplites were meant to board triremes and fight amphibious. Not only Athenians but also Lacedaemonians. One would wonder if this is also a factor that many soldiers of the end of the fifth century and the start of the 4th are shown in they funerary stelae unarmored.
Once again we are not sure, but it seems that a bit of everything holds some truth and it might have been no much different than in the past,with the front rankers well armoured and the rear rankers not so well armoured. Heck,we don't have a hint about this thing either,other than logic!
Hope to have confused you a bit more
Khaire
Giannis